Internet Draft Lloyd McIntyre
Internet Fax Working Group Stephen Zilles
Expires in six months July 30, 1997
File Format for Internet Fax
<DRAFT-IETF-FAX-TIFFPLUS-01.TXT>
Status
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Abstract
This Internet Draft describes the TIFF representation of the image data
specified by the ITU-T Recommendations for black-and-white and color
facsimile. The document provides a standard definition for TIFF-F (also
known as TIFF Class F), which is used for a subset of black-and-white
facsimile, and standard definitions for the TIFF representation of the
ITU-T Recommendations for facsimile, including color facsimile. For the
most part, existing TIFF constructs and fields are used; new TIFF fields
are introduced as necessary. The document also describes the refinement
of the registration of the MIME type image/tiff.
Table of Contents
1. INTRODUCTION........................................................3
1.1. Scope..........................................................3
1.2. Approach.......................................................4
1.3. Organization of this draft.....................................4
2. TIFF and Fax........................................................4
2.1. TIFF Overview..................................................4
2.1.1. File Structure.............................................5
2.1.2. Image Structure............................................6
2.1.3. Recommended TIFF File Structure for Fax Applications.......6
2.2 TIFF Fields for Fax Applications................................7
2.2.1. TIFF Fields required for all fax modes.....................8
2.2.2. Additional TIFF Fields required for all fax modes..........9
2.2.3. TIFF Fields recommended for all fax modes..................9
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2.2.4. New TIFF Fields recommended for fax modes......................10
3. Minimal Black-and-white Mode.......................................11
3.1. Overview......................................................12
3.2. Required TIFF Fields..........................................12
3.2.1 Baseline Fields............................................12
3.2.2 Extension Fields...........................................13
3.2.3 New Fields.................................................14
3.3. Recommended TIFF Fields.......................................14
3.4. End of Line Restrictions......................................14
3.5. Minimal Black-and-White Mode Summary..........................15
4. Extended Black-and-White mode......................................16
4.1. Overview......................................................16
4.2. Required TIFF Fields..........................................17
4.2.1. Baseline Fields...........................................17
4.2.2. Extension Fields..........................................18
4.2.3. New Fields................................................19
4.3. Recommended TIFF Fields.......................................19
4.3.1. Baseline Fields...........................................19
4.3.2. Extension Fields..........................................19
4.3.3. New Fields................................................19
4.4. TIFF Class F Definition.......................................19
4.5. Extended Black-and-White Fax Mode Summary.....................19
5. Base Color Fax Mode................................................22
5.1. Overview......................................................22
5.2. Required TIFF Fields..........................................22
5.2.1. Baseline Fields...........................................22
5.2.2. Extension Fields..........................................23
5.2.3. New Fields................................................24
5.3. Recommended TIFF Fields.......................................24
5.4. Base Color Fax Mode Summary...................................25
6. Lossless Color Mode................................................27
6.1. Overview......................................................27
6.1.1. Color Encoding............................................27
6.1.2. JBIG Encoding.............................................28
6.2. Required TIFF Fields..........................................28
6.2.1. Baseline Fields...........................................28
6.2.2. Extension Fields..........................................29
6.2.3. New Fields................................................29
6.3. Recommended TIFF Fields.......................................29
6.4. Lossless Color Fax Mode Summary...............................29
7. Mixed Raster Content Mode..........................................31
7.1 Overview.......................................................31
7.1.1. MRC 3-layer model.........................................32
7.1.2. A TIFF Representation for the MRC 3-layer model...........33
7.2. Required TIFF Fields..........................................34
7.2.1. Baseline Fields...........................................34
7.2.2. Extension Fields..........................................35
7.2.3. New Fields................................................36
7.3. Recommended TIFF Fields.......................................37
7.4. Rules and Requirements for Images.............................37
7.5. MRC Fax Mode Summary..........................................38
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8. Mime Sub-Type......................................................41
9. IANA Registration..................................................41
10. Security Considerations...........................................42
11. References........................................................42
12. Authors' Addresses................................................44
Appendix A: Summary of TIFF Fields for Internet Fax ..................44
Appendix B: Use of TIFF-F for Black-and-White Fax.....................49
1. Introduction
1.1 Scope
This document defines a TIFF-based file format specification and
corresponding MIME image/tiff content type consistent with the charter
of the IETF Internet Fax Working Group for enabling standardized
messaging-based fax over the Internet. In particular, it describes the
fields and parameters required to enable TIFF (Tag Image File Format) to
represent the data content and the structure generated by the suite of
ITU-T Recommendations for Group 3 facsimile. These standards, and the
TIFF fields described here, support five facsimile modes:
1. minimal black-and-white mode, using binary compression [T.4]
2. extended black-and-white mode, using binary compression [T.6, T.82]
3. lossy color and grayscale mode, using JPEG compression [T.42, T.81]
4. lossless color and grayscale mode, using JBIG compression [T.43,
T.82]
5. mixed raster content mode [MRC], using a combination of compression
methods in 1-4
This specification defines a minimal interchange set of fields that will
guarantee that, at least, binary black-and-white images will be
supported. Implementations are required to support this minimal
interchange set of fields.
This specification builds on a previously proposed standard [RFC1314]
and on work in progress [TIFF-F] for black-and-white fax to provide a
standard definition of TIFF Class F. It extends that work to make it
compatible with the relevant ITU-T Recommendations for color and
grayscale facsimile. It also proposes a modification to the image/tiff
content type that would accommodate the fax modes.
Within its defined scope of specifying a file format for Internet Fax,
this draft:
1. specifies TIFF structures for facsimile data,
2. defines ITU fax-compatible values for existing TIFF fields,
3. defines new TIFF fields and values for ITU-T compatibility.
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1.2 Approach
This specification uses the current TIFF spec [TIFF] as the basis for
describing color and grayscale facsimile modes. It takes advantage of
TIFF features and structures that bridge the data formats and
performance requirements of both legacy fax machines and host-based fax
applications. A TIFF-based approach also builds on an established base
of users and implementors and ensures backward compatibility with
existing TIFF-based IETF proposals and work in progress for Internet
fax.
The basic approach to using TIFF for facsimile data is to insert the fax
data stream in a TIFF file and to use TIFF fields to encode the
parameters that describe the properties of the data. TIFF constructs for
pages, images, and strips allow a TIFF file to preserve the fax data
stream structure, and the performance advantages that come with it.
1.3 Organization of this draft
Section 2 describes the general representation of fax data and
parameters in a TIFF file, including the TIFF fields that used in all
fax modes. Section 3 defines the representation of the minimal black-
and-white facsimile mode required of all implementations. Section 4
defines the optional extended black-and-white fax capabilities,
including TIFF-F. Section 5 defines the representation of lossy color
and grayscale facsimile. Section 6 defines the representation of
lossless color and grayscale facsimile and Section 7 defines the
representation of mixed raster content facsimile.
Sections 8 and 9 describe the MIME content type and IANA registration.
The three remaining sections give Security Considerations, References
and Authors' Addresses. Appendix A gives a summary of the TIFF fields
used or defined in this document. Appendix B summarizes and describes
the use of TIFF-F for storing black-and-white images from current
implementations. This profile summarizes the work in progress [TIFF-F]
for the minimal interchange black-and-white mode. To implement only the
minimal interchange black-and-white set of fields and values, one need
read only Sections 1, 2, 3, 8, 9, and 10.
2. TIFF and Fax
2.1. TIFF Overview
This section is based on the current TIFF specification [TIFF] and
selected TIFF Technical Notes [TTN1, TTN2]. Some features of TIFF are:
- TIFF is capable of describing bilevel, grayscale, palette-color,
and full-color image data in several color spaces.
- TIFF includes a number of compression schemes that allow developers
to choose the best space or time tradeoff for their applications.
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- TIFF is not tied to specific scanners, printers, or computer
display hardware.
- TIFF is portable. It does not favor particular operating systems,
file systems, compilers, or processors.
- TIFF is designed to be extensible-to evolve gracefully as new needs
arise.
- TIFF allows the inclusion of an unlimited amount of private or
special-purpose information.
This specification employs these features in using TIFF to represent
black-and-white and color facsimile data.
2.1.1. File Structure
TIFF is designed for scanned images, which makes it a good match for
facsimile documents, which are multi-page scanned raster images. Each
raster image consists of a number of rows or scanlines, each of which
has the same number of pixels, the unit of sampling. Each pixel has at
least one sample (exactly one for black-and-white images).
A TIFF file begins with an 8-byte image file header that describes the
byte order used within the file, includes a value that identifies the
file as a TIFF file, and then gives an offset that points to the first
image file directory (IFD). The IFD can be at any location in the file
after the header but must begin on a word boundary.
An IFD is a sequence of tagged fields, sorted in ascending order by tag
value. An IFD consists of a 2-byte count of the number of fields, a
sequence of field entries and a 4-byte offset to the next IFD. The
fields contain information about the image and pointers to the image
data. Each separate raster image in the file is represented by an IFD.
Each IFD field entry has 12 bytes, consisting of a 2-byte tag, 2 bytes
identifying the field type, 4 bytes giving the field count or number of
values, and the 4-byte offset of the field value. If, based on the type
and count, the value takes 4 bytes or less, then it replaces the offset
in the field entry. Details are given in the TIFF specification [TIFF].
A TIFF file can contain more than one IFD, where each IFD is a subfile
whose type is given in the NewSubfileType field. Multiple IFDs can be
organized either as a linked list, with the last entry in each IFD
pointing to the next IFD (the pointer in the last IFD is 0), or as a
tree, using the SubIFDs field in the primary IFD [TTN1]. The SubIFDs
field contains an array of pointers to child IFDs of the primary IFD.
Child IFDs describe related images, such as reduced resolution versions
of the primary IFD image. The same IFD can point both to a next IFD and
to child IFDs, and child IFDs can themselves point to other IFDs. A
Baseline TIFF reader is not required to read any IFDs beyond the first.
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All fax modes represent a multi-page fax image as a linked list of IFDs,
with a NewSubfileType field containing a bit that identifies the IFD as
one page of a multi-page document. Each IFD has a PageNumber field,
identifying the page number, starting at 0 for the first page. The Mixed
Raster Content fax mode, described in Section 7, uses child IFDs also.
2.1.2 Image Structure
An IFD stores an image as one or more strips. A strip consists of 1 or
more scanlines (rows) of raster image data in compressed form. An image
may be stored in a single strip or may be divided into several strips,
which would require less memory to buffer. (Baseline TIFF [TIFF]
recommends about 8k bytes per strip, but existing FAX usage is typically
one strip per image.)
Each IFD requires the StripOffsets field, which is an array of pointers
to the strip or strips that contain the actual image data. TIFF requires
that each strip, except the last, contain the same number of scanlines,
which is given in the RowsPerStrip field. This document introduces the
new StripRowCounts field that allows a variable number of scanlines per
strip, which is required by the Mixed Raster Content fax mode.
Image data is stored as uninterpreted, compressed data streams within a
strip. The formats of these streams follow the ITU-T Recommendations.
The Compression field in the IFD indicates the type of compression, and
other TIFF fields in the IFD describe image attributes, such as color
encoding and spatial resolution. Compression parameters are stored in
the compressed data stream, rather than in TIFF fields. This makes the
TIFF representation and compressed data format specification independent
of each another. This approach, modeled on [TTN2], allows TIFF to
gracefully add new compression schemes as they become available.
Some attributes can be specified both in the compressed data stream and
within a TIFF field. It is possible that the two values will differ.
When this happens for values required to interpret the data stream, then
the values in the data stream take precedence. For informational values
that are not required to interpret the data stream, such as author name,
then the TIFF field value takes precedence.
2.1.3 Recommended TIFF File Structure for Fax Applications
TIFF does not specify the ordering of IFDs, fields values and image data
in a file. It is up to individual applications to require or recommend
an ordering. In fax applications, it is recommended that the data be
ordered in a TIFF file the same way it is ordered in a fax transmission.
There would be an IFD for each page in a multi-page fax document, and
the IFDs would occur in the same order in the file as the pages occur in
the document. The IFD for page 1 would come first, the IFD for page 2
next, and so on.
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It is also recommended that the field values and image data of a given
IFD occur in the file between the offset to the next IFD and the start
of the next IFD. The fields values would come first, pointed to by the
value offsets of the field entries in the IFD, followed by the image
data, pointed to by the values of the StripOffsets field. If the image
data in the IFD is stored in multiple strips, then the strips would
occur in the file in the same order that the data they contain occurs in
the facsimile transmission, starting at the top of the page.
This specification introduces an optional new GlobalParametersIFD field,
defined in Section 2.2.4. This field has type IFD and contains
parameters describing the fax session. While it is often possible to
obtain these parameters by scanning the file, it is convenient to make
them available together in one place for fast and easy access. If the
GlobalParametersIFD occurs in a TIFF file, it is recommended that it be
located in the first IFD, immediately following the 8-byte image file
header.
2.2 TIFF Fields for Fax Applications
The TIFF specification is organized as a baseline and several
extensions, including technical notes that will be incorporated in the
next release of TIFF. The baseline and extensions have required and
optional fields.
Facsimile applications require (and recommend) a mixture of baseline and
extensions fields, as well as new fields that are not part of the TIFF
specification and that are defined in this document. Sections 3-7 for
each fax mode have sub-sections for required and recommended fields;
each sub-section organizes the fields according to whether they are
baseline, extension or new fields.
The fields required for facsimile have only a few legal values,
specified in the ITU-T Recommendations. Of these legal values, some are
required and some are optional, just as they are required (mandatory) or
optional in fax implementations that conform to the ITU-T
Recommendations. The required and optional values are noted in the
sections on the different fax modes.
This section describes the fields required or recommended by more than
one fax mode. The pattern for the description of TIFF fields in this
draft is
FieldName(TagValueInDecimal) = values. TYPE
WhetherRequiredByTIFForTIFFforFAX
Count = (omitted if =1) = (if not in current spec but available)
Explanation of the field, how it's used, and the values it can have.
Default value, if any
When a fields default value is the desired value, that field may be
omitted from the relevant IFD unless specifically required by the text
of this specification.
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2.2.1. TIFF Fields required for all fax modes
The TIFF fields listed in this section are required by all fax modes,
but do not have mode-specific values. The next sub-section lists the
required fields common to all fax modes and which do have mode-specific
values.
ImageLength(257). SHORT or LONG
RequiredByTIFFBaseline
Total number of scanlines in image.
No default, must be specified.
NewSubFileType(254) = (Bit 1=1). LONG
RequiredByTIFFforFAX, RequiredByTIFFBaseline
Bit 1 is 1 if the image is a single page of a multi-page document.
Default = 0 (no subfile bits on)
PageNumber(285) SHORT
RequiredByTIFFforFAX, TIFFExtension
Count = 2
The first number represents the page number (0 for the first page);
the second number is the total number of pages in the document. If
the second value is 0, then the total page count is not available.
ResolutionUnit(296) = 2, 3. SHORT
RequiredByTIFFBaseline
The unit of measure for resolution. 2 = inch, 3 = centimeter
Default = 2 (field may be omitted if this is the value)
RowsPerStrip(278). SHORT or LONG
RequiredByTIFFBaseline
The number of scanlines per TIFF strip. For a single strip image,
this is the same as the value of the ImageLength field.
Default = 2**32 - 1 (meaning all scanlines in one strip)
StripByteCounts(279). SHORT or LONG
RequiredByTIFFBaseline
Count = number of strips
For each strip, the number of bytes in that strip after compression.
No default, must be specified.
StripOffsets(273). SHORT or LONG
RequiredByTIFFBaseline
Count = number of strips
For each strip, the byte offset from the beginning of the file to
that strip.
No default, must be specified.
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2.2.2 Additional TIFF Fields required for all fax modes
The TIFF fields listed in this section are required by all fax modes,
but the values associated with them depend on the particular fax mode
being described. Therefore, only the fields are defined here; the values
applicable to a particular fax mode are described in Sections 3-7.
BitsPerSample(258) SHORT
RequiredByTIFFBaseline
Number of bits per image sample
Default = 1 (field may be omitted if this is the value)
Compression(259) SHORT
RequiredByTIFFBaseline
Compression method used for strip data
Default = 1 (no compression, so may not be omitted for FAX)
ImageWidth(256) SHORT or LONG
RequiredByTIFFBaseline
The number of samples per scanline (row) of the image
No default, must be specified.
PhotometricInterpretation(262) SHORT
RequiredByTIFFBaseline
The color space of the image data
No default, must be specified
SamplesPerPixel(277) SHORT
RequiredByTIFFBaseline
The number of components per pixel
Default =1 (field may be omitted if this is the value)
XResolution(282) RATIONAL
RequiredByTIFFBaseline
The horizontal resolution of the image is expressed in pixels per
resolution unit.
No default, must be specified
YResolution(283) RATIONAL
RequiredByTIFFBaseline
The vertical resolution of the image is expressed in pixels per
resolution unit.
No default, must be specified
2.2.3 TIFF Fields recommended for all fax modes
The TIFF fields listed in this section are recommended, but not
required, for use with all fax modes. Recommended fields that are mode-
specific are described in Section 3-7.
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DateTime(306) ASCII
OptionalInTIFFBaseline
Date/time in the format "YYYY:MM:DD HH:MM:SS" in 24-hour format.
No default.
DocumentName(269) ASCII
OptionalInTIFFExtension
The name of the scanned document. This is a TIFF extension field,
not a Baseline TIFF field.
No default.
ImageDescription(270) ASCII
OptionalInTIFFBaseline
A string describing the contents of the image.
No default.
Orientation(274) = 1-8 SHORT
RequiredByTIFFBaseline
1: 0th row represents the visual top of the image; the 0th column
represents the visual left side of the image. See the current TIFF
specification [TIFF] for further values.
Default = 1 (field may be omitted if this is the value)
Software(305) ASCII
OptionalInTIFFBaseline
The optional name and release number of the software package that
created the image.
No default.
2.2.4 New TIFF Fields recommended for fax modes
The new TIFF fields listed in this section are recommended for all fax
modes, but their support is not expected for the minimal black-and-white
fax mode described in Section 3. These fields describe "global"
parameters of the fax session that created the image data. They are
optional, not part of the current TIFF specification, and are defined in
this document.
The first new field, GlobalParametersIFD, is an IFD that contains global
parameters and is located in a Primary IFD.
GlobalParametersIFD(TagToBeAssigned). IFD
An IFD containing global parameters. It is recommended that a TIFF
writer place this field in the first IFD, where a TIFF reader would
find it quickly.
Each field in the GlobalParametersIFD is a TIFF field that is legal in
any IFD. Required baseline fields should not be located in the
GlobalParametersIFD, but should be in each image IFD. If a conflict
exists between fields in the GlobalParametersIFD and the image IFDs,
then the data in the image IFD shall prevail.
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Among the GlobalParametersIFD entries is a new ProfileType field which
generally describes information in this IFD and in the TIFF file.
ProfileType(TagToBeAssigned) LONG
The type of image data stored in this IFD:
0 = Unspecified
1 = Group 3 fax
Other values for further study
The following new global fields are defined in this document as IFD
entries for use with fax applications.
ITUFaxMode(TagToBeAssigned) = 0 - 4. BYTE
The currently defined values are:
0: does not conform to ITU-T Recommendations for facsimile,
1: minimal black & white lossless (ITU-T Rec. T.4),
2: extended black & white lossless (ITU-T Rec. T.6, T.85),
3: lossy color and grayscale (ITU-T Rec. T.42),
4: lossless color and grayscale (ITU-T Rec. T.43),
5: Mixed Raster Content (ITU-T Draft Rec. T.44).
Notes: In a file that uses an ITU encoding with non-ITU parameters,
e.g JPEG compression of CMYK data, ITUFaxMode is 0.
CodingMethods(TagToBeAssigned). LONG
This field indicates which coding methods are used in the file.
A bit value of 1 indicates:
Bit 0: unspecified compression,
Bit 1: 1-dimensional coding, ITU-T Rec. T.4 (MH - Modified Huffman),
Bit 2: 2-dimensional coding, ITU-T Rec. T.4 (MR - Modified Read),
Bit 3: 2-dimensional coding, ITU-T Rec. T.6 (MMR - Modified MR),
Bit 4: ITU-T Rec. T.82 coding, applying ITU-T Rec. T.85 (JBIG),
Bit 5: ITU-T Rec. T.81 (Baseline JPEG),
Bits 6-31: reserved for future use
Notes: There is a limit of 32 coding methods.
VersionYear(TagToBeAssigned) BYTE
Count: 4
The year of the standard specified by the ITUFaxMode field, given as
4 characters, e.g. '1997'; used in lossy and lossless color modes.
VersionNumber(TagToBeAssigned) BYTE
The version of the standard specified by the ITUFaxMode field. A
value of 0 indicates Version 1.0; used in Mixed Raster Content mode.
3. Minimal Black-and-White Fax Mode
Black-and-white mode is the binary fax application most users are
familiar with today. This mode is appropriate for black-and-white text
and line art. Black-and-white mode is divided into two levels of
capability. This section describes the minimal interchange set of TIFF
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fields that must be supported by all implementations in order to assure
that some form of image, albeit black-and-white, can be interchanged.
Section 4 describes extensions to the minimal interchange set of fields
that provide a richer set of black-and-white capabilities.
3.1. Overview
The minimal interchange portion of the black-and-white facsimile mode
supports the original Group 3 fax resolutions, commonly called
"standard" and "fine," and two compression methods: 1-dimensional
Modified Huffman (MH) and 2-dimensional Modified READ (MR) lossless
bilevel image coding [T.4]. (READ is an acronym for Relative Element
Address Designate.)
The minimal black-and-white mode corresponds to the original TIFF Class
F specification [TIFF-F0].
3.2. Required TIFF Fields
Besides the fields listed in Section 2.2.1, the minimal black-and-white
fax mode requires the following fields. The fields listed in Section
2.2.1 and the fields and fax-specific values specified in this sub-
section must be supported by all implementations.
3.2.1 Baseline Fields
BitsPerSample(258) = 1. SHORT
RequiredByTIFFBaseline
Binary data only.
Default = 1 (field may be omitted if this is the value)
Compression(259) = 3. SHORT
RequiredByTIFFBaseline
The value 3 is not a TIFF Baseline value [TIFF]; it is an extended
value. The T4Options field must be specified and its value
determines whether the data is encoded using the MH or MR encoding
of [T.4].
FillOrder(266) = 1, 2. SHORT
RequiredByTIFFBaseline
1 = Pixels are arranged within a byte such that pixels with lower
column values are stored in the higher-order bits of the bytes,
i.e., most significant bit first (MSB).
2 = Pixels are arranged within a byte such that pixels with lower
column values are stored in the lower-order bits of the bytes, i.e.,
least significant bit first (LSB).
The FillOrder field is used to describe how 1 bit pixels are stored
within a byte of compressed image data. There are two possible
FillOrders for each byte: 1) the pixels are stored with the lowest
numbered pixel in the byte in the Most Significant Bit (MSB) of the
byte, or 2) the pixels are stored with the lowest numbered pixel in
the byte in the Least Significant Bit (LSB) of the byte.
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NOTE: Baseline TIFF readers are only required to support the first
FillOrder where the lowest numbered pixel is stored in the MSB of the
byte. However, because many devices, such as modems, transmit the LSB
first when converting the data to serial form, it is common for black-
and-white FAX products to use the second FillOrder where the lowest
numbered pixel is stored in the LSB.
ImageWidth(256) = 1728, 2048, 2432. SHORT or LONG
RequiredByTIFFBaseline
This mode supports the fixed page widths: 1728, 2048, 2432.
No default, must be specified.
PhotometricInterpretation(262) = 0, 1. SHORT
RequiredByTIFFBaseline
0 = pixel value 1 means black, 1 = pixel value 1 means white
No default, must be specified
SamplesPerPixel(277) = 1. SHORT
RequiredByTIFFBaseline
The number of components per pixel; 1 for black-and-white
Default =1 (field may be omitted if this is the value)
XResolution(282). RATIONAL
RequiredByTIFFBaseline
The horizontal resolution of the image is expressed in pixels per
resolution unit. In pixels/inch, the allowed value is 204.
No default, must be specified
YResolution(283). RATIONAL
RequiredByTIFFBaseline
The vertical resolution of the image is expressed in pixels per
resolution unit. In pixels/inch, the allowed values are 98 and 196.
No default, must be specified
3.2.2 Extension Fields
T4Options(292) = (Bit 0 = 0 or 1, Bit 1 = 0, Bit 2 = 1) LONG
RequiredTIFFExtension (when Compression = 3)
Bit 0 = 1 indicates MR encoding, = 0 indicates MH encoding.
Bit 1 must be 0
Bit 2 = 1 indicates that fill bits are required, = 0 no fill bits
Default is all bits are 0 (therefore field must be specified)
The T4Options field is required if the Compression field has a value
of 3. The T4Options field specifies which encoding (MH or MR) is
used and whether fill bits are required. Fill bits are required in
the minimal interchange set. (See Section 3.4).
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3.2.3 New Fields
None.
3.3 Recommended TIFF Fields
There are three new optional fields for describing black & white fax
page quality. They are not defined in the current TIFF specification
[TIFF].
BadFaxLines(326). SHORT or LONG
The number of "bad" scan lines encountered by the facsimile device
during reception. A "bad" scanline is defined as a scanline that,
when decoded, comprises an incorrect number of pixels.
No default.
CleanFaxData(327) = 0, 1, 2. SHORT
Indicates if "bad" lines encountered during reception are stored in
the data, or if "bad" lines have been replaced by the receiver.
0 = No "bad" lines
1 = "bad" lines exist, but were regenerated by the receiver,
2 = "bad" lines exist, but have not been regenerated.
No default.
NOTE: Many facsimile devices do not actually output bad lines. Instead,
the previous good line is repeated in place of a bad line. Although this
substitution, known as line regeneration, results in a visual
improvement to the image, the data is nevertheless corrupted. The
CleanFaxData field describes the error content of the data. That is,
when the BadFaxLines and ImageLength fields indicate that the facsimile
device encountered lines with an incorrect number of pixels during
reception, the CleanFaxData field indicates whether these lines are
actually in the data or if the receiving facsimile device replaced them
with regenerated lines.
ConsecutiveBadFaxLines(328). SHORT or LONG
Maximum number of consecutive "bad" scanlines received. The
BadFaxLines field indicates only the quantity of bad lines. The
ConsecutiveBadFaxLines field is an indicator of the distribution of
bad lines and may therefore be a better general indicator of
perceived image quality.
No Default.
3.4 End of Line Restrictions
The minimal interchange set of fields and values for black-and-white fax
requires all EOLs in compressed image data must be byte-aligned. An EOL
is said to be byte-aligned when fill bits have been added as necessary
before EOL codes such that EOL always ends on a byte boundary, thus
ensuring an EOL-sequence of a one byte preceded by a zero nibble:
xxxx0000 00000001.
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Recall that Huffman encoding encodes bits, not bytes. This means that
the end-of-line token may end in the middle of a byte. In byte
alignment, extra zero bits (Fill) are added so that the first bit of
data following an EOL begins on a byte boundary. In effect, byte
alignment relieves application software of the burden of bit-shifting
every byte while parsing scan lines for line-oriented image manipulation
(such as writing a TIFF file).
3.5 Minimal Black-and-White Fax Mode Summary
The table below summarizes the TIFF fields that comprise the minimal
interchange set for black-and-white facsimile. The Baseline and
Extension fields must be supported by all implementations. For
convenience in the table, certain fields which have a value that is a
sequence of flag bits are shown taking integer values that correspond to
the flags that are set. An implementation should test the setting of the
relevant flag bits individually, however, to allow extensions to the
sequence of flag bits to be appropriately ignored. (See, for example,
T4Options below.)
Fields suffixed with an asterisk, "*", are optional fields and need not
be specified.
+---------------------------+--------------------------------+
| Baseline Fields | Values |
+---------------------------+--------------------------------+
| BitsPerSample | 1 |
+---------------------------+--------------------------------+
| Compression | 3: 1D Modified Huffman coding |
| | set T4Options = 4 |
| | 3: 2D Modified Read coding |
| | set T4Options = 5 |
+---------------------------+--------------------------------+
| FillOrder | 1: most significant bit first |
| | 2: least significant bit first |
+---------------------------+--------------------------------+
| ImageWidth | 1728, 2048, 2432 |
+---------------------------+--------------------------------+
| ImageLength | n: total number of scanlines |
| | in image |
+---------------------------+--------------------------------+
| NewSubFileType | 2: Bit 1 identifies single |
| | page of a multi-page document |
+---------------------------+--------------------------------+
| PageNumber | n,m: page number n followed by |
| | total page count m |
+---------------------------+--------------------------------+
| PhotometricInterpretation | 0: pixel value 1 means black |
| | 1: pixel value 1 means white |
+---------------------------+--------------------------------+
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+---------------------------+--------------------------------+
| ResolutionUnit | 2: inch |
| | 3: centimeter |
+---------------------------+--------------------------------+
| RowsPerStrip | n: number of scanlines per |
| | TIFF strip |
+---------------------------+--------------------------------+
| SamplesPerPixel | 1 |
+---------------------------+--------------------------------+
| StripByteCounts | <n>: number or bytes in TIFF |
| | strip |
+---------------------------+--------------------------------+
| StripOffsets | <n>: offset from beginning of |
| | file to each TIFF strip |
+---------------------------+--------------------------------+
| XResolution | 204 (pixels/inch) |
+---------------------------+--------------------------------+
| YResolution | 98, 196 (pixels/inch) |
+---------------------------+--------------------------------+
| Extension Fields |
+---------------------------+--------------------------------+
| T4Options | 4: Modified Huffman, fill bits |
| | required |
| | 5: 2D Modified Read, fill bits |
| | required |
+---------------------------+--------------------------------+
| New Fields |
+---------------------------+--------------------------------+
| BadFaxLines* | number of "bad" scanlines |
| | encountered during reception |
+---------------------------+--------------------------------+
| CleanFaxData* | 0: no "bad" lines |
| | 1: "bad" lines exist, but were |
| | regenerated by receiver |
| | 2: "bad" lines exist, but have |
| | not been regenerated |
+---------------------------+--------------------------------+
| ConsecutiveBadFaxLines* | Max number of consecutive |
| | "bad" lines received |
+---------------------------+--------------------------------+
4. Extended Black-and-white fax mode
4.1 Overview
The previous section described the minimal interchange set of TIFF
fields that must be supported by all implementations in order to assure
that some form of image, albeit black-and-white, can be interchanged.
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This section describes extensions to the minimal interchange set of
fields that provide a richer set of black-and-white capabilities. In
addition to the MH and MR encodings, Modified Modified READ (MMR) as
described in [T.6] is supported. Also available is the single-
progression sequential mode of JBIG [T.82], used in accordance with the
application rules given in ITU-T Rec. T.85 [T.85]. JBIG coding offers
improved compression for halftoned originals.
This section also provides a standard, updated definition of TIFF-F,
also known as TIFF Class F,equivalent to that found in [TIFF-F]. It has
been proposed to make [TIFF-F] an informational RFC.
4.2. Required TIFF Fields
This section lists the required fields and the values they must have to
be ITU-compatible. Besides the fields listed in Section 2.2.1, the
extended black-and-white fax mode requires the following fields.
4.2.1. Baseline Fields
BitsPerSample(258) = 1. SHORT
RequiredByTIFFBaseline
Binary data only.
Default = 1 (field may be omitted if this is the value)
Compression(259) = 3, 4, 9. SHORT
RequiredByTIFFBaseline
3 = 1- or 2- dimensional coding, must have T4Options field This is
a TIFF Extension value [TIFF].
4 = 2-dimensional coding, ITU-T Rec. T.6 (MMR - Modified Modified
Read, must have T6Options field)) This is a TIFF Extension value.
9 = ITU-T Rec. T.82 coding, applying ITU-T Rec. T.85 (JBIG) This is
a TIFF Extension value.
Default = 1 (and is not applicable; field must be specified)
ImageWidth(256) SHORT or LONG
RequiredByTIFFBaseline
This mode supports the following fixed page widths: 1728, 2048,
2432, 2592, 3072, 3456, 3648, 4096, 4864.
No default; must be specified
PhotometricInterpretation(262) = 0, 1. SHORT
RequiredByTIFFBaseline
0 = pixel value 1 means black 1 = pixel value 1 means white
No default, must be specified
SamplesPerPixel(277) = 1. SHORT
RequiredByTIFFBaseline
1 = monochrome, bilevel in this case (see BitsPerSample)
Default =1 (field may be omitted if this is the value)
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XResolution(282) RATIONAL
RequiredByTIFFBaseline
The horizontal resolution of the image is expressed in pixels per
resolution unit. In pixels/inch, the allowed values are: 200, 204,
300, 400, and 406.
No default, must be specified
YResolution(283) RATIONAL
RequiredByTIFFBaseline
The vertical resolution of the image is expressed in pixels per
resolution unit. In pixels/inch, the allowed values are: 98, 100,
196, 200, 300, 392, and 400 pixels/inch
No default, must be specified
NOTE: Not all combinations of XResolution, YResolution and ImageWidth
are legal. The following table gives the legal combinations for inch-
based resolutions and corresponding paper sizes [T.30].
+--------------+-----------------+---------------------------+
| XResolution x YResolution | ImageWidth |
+--------------+-----------------+---------------------------+
| 200x100,200x200,200x400 | 1728 | 2048 | 2432 |
+--------------+-----------------+---------------------------+
| 300 x 300 | 2592 | 3072 | 3648 |
+--------------+-----------------+---------------------------+
| 400 x 400 | 3456 | 4096 | 4864 |
+--------------+-----------------+---------------------------+
|Letter,A4| B4 | A3 |
+---------+--------+--------+
| Paper Size |
+---------------------------+
FillOrder(266) = 1, 2. SHORT
See 3.2.1 above
4.2.2. Extension Fields
T4Options(292) = (Bit 0 = 0 or 1, Bit 1 = 0, Bit 2 = 0 or 1) LONG
RequiredTIFFExtension (when Compression = 3)
T4Options was known as Group3Options in a prior version of [TIFF].
Bit 0 = 1 indicates MR encoding, = 0 indicates MH encoding.
Bit 1 must be 0
Bit 2 = 1 indicates that fill bits are required, = 0 no fill bits
Default is all bits are 0 (field must be specified for fax,see
3.2.2.)
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T6Options(293) = (Bit 0 = 0, Bit 1 = 0). LONG
RequiredTIFFExtension (when Compression = 4)
Used to indicate parameterization of 2D Modified Modified Read
compression.
Bit 0 must be 0.
Bit 1 = 0 indicates uncompressed data mode is not allow; = 1
indicates uncompressed data is allowed (see [TIFF]).
Default is all bits 0. For FAX, the field must be present and have
the value 0. The use of uncompressed data where compression would
expand the data size is not allowed for FAX.
4.2.3. New Fields
None.
4.3. Recommended TIFF Fields
4.3.1. Baseline Fields
See Section 2.2.3.
4.3.2. Extension Fields
See Section 2.2.3.
4.3.3. New Fields
See Sections 2.2.4 and 3.3.
4.4. TIFF-F Definition
TIFF-F, also known as TIFF Class F, was originally defined in 1989
[TIFF-F0]. Recent discussions and work have proposed updating it to
include MMR compression and higher resolutions, which the ITU-T has
approved for use with Group 3 facsimile. If JBIG compression
(Compression=9) and the global parameters are excluded, then the
extended black-and-white mode described in this section corresponds to
the most recent proposal for TIFF-F [TIFF-F]. Therefore, this section
provides a standard definition of TIFF-F.
4.5. Extended Black-and-white Fax Mode Summary
Recommended fields are shown with an asterisk *.
Required fields or values are shown with a double asterisk **. If the
double asterisk is on the field name, then all the listed values are
required of implementations; if the double asterisks are in the Values
column, then only the values suffixed with a double asterisk are
required of implementations.
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+---------------------------+--------------------------------+
| Baseline Fields | Values |
+---------------------------+--------------------------------+
| BitsPerSample | 1** |
+---------------------------+--------------------------------+
| Compression | 3**: 1D Modified Huffman and |
| | 2D Modified Read coding |
| | 4: 2D Modified Modified Read |
| | coding |
| | 9: JBIG |
+---------------------------+--------------------------------+
| DateTime* | {ASCII}: date/time in the |
| | 24-hour format |
| | "YYYY:MM:DD HH:MM:SS" |
+---------------------------+--------------------------------+
| FillOrder** | 1: most significant bit first |
| | 2: least significant bit first |
+---------------------------+--------------------------------+
| ImageDescription* | {ASCII}: A string describing |
| | the contents of the image. |
+---------------------------+--------------------------------+
| ImageWidth | 1728**, 2048**, 2432**, 2592, |
| | 3072, 3456, 3648, 4096, 4864 |
+---------------------------+--------------------------------+
| ImageLength** | n: total number of scanlines |
| | in image |
+---------------------------+--------------------------------+
| NewSubFileType** | 2: Bit 1 identifies single |
| | page of a multi-page document |
+---------------------------+--------------------------------+
| Orientation* | 1-8, Default 1 |
+---------------------------+--------------------------------+
| PhotometricInterpretation | 0: pixel value 1 means black |
| ** | 1: pixel value 1 means white |
+---------------------------+--------------------------------+
| ResolutionUnit** | 2: inch |
| | 3: centimeter |
+---------------------------+--------------------------------+
| RowsPerStrip** | n: number of scanlines per |
| | TIFF strip |
+---------------------------+--------------------------------+
| SamplesPerPixel** | 1 |
+---------------------------+--------------------------------+
| Software* | {ASCII}: name & release |
| | number of creator software |
+---------------------------+--------------------------------+
| StripByteCounts** | <n>: number or bytes in TIFF |
| | strip |
+---------------------------+--------------------------------+
| StripOffsets** | <n>: offset from beginning of |
| | file to each TIFF strip |
+---------------------------+--------------------------------+
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+---------------------------+--------------------------------+
| XResolution | 200, 204**, 300, 400, 406 |
| | (written in pixels/inch) |
+---------------------------+--------------------------------+
| YResolution | 98**, 196**, 100, |
| | 200, 300, 392, 400 |
| | (written in pixels/inch) |
+---------------------------+--------------------------------+
| Extension Fields |
+---------------------------+--------------------------------+
| T4Options | 0: required if Compression is |
| | Modified Huffman, no fill bits |
| | 1: required if Compression is |
| | 2D Modified Read, no fill bits |
| | 4**: required if Compression |
| | is Modified Huffman, fill bits |
| | 5**: required if Compression |
| | is 2D Modified Read, fill bits |
+---------------------------+--------------------------------+
| T6Options | 0: required if Compression is |
| | 2D Modified Modified Read |
+---------------------------+--------------------------------+
| DocumentName* | {ASCII}: name of scanned |
| | document |
+---------------------------+--------------------------------+
| PageNumber** | n,m: page number followed by |
| | total page count |
+---------------------------+--------------------------------+
| New Fields |
+---------------------------+--------------------------------+
| BadFaxLines* | number of "bad" scanlines |
| | encountered during reception |
+---------------------------+--------------------------------+
| CleanFaxData* | 0: no "bad" lines |
| | 1: "bad" lines exist, but were |
| | regenerated by receiver |
| | 2: "bad" lines exist, but have |
| | not been regenerated |
+---------------------------+--------------------------------+
| ConsecutiveBadFaxLines* | Max number of consecutive |
| | "bad" lines received |
+---------------------------+--------------------------------+
| GlobalParametersIFD* | <IFD>: global parameters IFD |
+---------------------------+--------------------------------+
| ProfileType* | n: type of data stored in |
| | TIFF file |
+---------------------------+--------------------------------+
| ITUFaxMode* | n: ITU-compatible fax mode |
+---------------------------+--------------------------------+
| CodingMethods* | n: compression algorithms used |
| | in file |
+---------------------------+--------------------------------+
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5. Base Color Fax Mode
5.1. Overview
This is the base mode for color and grayscale facsimile, which means
that all applications that support color fax must support this mode. The
basic approach is the lossy JPEG compression [T.81] of L*a*b* color data
[T.42]. Grayscale applications use the L* lightness component; color
applications use the L*, a* and b* components.
This mode uses a new PhotometricInterpretation field value to describe
the L*a*b* encoding specified in [T.42]. This encoding differs in two
ways from the other L*a*b* encodings used in TIFF [TIFF, TTN1]: it
specifies a different default range for the a* and b* components, based
on a comprehensive evaluation of existing hardcopy output, and it
optionally allows selectable range for the L*, a* and b* components.
5.2. Required TIFF Fields
This section lists the required fields, in addition to those given in
Section 2.2.1, and the values they must support to be compatible with
ITU-T Rec. T.42 and Annex E in ITU-T Rec. T.4.
5.2.1. Baseline Fields
ImageWidth(256). SHORT or LONG
This mode supports the following fixed page widths: 864, 1024, 1216,
1728, 2048, 2432, 2592, 3072, 3456, 3648, 4096, 4864.
BitsPerSample(258) = 8, 12. SHORT
Count = SamplesPerPixel
The base color fax mode requires 8 bits per sample, with 12 as an
option. 12 bits per sample is not baseline TIFF.
Compression(259) = 7. SHORT
Base color fax mode uses Baseline JPEG compression. Value 7
represents JPEG compression as specified in [TTN2].
PhotometricInterpretation(262) = 10. SHORT
Base color fax mode requires pixel values to be stored using the CIE
L*a*b* encoding defined in ITU-T Rec. T.42. This encoding is
indicated by the PhotometricInterpretation value 10, referred to as
ITULAB. With this encoding, the minimum sample value is
mapped to 0 and the maximum sample value is mapped to (2^n - 1),
i.e. the maximum value, where n is the BitsPerSample value.
The conversion from unsigned ITULAB-encoded samples values to signed
CIE L*a*b* values is determined by the Decode field; see Sec. 5.2.2
Note: PhotometricInterpretation values 8 and 9 specify encodings for
CIE L*a*b* [TIFF] and ICC L*a*b* [TTN1], but they are fixed
encodings, which use different minimum and maximum samples than the
T.42 default encoding. As currently defined, they are not able to
represent fax-encoded L*a*b* data.
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SamplesPerPixel(277) = 1, 3. SHORT
1: L* component only
3: L*, a*, b* components
Encoded according to PhotometricInterpretation field
XResolution(282). RATIONAL
YResolution(283). RATIONAL
The resolution of the image is expressed in pixels per resolution
unit. In pixels per inch, allowed XResolution values are: 100, 200,
300, and 400. The base color fax mode requires the pixels to be
square, hence YResolution must equal XResolution.
5.2.2 Extension Fields
The JPEG compression standard allows for the a*b* chroma components of
an image to be subsampled relative to the L* lightness component. The
extension fields ChromaSubSampling and ChromaPositioning define the
subsampling. They are the same as YCbCrSubSampling and YCbCrPositioning,
which have been renamed to reflect their applicability to other color
spaces.
ChromaSubSampling(530). SHORT
Count = 2
Specifies the subsampling factors for the chroma components of a
L*a*b* image. The two subfields of this field, ChromaSubsampleHoriz
and ChromaSubsampleVert, specify the horizontal and vertical
subsampling factors respectively.
SHORT 0: ChromaSubsampleHoriz = 1, 2, 4.
1: equal numbers of lightness and chroma samples horizontally,
2: twice as many lightness samples as chroma samples horizontally,
4: four times as many lightness samples as chroma samples
horizontally.
SHORT 1: ChromaSubsampleVert = 1, 2, 4.
1: equal numbers of lightness and chroma samples vertically,
2: twice as many lightness samples as chroma samples vertically,
4: four times as many lightness samples as chroma samples
vertically.
The default value for ChromaSubSampling is (2,2), which is the
default for chroma subsampling in color fax [T.42]. T.42 supports no
chroma subsampling, i.e. ChromaSubSampling = (1,1), as an option.
ChromaPositioning(531) = 1, 2. SHORT
Specifies the spatial positioning of chroma components relative to
the lightness component. See the current TIFF specification under
YCbCrPositioning for further information.
1: centered,
2: cosited.
Default = 1, which is what ITU-T T.4, Annex E specifies.
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Decode(TagToBeAssigned). SHORT
Count = 2 * SamplesPerPixel
Describes how to map image sample values into the range of values
appropriate for the current color space. In general, the values are
taken in pairs and specify the minimum and maximum output value for
each color component. For the base color fax mode, Decode maps from
the ITULAB encoding to L*a*b* and has a count of 6 values. In this
case, the input is an unsigned ITULAB-encoded value and the output
is a signed L*a*b* value.
L* = Decode[0] + Lsample x (Decode[1]-Decode[0])/(2^n -1)
a* = Decode[2] + asample x (Decode[3]-Decode[2])/(2^n -1)
b* = Decode[4] + bsample x (Decode[5]-Decode[4])/(2^n -1)
where Decode[0] is the minimum value for L*, Decode[1] is the
maximum value, etc. and n is the BitsPerSample field value, either 8
or 12. ITU-T Rec. T.42 specifies the ITULAB encoding in terms of a
range and offset for each component, which are related to the Decode
field values as follows:
Decode[0] = - (Lrange x Loffset) / 2^n
Decode[1] = Decode[0] + Lrange
NOTE: The following table gives the default range and offsets, as
specified in ITU-T Rec. T.42, and the corresponding minimum and maximum
L*a*b* component and ITULAB encoded values.
+-----------------------------------------------------------+
| Component | Rec. T.42 | L*a*b* | ITULAB |
| | defaults | values | encoding |
+-----------+------------------+------------+---------------+
| | Range | Offset | Min | Max | Min | Max |
+-----------+--------+---------+------+-----+-----+---------+
| L* | 100 | 0 | 0 | 100 | 0 | 2^n - 1 |
+-----------+--------+---------+------+-----+-----+---------+
| a* | 170 | 128 | -85 | 85 | 0 | 2^n - 1 |
+-----------+--------+---------+------+-----+-----+---------+
| b* | 200 | 96 | -75 | 125 | 0 | 2^n - 1 |
+-----------+--------+---------+------+-----+-----+---------+
The default value for Decode is (0, 100, -85, 85, -75, 125), which is
based on the ITULAB encoding, with PhotometricInterpretation 10,
SamplesPerPixel 3, and BitsPerSample 8.
5.2.3. New Fields
None.
5.3. Recommended TIFF Fields
See Sections 2.2.3. and 2.2.4.
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5.4 Base Color Fax Mode Summary
Recommended fields are shown with an asterisk *
+---------------------------+--------------------------------+
| Baseline Fields | Values |
+---------------------------+--------------------------------+
| BitsPerSample | 8: 8 bits per color sample |
| | 12: optional 12 bits/sample |
+---------------------------+--------------------------------+
| Compression | 7: JPEG |
+---------------------------+--------------------------------+
| DateTime* | {ASCII}: date/time in the |
| | 24-hour format |
| | "YYYY:MM:DD HH:MM:SS" |
+---------------------------+--------------------------------+
| ImageDescription* | {ASCII}: A string describing |
| | the contents of the image. |
+---------------------------+--------------------------------+
| ImageWidth | 864, 1024, 1216, 1728, 2048, |
| | 2432, 2592, 3072, 3456, 3648, |
| | 4096, 4864 |
+---------------------------+--------------------------------+
| ImageLength | n: total number of scanlines |
| | in image |
+---------------------------+--------------------------------+
| NewSubFileType | 2: Bit 1 identifies single page|
| | of a multi-page document |
+---------------------------+--------------------------------+
| Orientation* | 1-8, Default 1 |
+---------------------------+--------------------------------+
| PhotometricInterpretation | 10: ITULAB |
+---------------------------+--------------------------------+
| ResolutionUnit | 2: inch |
| | 3: centimeter |
+---------------------------+--------------------------------+
| RowsPerStrip | n: number of scanlines per |
| | TIFF strip |
+---------------------------+--------------------------------+
| SamplesPerPixel | 1: L* (lightness) |
| | 3: LAB |
+---------------------------+--------------------------------+
| Software* | {ASCII}: name & release number |
| | of creator software |
+---------------------------+--------------------------------+
| StripByteCounts | <n>: number or bytes in |
| | TIFF strip |
+---------------------------+--------------------------------+
| StripOffsets | <n>: offset from beginning |
| | of file to each TIFF strip |
+---------------------------+--------------------------------+
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+---------------------------+--------------------------------+
| XResolution | 100, 200, 300, 400 (written |
| | in pixels/inch) |
+---------------------------+--------------------------------+
| YResolution | equal to XResolution (pixels |
| | must be square) |
+---------------------------+--------------------------------+
| Extension Fields |
+---------------------------+--------------------------------+
| DocumentName* | {ASCII}: name of scanned |
| | document |
+---------------------------+--------------------------------+
| PageNumber | n,m: page number followed by |
| | total page count |
+---------------------------+--------------------------------+
| Decode | minL, maxL, mina, maxa, minb, |
| | maxb: minimum and maximum |
| | values for L*a*b* |
+---------------------------+--------------------------------+
| ChromaSubSampling | ChromaSubsampleHoriz: |
| | 1: equal numbers of lightness |
| | and chroma samples |
| | 2: twice as many lightness |
| | samples as chroma samples |
| | 4: four times as many |
| | lightness samples as chroma |
| | samples |
| | ChromaSubsampleVert: |
| | 1: equal numbers of lightness |
| | and chroma samples |
| | 2: twice as many lightness |
| | samples as chroma samples |
| | 4: four times as many lightness|
| | samples as chroma samples |
+---------------------------+--------------------------------+
| ChromaPositioning | 1: centered |
| | 2: cosited |
+---------------------------+--------------------------------+
| New Fields |
+---------------------------+--------------------------------+
| GlobalParametersIFD* | <IFD>: IFD containing |
| | global parameters |
+---------------------------+--------------------------------+
| ProfileType* | n: type of data stored in |
| | TIFF file |
+---------------------------+--------------------------------+
| ITUFaxMode* | n: ITU-compatible fax mode |
+---------------------------+--------------------------------+
| CodingMethods* | n: compression algorithms |
| | used in file |
+---------------------------+--------------------------------+
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+---------------------------+--------------------------------+
| VersionNumber* | n: version of ITU fax standard |
+---------------------------+--------------------------------+
| VersionYear* | byte sequence with year of |
| | ITU fax standard |
+---------------------------+--------------------------------+
6. Lossless Color Mode
6.1. Overview
This mode, defined in [T.43], uses JBIG to losslessly code three types
of color and grayscale images: one bit per color CMY, CMYK and RGB
images; a palettized (i.e. mapped) color image; and continuous tone
color and grayscale images. The last two are multi-level and use the
L*a*b* encoding specified in [T.42].
6.1.1. Color Encoding
While under development, this mode was called T.Palette, as one of its
major additions was palette or mapped color images. Baseline TIFF only
allows RGB color maps, but ITU-T Rec. T.43 requires L*a*b* color maps,
using the encoding specified in ITU-T Rec. T.42. Palette color images
are expressed with indices (bits per sample) of 12 bits or less, or
optionally 13 to 16 bits.
Enabling T.43 color maps in TIFF requires the extension field Indexed,
defined in [TTN1], and the PhotometricInterpretation field value 10,
defined in Section 5.2.1. The following table shows how T.43 image data
is interpreted, using the PhotometricInterpretation, SamplesPerPixel,
BitsPerSample and Indexed fields.
+----------------------------------------------------------+
| Image Type | PhotoInter- | Samples | Bits Per | Indexed |
| | pretation | PerPixel | Sample | |
|------------+-------------+----------+----------+---------|
| RGB | 2=RGB | 3 | 1 | 0 |
+----------------------------------------------------------+
| CMY | 5=CMYK | 3 | 1 | 0 |
+------------+-------------+----------+----------+---------+
| CMYK | 5=CMYK | 4 | 1 | 0 |
+------------+-------------+----------+----------+---------+
| Palette | 10=ITULAB | 1 | n | 1 |
+------------+-------------+----------+----------+---------+
| Grayscale | 10=ITULAB | 1 | 8, 12 | 0 |
+------------+-------------+----------+----------+---------+
| Color | 10=ITULAB | 3 | 8, 12 | 0 |
+------------+-------------+----------+----------+---------+
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6.1.2. JBIG Encoding
T.43 uses the single-progression sequential mode of JBIG, defined in
ITU-T Rec. T.82, in accordance with the application rules described in
ITU-T Rec. T.85. To code multi-level images using JBIG, which is a
bi-level compression method, an image is resolved into a set of
bit-planes using Gray code conversion, and each bit-plane is then JBIG
compressed. The Gray code conversion is part of the data stream
encoding, and is therefore invisible to TIFF.
6.2. Required TIFF Fields
This section lists the required fields, in addition to those in Section
2.2.1, and the values they must have to be compatible with ITU-T Rec.
T.43.
6.2.1. Baseline Fields
ImageWidth(256). SHORT or LONG
Same page widths as the base color mode; see Section 5.2.1.
BitsPerSample(258) = 1, 8, 9-16. SHORT
Count = SamplesPerPixel
RGB, CMY, CMYK: 1 bit per sample
Continuous tone (L*a*b*): 8 bits per sample, 12 bits optional
Palette color: 12 or fewer bits per sample, 13-16 bits optional
Note: More than 8 bits per sample is not baseline TIFF.
ColorMap(320). SHORT
Count = 3 * (2**BitsPerSample)
Lossless color fax mode supports palette-color (indexed) images
where the single component value is used as an index into a full
color lookup table stored in the ColorMap field. With lossless color
fax mode, only the ITULAB encoding with 8 bits per sample is
supported for palette-color images. To utilize a color map, the TIFF
Indexed field must be present. TIFF orders the color map values so
that all the L* values come first, followed by all the a* values and
then all the b* values. Because ITU-T Rec. T.43 specifies a "chunky"
ordering with the L*a*b*components of the first value, followed by
those of the second value, and so on, transferring color map values
between a TIFF file and fax data stream requires reordering values.
Compression(259) = 9. SHORT
9: ITU-T Rec. T.82 coding, applying ITU-T Rec. T.85 (JBIG)
FillOrder(266) = 1, 2. SHORT
This field is only relevant for 1 bit per sample color (RGB, CMY,
CMYK); see Section 3.2.1 for further information.
PhotometricInterpretation(262) = 2, 5, 10. SHORT
2: RGB
5: CMYK, including CMY
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10: ITULAB
Image data may also be stored as palette color images, where pixel
values are represented by a single component that is an index into a
color map using the ITULAB encoding. This color map is specified by
the ColorMap field. To use palette color images, set the
PhotometricInterpretation to 10,SamplesPerPixel to 1, and Indexed to
1. The color map is stored in the ColorMap field. See Section 6.1.1
for further discussion on the color encoding.
SamplesPerPixel(277) = 1, 3, 4. SHORT
1: Palette color image, or L*-only if Indexed = 0 and
PhotometricInterpretation is 10 (ITULAB).
3: RGB, or L*a*b*, or CMY if PhotometricInterpretation is 5 (CMYK).
4: CMYK.
XResolution(282). RATIONAL
YResolution(283). RATIONAL
The resolution of the image is expressed in pixels per resolution
unit. In pixels per inch, allowed XResolution values are: 100, 200,
300, and 400. The lossless color fax mode requires the pixels to be
square, hence YResolution must equal XResolution.
6.2.2. Extension Fields
Indexed(364) = 0, 1. SHORT
0: not a palette-color image
1: palette-color image
This field is used to indicate that the sample values are an index
into an array of color values specified in the ColorMap field.
Lossless color fax mode supports palette-color images with the
ITULAB encoding. The SamplesPerPixel value must be 1.
Decode(TagToBeAssigned) SHORT
Decode is used in connection with the ITULAB encoding of image data;
see Section 5.2.2.
6.2.3. New Fields
None.
6.3. Recommended TIFF Fields
See Sections 2.2.3. and 2.2.4.
6.4. Lossless Color Fax Mode Summary
Recommended fields are shown with an asterisk *
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+--------------------|--------------------------------------+
| Baseline Fields | Values |
+--------------------+--------------------------------------+
| BitsPerSample | 1: Binary RGB, CMY(K) |
| | 8: 8 bits per color sample |
| | 9-16: optional |
+--------------------+--------------------------------------+
| ColorMap | n: LAB color map |
+--------------------+--------------------------------------+
| Compression | 9: JBIG |
+--------------------+--------------------------------------+
| DateTime* | {ASCII}: date/time in the 24-hour |
| | format "YYYY:MM:DD HH:MM:SS" |
+--------------------+--------------------------------------+
| FillOrder | Applies only to 1 bit/sample |
| | encodings |
| | 1: Most significant bit first |
| | 2: Least significant bit first |
+--------------------+--------------------------------------+
| ImageDescription* | {ASCII}: A string describing the |
| | contents of the image. |
+--------------------+--------------------------------------+
| ImageWidth | 864, 1024, 1216, 1728, 2048, 2432, |
| | 2592, 3072, 3456, 3648, 4096, 4864 |
+--------------------+--------------------------------------+
| ImageLength | n: total number of scanlines in image|
+--------------------+--------------------------------------+
| NewSubFileType | 2: Bit 1 identifies single page of a |
| | multi-page document |
+--------------------+--------------------------------------+
| Orientation* | 1-8, Default 1 |
+--------------------+--------------------------------------+
| PhotometricInter- | 2: RGB |
| pretation | 5: CMYK |
| | 10: ITULAB |
+--------------------+--------------------------------------+
| ResolutionUnit | 2: inch |
| | 3: centimeter |
+--------------------+--------------------------------------+
| RowsPerStrip | n: number of scanlines per TIFF strip|
+--------------------+--------------------------------------+
| SamplesPerPixel | 1: L* (lightness) |
| | 3: LAB, RGB, CMY |
| | 4: CMYK |
+--------------------+--------------------------------------+
| Software* | {ASCII}: name & release number of |
| | creator software |
+--------------------+--------------------------------------+
| StripByteCounts | <n>: number or bytes in TIFF strip |
+--------------------+--------------------------------------+
| StripOffsets | <n>: offset from beginning of file to|
| | each TIFF strip |
+--------------------+--------------------------------------+
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+--------------------+--------------------------------------+
| XResolution | 100, 200, 300, 400 (written in |
| | pixels/inch) |
| YResolution | equal to XResolution (pixels must be |
| | square) |
+--------------------+--------------------------------------+
| Extension Fields |
+--------------------+--------------------------------------+
| Decode | minL, maxL, mina, maxa, minb, maxb: |
| |minimum and maximum values for L*a*b* |
+--------------------+--------------------------------------+
| DocumentName* | {ASCII}: name of scanned document |
+--------------------+--------------------------------------+
| PageNumber | n,m: page number followed by total |
| | page count |
+--------------------+--------------------------------------+
| Indexed | 0: not a palette-color image |
| | 1: palette-color image |
+--------------------+--------------------------------------+
| New Fields |
+--------------------+--------------------------------------|
| GlobalParameters | <IFD>: global parameters IFD |
| IFD* | |
+--------------------+--------------------------------------+
| ProfileType* | n: type of data stored in TIFF file |
+--------------------+--------------------------------------+
| ITUFaxMode* | n: ITU-compatible fax mode |
+--------------------+--------------------------------------+
| CodingMethods* | n:compression algorithms used in |
| | file |
+--------------------+--------------------------------------+
| VersionNumber* | n: version of ITU fax standard |
+--------------------+--------------------------------------+
| VersionYear* | byte sequence with year of ITU fax |
| | standard |
+--------------------+--------------------------------------+
7. Mixed Raster Content Mode
7.1. Overview
Unlike previous fax modes, which use a single coding method and spatial
resolution for an entire fax page, the Mixed Raster Content mode [MRC]
enables different coding methods and resolutions within a single page.
For example, consider a page that contains black-and-white text, which
is best coded with MMR or JBIG, a color bar chart, best coded with JBIG,
and a scanned color image, best coded with JPEG. Similarly, while
spatial resolution of 400 pixels per inch may be best for the black-and-
white text, 200 pixel per inch is usually sufficient for a color image.
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Rather than applying one coding method and resolution to all elements,
MRC allows multiple coders and resolutions within a page. By itself, MRC
does not define any new coding methods or resolutions. Instead it
defines a 3-layer model for structuring and combining the scanned image
data.
The ITU MRC document is currently a draft. However, its technical
content is stable, and editorial changes only are permitted between now
and its scheduled approval in October 1997. Upon approval, the MRC
document will become ITU-T Recommendation T.44.
7.1.1. MRC 3-layer model
The 3 layers of the MRC model are Foreground and Background, which are
both multi-level, and Mask, which is bi-level. Each layer may appear
only once on a page and is coded independently of the other two. In our
earlier example, the black-and-white text could be in the Mask layer,
the color chart in the Foreground layer, and the color image in the
Background layer. The distribution of content among layers is determined
by the writer, as is the choice of compression method, color encoding
and spatial resolution for a layer.
The final image is obtained by using the Mask layer to select pixels
from the other two layers. When the Mask layer pixel value is 1, the
corresponding pixel from the Foreground layer is selected; when it is 0,
the corresponding pixel from the Background layer is selected. Details
are given in Appendix 1 of [MRC].
Not all pages, and not all parts of a page, require 3 layers. If there
is only one layer present, then that layer is the primary image and must
be page size. If there is more than one layer, then the Mask must be one
of the layers, in which case it is the primary image and must be page
size.
MRC allows a page to be split into strips, with a variable number of
scanlines in a strip. A strip can have 1, 2 or 3 layers. A single,
stripped layer may be stored as a single, stripped image in an IFD,
e.g., all strips associated with the Background layer may be treated as
a single image. Alternatively, each strip associated with a layer may be
stored as a separate image or IFD, e.g., the Background layer can be
composed of several images that are offset vertically with respect to
the page. In this case, there can be no overlap between images
associated with a single layer.
Furthermore, color fax also requires the spatial resolutions of
Background and Foreground images to be integer factors of the Mask image
resolution. For example, if the Mask Layer is 400 pixels per inch, then
the Background layer can be 100, 200 or 400 pixels per inch.
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7.1.2. A TIFF Representation for the MRC 3-layer model
In the TIFF representation of the 3-layer MRC model, each page is
represented by a single IFD, called the Primary IFD, that represents the
Mask layer (unless the Foreground or Background is the single layer
present), and a set of child IFDs that are referenced through the
SubIFDs extension field. The structure of SubIFDs is described in
[TTN1]. To distinguish MRC-specific SubIFDs from other SubIFDs, the
NewSubFileType field will have Bit 4 ON, indicating an MRC-related IFD.
A new ImageLayer field is also introduced that consists of two values
which identify the layer (Foreground, Background, or Mask) and the order
within the layer (first, second, ... image of the layer); see Section
7.2.3.
Because MRC allows strips with variable numbers of scanlines, a new
StripRowCounts field is introduced to replace the RowsPerStrip field in
this mode. The StripRowCounts field allows each layer, with a variable
number of scanlines in each strip, to be represented by a single IFD.
Alternatively, each strip in the Foreground and Background layers can be
represented by a single IFD at the option of the writer. In all cases,
the Mask layer is required to be represented by a single IFD.
The use of SubIFDs to store child IFDs is described in [TTN1]. An
example is shown graphically below. The Primary IFD associated with page
1 (PrimaryIFD 0) points to page 2 (PrimaryIFD 1) with the nextIFD
offset. The Primary IFD, corresponding to the Mask layer
(ImageLayer=[2,1]), contains a SubIFDs field that points to a list of
child IFDs. The first child IFD represents one image of the Background
layer, i.e., ImageLayer=[1,1]. This child IFD points to the second child
IFD via the nextIFD offset. This child represents the second Background
layer image, ImageLayer=[1,2]. Finally, the second child points to the
third child, which corresponds to the single Foreground layer image,
ImageLayer=[3,1]. The next IFD offset associated with this Foreground
image is 0, indicating no more child IFDs exist. Each primary IFD has
the NewSubFileType set to 18, indicating the IFD is MRC-specific (bit 4)
and that it is a single page of a multi-page document (bit 1). Each
child IFD has the NewSubFileType set to 16, indicating the IFD is
MRC-specific. Note: the 'V' character should be read as a down-pointing
arrow.
(nextIFD)
PRIMARY IFD 0 ------------> PRIMARY IFD 1--> ...
ImageLayer = [2,1]
NewSubFileType = 18
SubIFDs
|
V
Child IFD
ImageLayer = [1,1]
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NewSubFileType = 16
|
|(nextIFD)
|
V
Child IFD
ImageLayer = [1,2]
NewSubFileType = 16
|
|(nextIFD)
|
V
Child IFD
ImageLayer = [3,1]
NewSubFileType = 16
|
|(nextIFD)
|
V
0
In the example above, the SubIFDs field of the Primary IFD points to the
first IFD in a list of child IFDs. TIFF allows the SubIFDs field to
point to an array of IFDs, each of which can be the first of a list of
IFDs. An MRC-enabled TIFF reader must scan all available child IFDs to
locate and identify IFDs associated with MRC layers.
In the case where the Background or Foreground layers are described with
multiple IFDs, the XPosition and YPosition TIFF fields specify the
offset to the upper-left corner of the IFD with respect to the Mask
layer; see Section 7.2.2. When there is only a single layer (Mask,
Foreground, or Background), it is stored as the Primary IFD.
7.2. Required TIFF Fields
This section describes the TIFF fields required, in addition to those in
Section 2.2.1, to store MRC mode fax images. Since MRC mode stores the
fax data as a collection of images corresponding to layers or parts of
layers, the compression mechanisms, color encodings and spatial
resolutions used by previous modes apply to MRC. Therefore, the
descriptions here will typically reference the appropriate earlier
section. Fields and values specific to MRC mode are pointed out.
7.2.1. Baseline Fields
ImageWidth(256). SHORT or LONG
Same page widths as the base color mode; see Section 5.2.1.
BitsPerSample(258) = 1, 8, 9-16 SHORT
Compression(259) = 3, 4, 7, 9. SHORT
SamplesPerPixel(277) = 1, 3, 4. SHORT
FillOrder(266) = 1, 2. SHORT
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PhotometricInterpretation(262) = 0, 1, 2, 5, 10. SHORT
For Mask layer, see Section 3.2.1.
For Foreground and Background layers, see Sections 5.2.1 and 6.2.1.
ColorMap(320). SHORT
Count = 3 * (2**BitsPerSample)
Used when Foreground or Background layer is a palette-color image;
see Section 6.2.1.
NewSubFileType(254) = 16, 18. LONG
For MRC fax mode, the NewSubFileType field has two bits that are
required.
Bit 1 indicates a single page of a multi-page document and must be
set for the Primary IFDs;
Bit 4 indicates MRC imaging model as described in ITU-T
Recommendation Draft T.44 [MRC], and must be set for Primary IFDs
and all MRC-specific child IFDs.
StripRowCounts(559). LONG
Count = number of strips
The number of scanlines stored in a strip. MRC allows each fax strip
to store a different number of scanlines, up to a specified maximum
strip size, which is either 256 scanlines or the page length in
scanlines, depending on the implementation. This field replaces
RowsPerStrip for IFDs with variable-sized strips. Only one of the
two fields, StripRowCounts and RowsPerStrip, may be used in an IFD.
XResolution(282). RATIONAL
YResolution(283). RATIONAL
The resolution of the image is expressed in pixels per resolution
unit. In pixels per inch, allowed XResolution values are: 100, 200,
300, and 400. MRC color fax mode requires the pixels to be square,
hence YResolution must equal XResolution.
7.2.2. Extension Fields
ChromaSubSampling(530). SHORT
ChromaPositioning(531). SHORT
Decode(TagToBeAssigned). SHORT
For Foreground and Background layers, see Section 5.2.2.
Indexed(346) = 0, 1. SHORT
For Foreground and Background layers: 1 indicates a palette-color
image, see Section 6.2.2.
T4Options(292) = 0, 1, 4, 5. SHORT
T6Options(293) = 0. SHORT
For Mask layer, see Section 4.2.2.
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DefaultImageColor(TagToBeAssigned). SHORT or LONG
Count = SamplesPerPixel
In areas where no image data is available, a default color is needed
to specify the color value. If the StripByteCounts value for a strip
is 0, then the color for that strip must be defined by a default
image color.
The DefaultImageColor field uses the same encoding as the image
data, and its value is therefore interpreted using the
PhotometricInterpretation, SamplesPerPixel, BitsPerSample, and
Indexed fields. If the fax data stream requires a different
encoding, then transferring the default color value between a TIFF
file and fax data stream requires a color conversion.
The default value for the DefaultImageColor field is white.
SubIFDs(330). IFD
Count = number of child IFDs
Each value is an offset from the beginning of the TIFF file to a
child IFD [TTN1].
XPosition(286). RATIONAL
YPosition(287). RATIONAL
Specifies the horizontal and vertical offsets of the top-left of the
IFD from the top-left of the Primary IFD in page resolution units.
For example, if the Primary IFD is at 400 pixels per inch, and a
foreground layer IFD is at 200 pixels per inch and located at pixel
coordinate (345, 678) with respect to the Primary IFD, the XPosition
value is 345/400 and the YPosition value is 678/400.
Color fax does not currently allow overlap of any component images
within a single layer.
Default values for XPosition and YPosition are 0.
7.2.3. New Fields
The Mixed Raster Content mode requires one new field: ImageLayer.
ImageLayer (34732). SHORT or LONG.
Count = 2
Image layers are defined such that layer 1 is the Background layer,
layer 3 is the Foreground layer, and layer 2 is the Mask layer that
selects pixels from the Background and Foreground layers. The
ImageLayer tag contains two values, describing the layer to which
the image belongs and the order in which it is imaged.
ImageLayer[0]: 1, 2, 3.
1: Image is a Background image, i.e., the image that will appear
whenever the Mask contains a value of 0. Background images
typically contain low-resolution, continuous-tone imagery.
2: Image is the Mask layer. In MRC, if the Mask layer is present, it
must be the Primary IFD and be full page in extent (no gaps.)
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3: Image is a Foreground image, i.e., the image that will appear
whenever the Mask contains a value of 1. The Foreground image
generally defines the color of text or lines, but may also
contain high-resolution imagery.
ImageLayer[1]:
1: first image to be imaged in this layer (e.g., first fax strip),
2: second image to be imaged in this layer (e.g., second fax strip),
3: ...
Value describing the image order. In MRC, this may be considered the
strip number. Since MRC mode currently does not allow overlap
between images within a layer, the order value does not have any
visual effect.
In MRC fax mode, it is possible that only a single layer is transmitted.
For example, if a page contains only a single continuous-tone
photograph, then only the Background layer may be transmitted. In this
case, the Background layer will be full page and stored as the Primary
IFD. ImageLayer[0] will be 1 indicating Background; ImageLayer[1] will
be 1 since the layer must be full page and there can be no other IFDs
associated with that layer. No Mask layer will exist.
7.3. Recommended TIFF Fields
See Sections 2.2.3. and 2.2.4.
7.4. Rules and Requirements for Images
The MRC mode defines a fundamental set of rules for images in the
3-layer representation.
1. If more than one layer exists, then the binary Mask layer must be
present and it is the primary image. If only one layer exists, then
the image corresponding to that layer is the primary image.
2. The Primary image defines and extends to the entire page boundary;
all attached model images cannot extend beyond the Primary image.
Resolution differences may cause some pixels to "hang over" the page
boundary, but no new pixels should exist completely beyond the page
extent
3. The Background and Foreground images may use any color encoding
defined in Sections 5 and 6, and may optionally cover only a portion
of the region defined by the Primary image, unless one of them is the
primary image.
4. Each Primary IFD and each MRC-specific SubIFD must have an ImageLayer
field to specify which layer the IFD belongs to, and the imaging
order of that IFD within the layer.
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5. Each Primary IFD must have a NewSubFileType field value set to 18,
indicating a single page of a multi-page document (bit 1) and MRC
mode (bit 4).
6. Each MRC-specific child IFD must have a NewSubFileType field value
set to 16, indicating MRC mode (bit 4).
7. In MRC mode, each layer is transmitted as a sequence of strips. It is
possible that each strip of each layer can be stored as a separate
IFD. In this case, the SubIFDs structure pointed to by the Primary
IFD will contain several IFDs that have an ImageLayer field with the
layer identified as either Background (layer 1) or Foreground (layer
3). There may be no overlap in the vertical direction between IFDs
associated with a single layer, although there may be a gap from one
of these images to the next. The TIFF XPosition and YPosition fields
are used to indicate the placement of these images with respect to
the primary image.
8. The resolution of Background and Foreground images must each be an
integer factor of the Primary image. For example, if the Primary
image is 400 pixels/inch, then the model images may be at 400
pixels/inch (400 / 1), 200 pixels/inch (400 / 2), 100 pixels/inch
(400 / 4), etc.
7.5. MRC Fax Mode Summary
Recommended fields are shown with an asterisk *
+------------------+-----------------------------------------+
| Baseline Fields | Values |
|------------------|-----------------------------------------|
| BitsPerSample | 1: binary mask |
| | 8: 8 bits per color sample |
| | 9-16: optional 12 bits/sample |
+------------------+-----------------------------------------+
| ColorMap | n: LAB color map |
+------------------+-----------------------------------------+
| Compression | 3: Modified Huffman and Modified Read |
| | 4: Modified Modified Read |
| | 7: JPEG |
| | 9: JBIG |
+------------------+-----------------------------------------+
| DateTime* | {ASCII): date/time in the 24-hour format|
| | "YYYY:MM:DD HH:MM:SS" |
+------------------+-----------------------------------------|
| FillOrder | Applies only to 1 bit/sample encodings |
| | 1: Most significant bit first |
| | 2: Least significant bit first |
+------------------+-----------------------------------------+
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+------------------+-----------------------------------------|
| ImageDescription*| {ASCII}: A string describing the |
| | contents of the image. |
+------------------+-----------------------------------------+
| ImageWidth | 864, 1024, 1216, 1728, 2048, 2432, 2592,|
| | 3072, 3456, 3648, 4096, 4864 |
+------------------+-----------------------------------------+
| ImageLength | n: total number of scanlines in image |
+------------------+-----------------------------------------|
| NewSubFileType | 16, 18: |
| | Bit 1 indicates single page of a multi- |
| | page document on Primary IFD |
| | Bit 4 indicates MRC model |
+------------------+-----------------------------------------+
| Orientation* | 1-8, Default 1 |
+------------------+-----------------------------------------+
| PhotometricInter | 0: WhiteIsZero |
| pretation | 1: BlackIsZero |
| | 2: RGB |
| | 5: CMYK |
| | 10: ITULAB |
+------------------+-----------------------------------------+
| ResolutionUnit | 2: inch |
| | 3: centimeter |
+------------------+-----------------------------------------+
| RowsPerStrip | n: number of scanlines in each strip |
+------------------+-----------------------------------------+
| SamplesPerPixel | 1: L* (lightness) |
| | 3: RGB, LAB, CMY |
| | 4: CMYK |
+------------------+-----------------------------------------+
| Software* | {ASCII}: name & release number of |
| | creator software |
+------------------+-----------------------------------------+
| StripByteCounts | <n>: number or bytes in TIFF strip |
+------------------+-----------------------------------------+
| StripOffsets | <n>: offset from beginning of file to |
| | each TIFF strip |
+------------------+-----------------------------------------|
| XResolution | 100, 200, 300, 400 (written in |
| | pixels/inch) |
+------------------+-----------------------------------------|
| YResolution | equal to XResolution (pixels must be |
| | square) |
+------------------+-----------------------------------------+
| Extension Fields |
+------------------+-----------------------------------------+
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+------------------+-----------------------------------------+
| T4Options | 0: required if Compression is |
| | Modified Huffman, no fill bits |
| | 1: required if Compression is 2D |
| | Modified Read, no fill bits |
| | 4: required if Compression is Modified |
| | Huffman, fill bits |
| | 5: required if Compression is 2D |
| | Modified Read, fill bits |
+------------------+-----------------------------------------+
| T6Options | 0: required if Compression is 2D |
| | Modified Modified Read |
+------------------+-----------------------------------------+
| DocumentName* | {ASCII}: name of scanned document |
+------------------+-----------------------------------------+
| PageNumber | n,m: page number followed by total page |
| | count |
+------------------+-----------------------------------------+
| Decode | minL, maxL, mina, maxa, minb, maxb: |
| | minimum and maximum values for L*a*b* |
+------------------+-----------------------------------------+
| ChromaSubSampling| ChromaSubsampleHoriz: |
| | 0: equal numbers of lightness and chroma|
| | samples |
| | 1: twice as many lightness samples as |
| | chroma samples |
| | 2: four times as many lightness samples |
| | as chroma samples |
| | ChromaSubsampleVert: |
| | 0: equal numbers of lightness and chroma|
| | samples |
| | 1: twice as many lightness samples as |
| | chroma samples |
| | 2: four times as many lightness samples |
| | as chroma samples |
+------------------+-----------------------------------------+
| ChromaPositioning| 1: centered |
| | 2: cosited |
+------------------+-----------------------------------------+
| DefaultImageColor| <n>: background color |
+------------------+-----------------------------------------+
| Indexed | 0: not a palette-color image |
| | 1: palette-color image |
+------------------+-----------------------------------------+
| SubIFDs | <IFD>: byte offset to fg/bg IFDs |
+------------------+-----------------------------------------+
| XPosition | horizontal offset in primary IFD |
| | coordinates |
+------------------+-----------------------------------------+
| YPosition | vertical offset in primary IFD |
| | coordinates |
+------------------+-----------------------------------------+
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+------------------+-----------------------------------------+
| New Fields |
+------------------+-----------------------------------------+
| ImageLayer | n, m: layer number, imaging sequence |
| | (e.g., strip number) |
+------------------+-----------------------------------------+
| GlobalParameters | <IFD>: global parameters IFD |
| IFD* | |
+------------------+-----------------------------------------+
| ProfileType* | n: type of data stored in TIFF file |
+------------------+-----------------------------------------+
| ITUFaxMode* | n: ITU-compatible fax mode |
+------------------+-----------------------------------------+
| CodingMethods* | n: compression algorithms used in file |
+------------------+-----------------------------------------+
| VersionNumber* | n: version of ITU fax standard |
+------------------+-----------------------------------------+
| VersionYear* | byte sequence with year of ITU fax |
| | standard |
+------------------+-----------------------------------------+
8. MIME sub-type
The draft proposes to redefine the image/tiff sub-type to correspond to
the current TIFF specification, Revision 6.0, dated June 3, 1992, with
the addition of the new fields defined here. Further, an optional
application parameter is proposed that would distinguish subsets of the
image/tiff subtype corresponding to the TIFF representation of the fax
modes defined in this document.
9. IANA Registration
To: ietf-types@iana.org
Subject: Registration of Standard
MIME media type image/tiff
MIME media type name: image
MIME subtype name: tiff
Required parameters: none
Optional parameters: application
The value of the application parameter of image/tiff is denoted by a
single letter. Six values are proposed.
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B - minimal black-and-white fax mode (defined in Section 3).
F - extended black-and-white fax mode (defined in Section 4,
excluding Compression=9)
J - extended black-and-white fax mode (defined in Section 4,
including Compression=9)
C - lossy color mode (defined in Section 5)
P - lossless color mode (defined in Section 6)
M - Mixed Raster Content mode (defined in Section 7)
There is no default value for application, and in the absence of the
application parameter, the user should assume baseline TIFF.
Encoding considerations: binary or base64 generally preferred
Security considerations: The security issues associated with this type
have not been assessed.
Interoperability considerations: These application parameter values for
image/tiff are intended for interoperability between ITU and Internet
fax data formats.
Published specification: TIFF Revision 6.0 is available at: ftp://
ftp.adobe.com/pub/adobe/devrelations/devtechnotes/pdffiles/tiff6.pdf,
http://www.adobe.com/supportservice/devrelations/PDFS/TN/TIFF6.pdf
Person & e-mail address to contact for further information:
Lloyd McIntyre: lmcintyre@adoc.xerox.com
Steve Zilles: szilles@adobe.com
Intended usage: Common
10. Security Considerations
Security issues have not been assessed in this document.
11. References
[MRC] TD1018, "Revision of D25, forming the basis for Draft Rec. T.44
(MRC-Colour)", ITU-T Study Group 8, February 1997 currently available at
http://www.xerox.com/xis/mrc/
[RFC1314] Katz, A., and Cohen, D., "A File Format for the Exchange of
Images in the Internet", RFC 1314, USC Information Sciences Institute,
April 1992
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[RFC1528] Malamud, C., and Rose, M., Principles of Operation for the
TPC.INT Subdomain: Remote Printing -- Technical Procedures, RFC 1528,
October 1993; also Rose, M., Registration of new MIME content-
type/subtype, July 31, 1993, ftp://ftp.isi.edu/in-notes/iana/
assignments/media-types/image/tiff
[T.4] ITU-T Recommendation T.4, Standardization of group 3 facsimile
apparatus for document transmission, July 1996
[T.6] ITU-T Recommendation T.6, Facsimile coding schemes and coding
control functions for group 4 facsimile apparatus, November 1988
[T.42] ITU-T Recommendation T.42, Continuous-tone colour representation
method for facsimile, February 1996
[T.43] ITU-T Recommendation T.43, Colour and gray-scale image
representations using lossless coding scheme for facsimile, February
1997
[T.81] ITU-T Recommendation T.81, Information technology - Digital
compression and coding of continuous-tone still images - Requirements
and guidelines, September 1992
[T.82] ITU-T Recommendation T.82, Information technology - Coded
representation of picture and audio information - Progressive bi-level
image compression, March 1995
[T.85] ITU-T Recommendation T.85, Application profile for Recommendation
T.82 - Progressive bi-level image compression (JBIG coding scheme) for
facsimile apparatus, August 1995
[TIFF] Tag Image File Format, Revision 6.0, Adobe Developers
Association, June 3, 1992, ftp://ftp.adobe.com/pub/adobe/devrelations/
devtechnotes/pdffiles/tiff6.pdf
[TIFF-F] work in progress, Tag Image File Format (TIFF) - Class F, May
30, 1997, ftp://ftp.ietf.org/internet-drafts/draft-ietf-fax-tiff-02.txt
[TIFF-F0] TIFF Class F specification, Apr 28, 1990,
ftp://ftp.faximum.com/pub/documents/tiff_f.txt
[TTN1] Adobe PageMaker 6.0 TIFF Technical Notes, Sept. 14, 1995,
http://www.adobe.com/supportservice/devrelations/PDFS/TN/TIFFPM6.pdf
[TTN2] Draft TIFF Technical Note 2, Replacement TIFF/JPEG specification,
March 17, 1995, ftp://ftp.sgi.com/graphics/tiff/TTN2.draft.txt
The ITU-T Recommendations are available at http://www.itu.ch.
[MRC] is currently available at http://www.xerox.com/xis/mrc/
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12. Authors' Addresses
Lloyd McIntyre
Xerox Corporation
Mailstop PAHV-305
3400 Hillview Ave.
Palo Alto, CA 94304 USA
Voice: +1-415-813-6762 Fax: +1-415-813-6792
Email: lmcintyre@adoc.xerox.com
Stephen Zilles
Adobe Systems Inc.
Mailstop W14
345 Park Avenue
San Jose, CA 95110-2704
Voice: +1-408-536-4766
Fax: +1-408-536-4042
Email: szilles@adobe.com
Appendix A: Summary of TIFF Fields for Internet Fax
This annex includes tables which list by mode the TIFF fields used in
the proposed fax file format. The fields are organized into 3
categories:
1) TIFF Baseline Fields
2) TIFF Extension Fields
3) New Fields.
The tables include the allowed values for each fax mode.
Entries other than explicit numbers are described by:
n - single number
n, m - 2 numbers
a, b, c - 3 numbers
r - rational number
<n> - array of numbers
<b> - byte sequence
{ASCII} - string
<IFD> - array of IFD byte offsets
A blank entry in the table indicates the field is not used by that
particular fax mode.
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Table A.1 TIFF Baseline Fields
+----------------------------------------------------------+
| Fax Mode |
+----------+----------------------------------------------------------|
| Field | B&W | B&W | Color | Color | Mixed Raster|
| | Minimal | Extended | Lossy | Lossless | Content |
+----------+---------------------+----------+-----------+-------------+
| BitsPer | | | | | |
| Sample | 1 | 1 | 8, 12 | 1, 8, 9-16| 1, 8, 9-16 |
+----------+---------------------+----------+-----------|-------------|
| ColorMap | | | | <n> | <n> |
+----------+---------------------+----------+-----------|-------------|
| Compres- | | | | | |
| sion | 3 | 3, 4 | 7 | 9 | 3, 4, 7, 9 |
+----------+---------------------+----------+-----------+-------------+
| DateTime | {ASCII} | {ASCII} | {ASCII} | {ASCII} |{ASCII} |
+----------+---------------------+--------- +-----------|-------------|
| FillOrder| 1, 2 | 1, 2 | | 1, 2 | 1, 2 |
+----------+---------------------+----------+-----------+-------------+
| ImageDes-| {ASCII} | {ASCII} | {ASCII} | {ASCII} | {ASCII} |
| cription | | | | | |
+----------+---------------------+----------+-----------+-------------|
| Image- | 1728, |1728, 2048 |864, 1024 |864, 1024 |864, 1024 |
| Width | 2048, |2432, 2592 |1216, 1728|1216, 1728 |1216, 1728 |
| | 2432 |3072, 3456 |2048, 2432|2048, 2432 |2048, 2432 |
| | |3648, 4096 |2592, 3072|2592, 3072 |2592, 3072 |
| | |4864 |3456, 3648|3456, 3648 |3456, 3648 |
| | | |4096, 4864|4096, 4864 |4096, 4864 |
+----------+---------------------+----------+-----------+-------------+
| Image- | | | | | |
| Length | n | n | n | n | n |
+----------+---------------------+----------+-----------+-------------+
| NewSub- | | | | | |
| FileType | 2 | 2 | 2 | 2 | 16, 18 |
+----------+---------------------+----------+-----------+-------------+
| Orien- | | | | | |
| tation | 1 | 1 | 1 |1 | 1 |
+----------+---------------------+----------+-----------+-------------+
| Photo- | | | | | |
| metric- | | | | | |
| Interp- | | | | | |
| retation | 0, 1 | 0, 1 | 10 | 2, 5, 10 | 0,1,2,5,10 |
+----------+---------------------+----------+-----------+-------------+
| Resolu- | | | | | |
| tionUnit | 2, 3 | 2, 3 | 2, 3 | 2, 3 | 2, 3 |
+----------+---------------------+----------+-----------+-------------+
| RowsPer- | | | | | |
| Strip | n | n | n | n | |
+----------+---------------------+----------+-----------+-------------+
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+----------+---------------------+----------+-----------+-------------+
| Samples- | | | | | |
| PerPixel | 1 | 1 | 1, 3 | 1, 3, 4 | 1, 3, 4 |
+----------+---------------------+----------+-----------+-------------+
| Software | {ASCII} | {ASCII} | {ASCII} | {ASCII} | {ASCII} |
+----------+---------------------+----------+-----------+-------------+
| Strip- | | | | | |
| Byte- | | | | | |
| Counts | <n> | <n> | <n> | <n> | <n> |
+----------+---------------------+----------+-----------+-------------+
| Strip- | | | | | |
| Offsets | <n> | <n> | <n> | <n> | <n> |
+----------+---------------------+----------+-----------+-------------+
| XResolu- | 204 | 200, 204, | 100, 200,| 100, 200, | 100, 200, |
| tion | | 300, 400, | 300, 400 | 300, 400 | 300, 400 |
| | | 406 | | | |
+----------+---------------------+----------+-----------+-------------+
| YResolu- | 98, 196 | 98, 196, | 100, 200,| 100, 200, | 100, 200, |
| tion | | 100, 200, | 300, 400 | 300, 400 | 300, 400 |
| | | 300, 392, | | | |
| | | 400 | | | |
+----------+---------------------+----------+-----------+-------------+
Table A.2 TIFF Extension Fields
+---------------------------------------------------------+
| Fax Mode |
+-----------+---------------------------------------------------------|
| Field | B&W | B&W | Color | Color |Mixed Raster|
| | Minimal| Extended | Lossy | Lossless | Content |
+-----------+--------+-----------+----------+----- ------+------------+
| T4Options | 4, 5 | 0, 1, 4, 5| | | 0, 1, 4, 5 |
+-----------+--------+-----------+----------+------------+------------+
| T6Options | | 0 | | | 0 |
+-----------+--------+-----------+----------+------------+------------+
| Document- | | | | | |
| Name | {ASCII}| {ASCII} | {ASCII} | {ASCII} | {ASCII} |
+-----------+--------+-----------+----------+------------+------------+
| Page- | | | | | |
| Number | n, m | n, m | n, m | n, m | n, m |
+-----------+--------+-----------+----------+------------+------------+
| XPosition | | | | | r |
+-----------+--------+-----------+----------+------------+------------+
| YPosition | | | | | r |
+-----------+--------+-----------+----------+------------+------------+
| SubIFDs | | | | | <IFD> |
+-----------+--------+-----------+----------+------------+------------+
| Indexed | | | | 0, 1 | 0, 1 |
+-----------+--------+-----------+----------+------------+------------+
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+-----------+--------+-----------+----------+------------+------------+
| Chroma- | | | | | |
| SubSampl- | | | | | |
| ing | | | n, m | | n, m |
+-----------+--------+-----------+----------+------------+------------+
| Chroma- | | | | | |
| Position- | | | | | |
| ing | | | 1, 2 | | 1,2 |
+-----------+--------+-----------+----------+------------+------------+
| Decode | | | <n> | <n> | <n> |
+-----------+--------+-----------+----------+------------+------------+
| Default- | | | | | |
| ImageColor| | | | | <n> |
+-----------+--------+-----------+----------+------------+------------+
| Strip- | | | | | |
| RowCounts | | | | | <n> |
+-----------+--------+-----------+----------+------------+------------+
Table A.3 New Fields
+---------------------------------------------------------+
| Fax Mode |
+-----------+---------------------------------------------------------|
| Field | B&W | B&W | Color | Color |Mixed Raster|
| | Minimal| Extended | Lossy | Lossless | Content |
+-----------+--------+-----------+----------+----- ------+------------+
| BadFax- | | | | | |
| Lines | n | n | | | |
+-----------+--------+-----------+----------+------------+------------+
| CleanFax- | | | | | |
| Data | 0,1,2 | 0,1,2 | | | |
+-----------+--------+-----------+----------+------------+------------+
| Consecu- | | | | | |
| tiveBad- | | | | | |
| FaxLines | n | n | | | |
+-----------+--------+-----------+----------+------------+------------+
| Image- | | | | | |
| Layer | | | | | n, m |
+-----------+--------+-----------+----------+------------+------------+
| Global- | | | | | |
| Parame- | | | | | |
| tersIFD | | <IFD> | <IFD> | <IFD> | <IFD> |
+-----------+--------+-----------+----------+------------+------------+
| Profile- | | | | | |
| Type | | n | n | n | n |
+-----------+--------+-----------+----------+------------+------------+
| ITUFax- | | | | | |
| Mode | | n | n | n | n |
+-----------+--------+-----------+----------+------------+------------+
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+-----------+--------+-----------+----------+------------+------------+
| Coding- | | | | | |
| Method | | n | n | n | n |
+-----------+--------+-----------+----------+------------+------------+
| Version- | | | | | |
| Number | | n | n | n | n |
+-----------+--------+-----------+----------+------------+------------+
| Version- | | | | | |
| Year | | <b> | <b> | <b> | <b> |
+-----------+--------+-----------+----------+------------+------------+
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Appendix B: Use of TIFF-F for Black-and-White Fax
B.1 Overview
Section 4 provides the standard definition of image/tiff, application=F,
known commonly as TIFF-F and historically as TIFF Class F (see Section
4.4). Although Section 4 is a necessary and sufficient definition of
TIFF-F, there is much lore associated with TIFF-F. This appendix, based
on [TIFF-F], captures this lore and provides background information for
implementors.
B.2 TIFF-F Fields
The following table summarizes the required and optional (or
recommended) fields for TIFF-F (See Section 4.). The optional fields are
shown with an asterisk * For convenience, a minimum subset of values is
also shown; these values should provide maximum portability with
historical TIFF-F readers.
Field | Values | Minimum | Comment
------------------|-------------|--------------|----------------------
BadFaxLines* |>=0 | |number of bad fax lines
BitsPerSample | 1 | 1 |one bit per sample
CleanFaxData* | 0,1,2 | 0,1,2 |status of bad lines
Compression | 3,4 | 3 |3 for T.4 (MH, MR)
| | |4 for T.6 (MMR)
ConsecutiveBad- |>=0 | |max number of bad lines
Lines* | | |
DateTime* | ASCII | |"YYYY:MM:DD:HH:MM:SS"
DocumentName* | ASCII | |informational field
FillOrder | 2,1 | 2 |LSB first or MSB first
ImageDescription* | ASCII | |informational field
ImageWidth | 1728, 2048, | 1728, 2048 |depends on XResolution
| 2432, 2592, | |
| 3072, 3648, | |
| 3456, 4096, | |
| 4864 | |
ImageLength | >0 | |required
NewSubFileType | 2 | 2 |single page of
| | |multipage file
Orientation* | 1 | 1 |1st row=top left,
| | | 1st col=top
PageNumber | X/X | 0/1 |pg/tot, 0 base,
| | | tot in 1st IFD
PhotometricInterp | 0,1 | 0 |0 is white
ResolutionUnit | 2,3 | 2 |inches (default)
RowsPerStrip |=ImageLength |=ImageLength |required
| or other | |
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SamplesPerPixel | 1 | 1 |one sample per pixel
Software* | ASCII | |informational field
StripByteCounts | >0 | |required
StripOffsets | >0 | |required
T4Options | 4,5 | 4 |MH, MR: incl. if not MMR
T6Options | 0 | |MMR: incl. only if MMR
XResolution | 204,200,300 | 204 |If unit is per inch
| 400,406 | |
| 77 | | If unit is per cm
YResolution | 196,98,100, | 196,98 |If unit is per inch
| 200,300,392,| |
| 400 | |
| 77,38.5 | | If unit is per cm
------------------|-------------|--------------|--------------------
B.3 Implementation Notes
B.3.1 Strips
In general, TIFF files divide an image into "strips," also known as
"bands." Each strip contains a few scanlines of the image. By using
strips, a TIFF reader need not load the entire image into memory, thus
enabling it to fetch and decompress small random portions of the image
as necessary.
The number of scanlines in a strip is described by the RowsPerStrip
value and the number of bytes in the strip after compression by the
StripByteCount value. The location in the TIFF file of each strip is
given by the StripOffsets values.
Strip size is application dependent. Existing TIFF-F usage is typically
one strip per page in multi-page TIFF-F. See Sections 2.1.2 and 2.1.3.
B.3.2 Bit Order
The current TIFF specification [TIFF] does not require a Baseline TIFF
reader to support FillOrder=2, i.e. lowest numbered 1-bit pixel in the
least significant bit of a byte. It further recommends that FillOrder=2
be used only in special purpose applications.
Facsimile data appears on the phone line in bit-reversed order relative
to its description in ITU-T Recommendation T.4. Therefore, a wide
majority of facsimile applications choose this natural order for data in
a file. Nevertheless, TIFF F readers must be able to read data in both
bit orders and support FillOrder values of 1 and 2. See Section 3.2.1.
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B.3.3. Multi-Page Files
Many existing applications already read TIFF-F-like files, but do not
support multiples pages using a linked list of IFDs. A Baseline TIFF
reader is not required to read any IFDs beyond the first one. Since a
multi-page format greatly simplifies file management in fax application
software, TIFF-F specifies multi-page documents and uses NewSubfileType
= 2.
B.3.4. Compression
Group 3 facsimile uses three compression methods. The ITU-T T.4
Recommendation [T.4] defines a one-dimensional compression method known
as Modified Huffman (MH) and a two-dimensional method known as Modified
READ (MR) (READ is short for Relative Element Address Designate). In
1984, a somewhat more efficient compression method known as Modified
Modified READ (MMR) was defined in the ITU-T T.6 Recommendation [T.6].
MMR was originally defined for use with Group 4 facsimile, so that this
compression method has been commonly called Group 4 compression. In
1991, the MMR method was approved for use in Group 3 facsimile and has
since been widely utilized.
TIFF-F supports these three compression methods. The most common
practice is the one-dimensional Modified Huffman (MH) compression
method. This is specified by setting the Compression field value to 3
and then setting bit 0 of the T4Options field to 0. Alternatively, the
two dimensional Modified READ (MR) method, which is much less frequently
used in historical TIFF-F implementations, may be selected by setting
bit 0 of the T4Options field to 1. The value of Bit 2 in this field is
determined by the use of fill bits.
Depending upon the application, the more efficient two-dimensional
Modified Modified Read (MMR)compression method from T.6 may be selected
by setting the Compression field value to 4 and then setting the
first two bits (and all unused bits) of the T6Options field to 0.
Baseline TIFF also permits use of Compression=2 to specify Modified
Huffman compression, but the data is presented in a form that is not
byte aligned. As a result, TIFF-F uses Compression=3 instead of
Compression=2 to specify Modified Huffman compression.
Uncompressed data is not supported in TIFF-F. Therefore, Bit 1 in both
the T4Options and T6Options fields must always be 0.
Since two-dimensional encoding is not required for Group 3
compatibility, some historic TIFF F readers have not been able to read
such files. However, for maximum efficiency, images should be
compressed using T.6 MMR compression when possible. For maximum
portability, applications also need to be able to read and write one-
dimensional (Modified Huffman) files. Some TIFF-F readers will also
support two-dimensional Modified READ files. Applications that wish to
have the maximum flexibility in reading TIFF F files should support all
three of the supported compression methods.
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B.3.5. Example Use of Page-quality Fields
Here are examples for writing the CleanFaxData, BadFaxLines, and
ConsecutiveBadFaxLines fields:
1. Facsimile hardware does not provide page quality information: Do not
write page-quality fields.
2. Facsimile hardware provides page quality information, but reports no
bad lines. Write only BadFaxLines = 0.
3. Facsimile hardware provides page quality information, and reports
bad lines. Write both BadFaxLines and ConsecutiveBadFaxLines. Also
write CleanFaxData = 1 or 2 if the hardware's regeneration
capability is known.
4. Computer generated file: write CleanFaxData = 0.
5. Source image data stream is error-corrected or otherwise guaranteed
to be error-free: Do not write page-quality fields.
B.3.6 Resolution
Almost all facsimile products support both "standard" (98 dpi)
resolution and "fine" (196 dpi) vertical resolution. Therefore, fine-
resolution files are quite portable in the real world.
In 1993, the ITU-T added support for higher resolutions in the T.30
Recommendation, including 200 x 200, 300 x 300, 400 x 400 in dots per
inch based units. At the same time, support was added for metric
dimensions which are equivalent to the following inch based resolutions:
392v x 203h and 392v x 406h. Therefore, the full set of inch-based
equivalents of the new resolutions are supported in the TIFF-F, since
they may appear in some image data streams received from Group 3
facsimile devices. However, many facsimile terminals and older versions
of TIFF-F readers are likely to not support the use of these higher
resolutions.
It is permissible for applications to treat the following XResolution
values as being equivalent: <204,200> and <400,406>. In a similar
respect, the following YResolution values may also be treated as being
equivalent: <98, 100>, <196, 200>, and <392, 400>. These equivalencies
were allowed by [T.30] to permit conversions between inch and metric
based facsimile terminals.
In a similar respect, the optional support of metric based resolutions
in the TIFF-F reader (i.e. 77 x 38.5 cm) is included for completeness,
since they are used in some legacy TIFF-F applications, but this use is
not recommended for the creation of TIFF-F files by a writer.
Historical TIFF-F documents also included the following values related
to A5 and A6 widths: 816 and 1216. Per the most recent version of
[T.4], A5 and A6 documents are no longer supported in Group 3 facsimile,
so the related width values are now obsolete.
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B.3.7. EOL
As illustrated in FIGURE 1/T.4 in [T.4], facsimile documents encoded
with Modified Huffman begin with an EOL, which in TIFF-F is byte-
aligned. The last line of the image is not terminated by an EOL. In a
similar respect, images encoded with Modified READ two dimensional
encoding begin with an EOL, followed by a tag bit. The EOL/tag bit
combination is byte aligned in TIFF-F. See Section 3.4.
B.3.8. RTC Exclusion
Aside from EOLs, TIFF F files contain only image data. This means that
the Return To Control sequence (RTC) is specifically excluded.
B.3.9. Use of EOFB for T.6 Compressed Images
TIFF-F pages which are encoded with the T.6 Modified Modified READ
compression method should include an "end-of-facsimile-block" (EOFB)
code at the end of each coded strip. Per [TIFF], the EOFB code is
followed by pad bits as needed to align on a byte boundary. TIFF
readers should ignore any bits other than pad bits beyond the EOFB.
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