Internet Fax Working Group Lloyd McIntyre
Internet Draft Stephen Zilles
Expires in six months March 20, 1997
File Format for Internet Fax
<draft-ietf-fax-tiffplus-00.txt>
Status
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Abstract
This Internet Draft describes the TIFF representation of the
image data specified by the ITU-T Recommendations and proposals
for black-and-white and color facsimile. For the most part,
existing TIFF constructs and fields are used; new TIFF fields are
introduced only when necessary. Black-and-white facsimile is
already described by TIFF Class F, and this Draft builds on that
to add color fax capability and to establish a structure for
future enhancements.
Table of Contents
1. INTRODUCTION..................................................3
1.1. Scope....................................................3
1.2. Rationale................................................3
1.3. Organization of This Draft...............................4
2. TIFF and Fax..................................................4
2.1. TIFF Overview............................................4
2.1.1. File Structure.......................................4
2.1.2. Image Structure......................................5
2.2 Fax Requirements..........................................6
2.2.1. TIFF fields required for all fax modes...............6
2.2.2. TIFF fields recommended for all fax modes............7
2.2.3. New TIFF fields for all fax modes....................7
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3. Base B&W Fax Mode.............................................9
3.1. Overview.................................................9
3.2. Required TIFF Fields.....................................9
3.2.1. Baseline fields.....................................10
3.2.2. Extension fields....................................10
3.2.3. New fields..........................................11
3.3. Recommended TIFF Fields.................................11
3.3.1. Baseline fields.....................................11
3.3.2. Extension fields....................................11
3.3.3. New fields..........................................11
3.4. Base B&W Fax Mode Summary...............................12
4. Base Color Fax Mode..........................................14
4.1. Overview................................................14
4.2. Required TIFF Fields....................................14
4.2.1. Baseline Fields.....................................15
4.2.2. Extension fields....................................16
4.2.3. New fields..........................................18
4.3. Recommended TIFF Fields.................................18
4.4. Base Color Fax Mode Summary.............................18
5. Lossless Color Mode..........................................20
5.1. Overview................................................20
5.1.1. Color Encoding......................................21
5.1.2. JBIG Encoding.......................................21
5.2. Required TIFF Fields....................................21
5.2.1. Baseline fields.....................................22
5.2.2. Extension fields....................................23
5.2.3. New fields..........................................23
5.3. Recommended TIFF Fields.................................23
5.4. Lossless Color Fax Mode Summary.........................24
6. Mixed Raster Content Mode....................................26
6.1 Overview.................................................26
6.1.1. MRC 3-layer model...................................26
6.1.2. A TIFF Representation for the MRC 3-layer model.....27
6.2. Required TIFF Fields....................................29
6.2.1. Baseline fields.....................................29
6.2.2. Extension fields....................................30
6.2.3. New fields..........................................31
6.3. Recommended TIFF Fields.................................32
6.4. Rules and Requirements for Images.......................32
6.5. MRC Fax Mode Summary....................................33
7. Mime Sub-Type................................................36
8. IANA Registration............................................36
9. Security Considerations......................................37
10. References..................................................38
11. Authors' Addresses..........................................39
Annex A: Summary of TIFF Fields for Internet Fax ...............39
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1. Introduction
1.1 Scope
This document defines a TIFF-based file format specification and
corresponding IETF 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 content and the structure of the data
generated by the suite of existing and developing ITU-T
Recommendations for Group 3 facsimile. These standards, and the
TIFF fields described here, support four facsimile modes:
1. base black-and-white mode, using binary compression [T.4,
T.6, T.82]
2. lossy color and grayscale mode, using JPEG compression
[T.42, T.81]
3. lossless color and grayscale mode, using JBIG compression
[T.43, T.82]
4. mixed raster content mode [MRC], using a combination of
existing compression methods
This specification builds on a previously proposed standard
[RFC1314] and on work in progress [TIFF-F] for the base black-and-
white mode. 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 required for
compatibility with ITU fax.
1.2 Rationale
This specification uses the current TIFF spec [TIFF] as the basis
for describing color and grayscale facsimile modes. By doing so,
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.
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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
would be used in all four fax modes. Sections 3-6 each describe
the representation of a single fax mode. Each section starts with
an overview of the applicable ITU-T Recommendation and then
describes the required and optional TIFF fields for representing
the ITU data stream. The fields in each subsection are organized
according to whether they are baseline or extensions in the
current TIFF specification or that will appear in the next
specification, or are new and defined in this document.
Sections 7 and 8 describe the MIME content type and IANA
registration. The remaining sections are required in
all Internet Drafts. Annex A gives a summary of the TIFF fields
used or defined in this document.
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].
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. 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, containing information about the
image and pointers to the image data.
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
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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 a reduced resolution version of the primary IFD image. The same
IFD can point to both 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.
This specification requires the use of subfiles or multiple IFDs.
All four 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. Only the Mixed Raster Content fax mode,
described in Section 6, requires child IFDs.
2.1.2 Image Structure
An IFD can store an image either as a single strip or split into
individually addressable strips, for improved performance at the
option of the writer. 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, given in the
RowsPerStrip field. This document introduces the new
StripRowCounts field that allows a variable number of scanlines
per strip. This feature is required by the Mixed Raster Content
fax mode.
Whether stripped or not, the image data is stored as uninterpreted
data streams. The formats of these streams follow the ITU-T
Recommendations. The compressed data stream is placed
in a strip within an IFD, with the Compression field used to
indicate the type of compression, and other TIFF fields used to
describe image attributes, such as color encoding and spatial
resolution. Compression parameters are stored in the compressed
data stream, rather than in additional TIFF fields. This makes the
TIFF and compression data format specifications independent of one
another. This approach, modeled on [TTN2], allows TIFF to
gracefully add new compression schemes as they become available.
To reduce overhead in TIFF files with multiple JPEG segments, one
can allow JPEG tables to be stored just once in a new
JPEGTables field [TTN2]. While this specification does not use
this field, this approach can be adapted for use with other
compression schemes, when all the data streams refer to the same
table and it would reduce file size to store the table once,
rather than replicate it throughout the file.
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For attributes that can be specified both places, i.e. in the 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 would take
precedence. For informational values that are not required to
interpret the data stream, such as author name, then the TIFF
field value normally would take precedence.
2.2 Fax requirements
2.2.1. TIFF fields required for all fax modes
The TIFF fields listed in this section are common to all fax modes
and required by all fax modes. Other TIFF fields are also common
to all modes, but the values associated with the field are mode-
specific. Such fields are described in the relevant sections
below.
The pattern for the field descriptions in this draft is
FieldName = values. TYPE
Count = (omitted if =1)
Tag = (if not in current spec but available)
Explanation of the field, how its used and the values it can
have.
ImageWidth. SHORT or LONG
Total number of pixels in a scanline.
ImageLength. SHORT or LONG
Total number of scanlines in image.
NewSubFileType = 2. LONG
2 = Bit 1 identifies the image as a single page of a
multi-page document.
PageNumber. SHORT
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. This field is a TIFF extension, not a
Baseline TIFF field.
ResolutionUnit = 2, 3. SHORT
2 = inch
3 = centimeter
The unit of measure for resolution.
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RowsPerStrip. SHORT or LONG
The number of scanlines per TIFF strip.
StripByteCounts. SHORT or LONG
Count = number of strips
For each strip, the number of bytes in that strip.
StripOffsets. SHORT or LONG
Count = number of strips
For each strip, the byte offset from the beginning of the file
to that strip.
2.2.2 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 the relevant sections below.
DateTime. ASCII
Date/time in the format "YYYY:MM:DD HH:MM:SS" in 24-hour
format.
DocumentName. ASCII
The name of the scanned document. This field is a TIFF
extension, not a Baseline TIFF field.
ImageDescription. ASCII
A string describing the contents of the image.
Orientation = 1-8. SHORT
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
Software. ASCII
The optional name and release number of the software package
that created the image.
2.2.3. New TIFF fields for all fax modes
Almost all the fields recommended in the previous section apply to
the collection of images that make up a TIFF-based fax file,
rather than to any one specific image in that file. It makes sense
to separate these "global" fields from the image-specific fields
in an IFD. It would also be useful to have a field, analogous to
the Software field, that identifies the fax mode used to create
the image data.
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For these reasons, several new fields, which are not part of the
current TIFF specification, 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. IFD
An IFD containing global parameters. A TIFF writer would
likely place this field in the first IFD, where a 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
image IFDs, it is valid to use the data closest to the image data,
otherwise the image IFD is ill-formed and should be ignored.
Among the GlobalParametersIFD entries is a new ProfileType field
which generally describes information in this IFD and in the TIFF
file.
ProfileType: LONG
Tag: to be assigned
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 = 0 - 4. BYTE
Tag: to be assigned
The currently defined values are:
0: does not conform to ITU-T Recommendations for facsimile,
1: black & white lossless (ITU-T Rec. T.4, T.6),
2: lossy color and grayscale (ITU-T Rec. T.42),
3: lossless color and grayscale (ITU-T Rec. T.43),
4: Mixed Raster Content (ITU-T Draft Rec. T.44).
Notes: A file can use an ITU encoding with non-ITU parameters,
such as using JPEG to compress a CMYK image. In this case,
ITUFaxMode is 0.
CodingMethods. LONG
Tag: to be assigned
This field indicates which coding methods are used in the
file. A bit value of 1 indicates:
Bit 0: unspecified compression,
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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
Modified Read),
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 compression types to identify
standard compression methods.
VersionYear. BYTE
Tag: to be assigned
Count: 4
The year of the standard specified by the ITUFaxMode field,
given as 4 characters, e.g. '1997'; used with T.42 and T.43.
VersionNumber BYTE
Tag: to be assigned
The version of the standard specified by the ITUFaxMode field.
A value of 0 indicates Version 1.0; used with MRC (Draft
T.44).
3. Base B&W Fax Mode
3.1. Overview
Base 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. It offers several options for lossless
bi-level image coding, including 1-dimensional Modified Huffman
(MH) and 2-dimensional Modified Read (MR), both described in
[T.4], and Modified Modified Read (MMR), described in [T.6]. (Read
is an acronym for Relative Element Address Designate.)
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 is based on the TIFF Class F definition.
3.2. Required TIFF fields
This section lists the required fields and the values they must
have to be ITU-compatible.
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3.2.1. Baseline fields
ImageWidth. SHORT or LONG
This mode supports the following fixed page widths: 1728,
2048, 2432, 2592, 3072, 3456, 3648, 4096, 4864.
BitsPerSample = 1. SHORT
Binary data only.
Compression = 2, 3, 4, 9. SHORT
2 = 1-dimensional coding, ITU-T Rec. T.4 (MH - Modified
Huffman)
3 = 2-dimensional coding, ITU-T Rec. T.4 (MR - Modified Read)
4 = 2-dimensional coding, ITU-T Rec. T.6 (MMR - Modified
Modified Read)
9 = ITU-T Rec. T.82 coding, applying ITU-T Rec. T.85 (JBIG)
FillOrder = 1, 2. SHORT
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).
Most b&w fax products use LSB first.
PhotometricInterpretation = 0, 1. SHORT
0 = pixel value 1 means black
1 = pixel value 1 means white
SamplesPerPixel = 1. SHORT
1 = monochrome, bi-level in this case (see BitsPerSample)
XResolution. RATIONAL
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.
YResolution. RATIONAL
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
3.2.2. Extension fields
T4Options = 0, 1, 4, 5. LONG
0: Required if Compression field has a value of 2, indicating
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Modified Huffman encoding, and if fill bits have not been
added to insure EOL codes end on a byte boundary.
1: Required if Compression field has a value of 3, indicating
Modified Read encoding, and if fill bits have not been
added to insure EOL codes end on a byte boundary.
4: Required if Compression field has a value of 2, indicating
Modified Huffman encoding, and if fill bits have been
added to insure EOL codes end on a byte boundary.
5: Required if Compression field has a value of 3, indicating
Modified Read encoding, and if fill bits have been added
to insure EOL codes end on a byte boundary.
T6Options = 0. LONG
Required if Compression field has a value of 4, indicating 2D
Modified Modified Read.
3.2.3. New fields
No new extensions are required for black & white fax.
3.3 Recommended TIFF fields
3.3.1. Baseline fields
See Section 2.2.2.
3.3.2. Extension fields
See Section 2.2.2.
3.3.3. New fields
There are three new optional fields for describing black & white
fax page quality. They are not defined in the current TIFF
specification.
BadFaxLines. SHORT or LONG
The number of "bad" scan lines encountered by the facsimile
during reception. A "bad" scanline is defined as a scanline
that, when decoded, comprises an incorrect number of pixels.
CleanFaxData = 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.
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ConsecutiveBadFaxLines. SHORT or LONG
Maximum number of consecutive "bad" scanlines received.
3.4. Base B&W Fax Mode Summary
Recommended fields are shown with an asterisk *
+---------------------------+--------------------------------+
| Baseline Fields | Values |
+---------------------------+--------------------------------+
| BitsPerSample | 1 |
+---------------------------+--------------------------------+
| Compression | 2: 1D Modified Huffman coding |
| | 3: 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 |
+---------------------------+--------------------------------+
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+---------------------------+--------------------------------+
| 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 |
+---------------------------+--------------------------------+
| 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 |
+---------------------------+--------------------------------+
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+---------------------------+--------------------------------+
| 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 |
+---------------------------+--------------------------------+
| VersionNumber* | n: version of ITU fax standard |
+---------------------------+--------------------------------+
| VersionYear* | byte sequence with year of |
| | ITU fax standard |
+---------------------------+--------------------------------+
4. Base Color Fax Mode
4.1. 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 CIELAB 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 CIELAB encoding specified in [T.42]. Compared to the
other two CIELAB-based values for this field, ITU-T Rec. T.42
specifies a different default range for the a* and b* components,
based on a comprehensive evaluation of existing hardcopy output.
It optionally allows a selectable range for the L*, a* and b*
components.
4.2. Required TIFF fields
This section lists the required fields and the applications they
must support to be compatible with ITU-T Rec. T.42 and Annex E in
ITU-T Rec. T.4.
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4.2.1. Baseline Fields
ImageWidth. SHORT or LONG
This mode supports the following fixed page widths: 864, 1024,
1216, 1728, 2048, 2432, 2592, 3072, 3456, 3648, 4096, 4864.
BitsPerSample = 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 = 7. SHORT
Base color fax mode uses Baseline JPEG compression. Value 7
represents JPEG compression as specified in [TTN2], which also
defines a new extension field JPEGTables that is not used
here.
PhotometricInterpretation = 10. SHORT
Base color fax mode requires pixel values to be stored as
CIELAB, specified by the PhotometricInterpretation value 10,
referred to as ITULAB and which is based on the sample value
encoding defined in ITU-T Rec. T.42. 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 BitsPerSample.
The conversion from unsigned ITULAB-encoded samples values to
signed CIELAB values is determined by the Decode field; see
Section 4.2.2
PhotometricInterpretation values 8 and 9 also specify CIELAB
encodings, but with different, fixed ranges than ITU-T Rec.
T.42, and so are not used for color fax.
Note: The intent of PhotometricInterpretation 10 is to broaden
the scope of the CIELAB parameter space and not just support
color fax. Therefore, the name and default Decode values are
still under discussion.
SamplesPerPixel = 1, 3. SHORT
1: L* component only
3: L*, a*, b* components
Encoded according to PhotometricInterpretation field
XResolution. RATIONAL
YResolution. RATIONAL
The resolution of the image is expressed in pixels per
resolution unit. In pixels per inch, allowed XResolution
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values are: 100, 200, 300, and 400. The base color fax mode
requires the pixels to be square, hence YResolution must equal
XResolution.
4.2.2 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. SHORT
Count = 2
Specifies the subsampling factors for the chroma components of
a CIELAB 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 (1,1) for
LAB-based images, which is an option for color fax. The color
fax default is (2,2), which must be specified explicitly in
the IFD.
ChromaPositioning = 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. SSHORT
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 CIELAB
and has a count of 6 values. In this case, the input is an
unsigned ITULAB-encoded value and the output is a signed
CIELAB 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
The following table gives the default range and offsets, as
specified in ITU-T Rec. T.42, and the corresponding minimum
and maximum CIELAB component and ITULAB encoded values.
+-----------------------------------------------------------+
| Component | Rec. T.42 | CIELAB | 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 |
+-----------+--------+---------+------+-----+-----+---------+
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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.
Note: The intent of PhotometricInterpretation 10 is to broaden
the scope of the CIELAB parameter space and not just support
color fax. Therefore, the name and default Decode values are
still under discussion.
4.2.3. New fields
None.
4.3. Recommended TIFF fields
See Sections 2.2.2. and 2.2.3.
4.4 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 |
+---------------------------+--------------------------------+
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+---------------------------+--------------------------------+
| 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 |
+---------------------------+--------------------------------+
| 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 CIELAB |
+---------------------------+--------------------------------+
| ChromaSubSampling | ChromaSubsampleHoriz: |
| | 0: equal numbers of lightness |
| | and chroma samples |
| | 1: twice as many lightness |
| | samples as chroma samples |
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| | 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 |
+---------------------------+--------------------------------+
| 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 |
+---------------------------+--------------------------------+
| VersionNumber* | n: version of ITU fax standard |
+---------------------------+--------------------------------+
| VersionYear* | byte sequence with year of |
| | ITU fax standard |
+---------------------------+--------------------------------+
5. Lossless Color Mode
5.1. 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 CIELAB encoding specified in [T.42].
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5.1.1. 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
CIELAB 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.
To enable T.43 color maps in TIFF requires the extension field
Indexed, defined in [TTN1], and the PhotometricInterpretation
field value 10, defined in Section 4.2.1. 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 |
+------------+-------------+----------+----------+---------+
5.1.2. 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.
5.2. Required TIFF Fields
This section lists the required fields and the values they must
have to be compatible with ITU-T Rec. T.43.
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5.2.1. Baseline fields
ImageWidth. SHORT or LONG
Same page widths as the base color mode; see Section 4.2.1.
BitsPerSample = 1, 8, 9-16. SHORT
Count = SamplesPerPixel
RGB, CMY, CMYK: 1 bit per sample
Continuous tone (CIELAB): 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. 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 = 9. SHORT
9: ITU-T Rec. T.82 coding, applying ITU-T Rec. T.85 (JBIG)
FillOrder = 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 = 2, 5, 10. SHORT
2: RGB
5: CMYK, including CMY
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,
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SamplesPerPixel to 1, and Indexed to 1. The color map is
stored in the ColorMap field. See Section 5.1.1 for further
discussion on the color encoding.
SamplesPerPixel = 1, 3, 4. SHORT
1: Palette color image, or L*-only if Indexed = 0 and
PhotometricInterpretation is 10 (ITULAB).
3: RGB, or CIELAB, or CMY if PhotometricInterpretation is 5
(CMYK).
4: CMYK.
XResolution. RATIONAL
YResolution. 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.
5.2.2. Extension fields
Indexed = 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 SHORT
Decode is used in connection with the ITULAB encoding of image
data; see Section 4.2.2.
5.2.3. New Fields
None.
5.3. Recommended TIFF fields
See Sections 2.2.2. and 2.2.3.
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5.4. Lossless Color Fax Mode Summary
Recommended fields are shown with an asterisk *
+--------------------|--------------------------------------+
| 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 |
+--------------------+--------------------------------------+
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+--------------------+--------------------------------------+
| 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 |
+--------------------+--------------------------------------+
| Decode | minL, maxL, mina, maxa, minb, maxb: |
| |minimum and maximum values for CIELAB |
+--------------------+--------------------------------------+
| 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 |
+--------------------+--------------------------------------+
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6. Mixed Raster Content Mode
6.1. 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.
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.
6.1.1. 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].
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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.
6.1.2. 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 6.2.3.
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.
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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]
NewSubFileType = 16
|
|(nextIFD)
|
V
Child IFD
ImageLayer = [1,2]
NewSubFileType = 16
|
|(nextIFD)
|
V
Child IFD
ImageLayer = [3,1]
NewSubFileType = 16
|
|(nextIFD)
|
V
0
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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 6.2.2.
When there is only a single layer (Mask, Foreground, or
Background), it is stored as the Primary IFD.
6.2. Required TIFF Fields
This section describes the TIFF fields required 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.
6.2.1. Baseline fields
ImageWidth. SHORT or LONG
Same page widths as the base color mode; see Section 4.2.1.
BitsPerSample = 1, 8, 9-16 SHORT
Compression = 2, 3, 4, 7, 9. SHORT
SamplesPerPixel = 1, 3, 4. SHORT
FillOrder = 1, 2. SHORT
PhotometricInterpretation = 0, 1, 2, 5, 10. SHORT
For Mask layer, see Section 3.2.1.
For Foreground and Background layers, see Sections 4.2.1 and
5.2.1.
ColorMap. SHORT
Count = 3 * (2**BitsPerSample)
Used when Foreground or Background layer is a palette-color
image; see Section 5.2.1.
NewSubFileType = 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
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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. LONG
Tag = 559 (0x022F)
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. RATIONAL
YResolution. 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.
6.2.2. Extension fields
ChromaSubSampling. SHORT
ChromaPositioning. SHORT
Decode. SHORT
For Foreground and Background layers, see Section 4.2.2.
Indexed = 0, 1. SHORT
For Foreground and Background layers: 1 indicates a palette-
color image, see Section 5.2.2.
T4Options = 0, 1, 4, 5. SHORT
T6Options = 0. SHORT
For Mask layer, see Section 3.2.2.
DefaultImageColor. 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
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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.
XPosition. RATIONAL
YPosition. 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.
6.2.3. New fields
MRC Color fax mode requires one new field: ImageLayer.
ImageLayer. SHORT or LONG.
Count = 2
Tag = 0x87ac
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.)
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.
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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.
6.3. Recommended TIFF fields
See Sections 2.2.2. and 2.2.3.
6.4. 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 4 and 5, 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.
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).
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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.
6.5. 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 | 2: Modified Huffman |
| | 3: 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 |
+------------------+-----------------------------------------+
| ImageDescription*| {ASCII}: A string describing the |
| | contents of the image. |
+------------------+-----------------------------------------+
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+------------------+-----------------------------------------+
| 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 |
+------------------+-----------------------------------------+
| T4Options | 0: required if Compression is |
| | Modified Huffman, no fill bits |
| | 1: required if Compression is 2D |
| | Modified Read, no fill bits |
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| | 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 CIELAB |
+------------------+-----------------------------------------+
| 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 |
+------------------+-----------------------------------------+
7. 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.
8. 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 or a pair of letters. Three
applications are proposed.
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B - multi-page bi-level monochrome images [RFC1314].
F - the current reference for the image/tiff subtype
[RFC1528], with adds from work in progress [TIFF-F]
FX - this specification, with ITU-fax-compatible parameter
values
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: image/tiff, class=FX is 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
Person & e-mail address to contact for further information:
Lloyd McIntyre <lmcintyre@adoc.xerox.com>
Steve Zilles <szilles@adobe.com>
Intended usage: Common
9. Security Considerations
Security issues have not been assessed in this document.
10. 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 Draft Recommendation T.43, T.Palette-Colour - 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
[TIFF-F] work in progress, Tag Image File Format (TIFF) - Class
F, Jan. 1997, ftp://ftp.ietf.org/internet-drafts/draft-ietf-
fax-tiff-00.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
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T.4, T.6, T.42, T.81, T.82 and T.85 are available at
http://www.itu.ch.
[MRC] is currently available at http://www.xerox.com/xis/mrc/
11. 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
Annex A: Summary of TIFF Fields for Internet Fax
This annex includes tables which list all relevant 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 relevant to 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
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The absence of an entry indicates the field is not part of the
current proposal.
Table A.1 TIFF Baseline Fields
+----------+-------------------------------------------------+
| | Fax Modes |
+----------+-------------------------------------------------|
| Field | Bi-level | Color | Color | Mixed Raster|
| | | (Lossy) | (Lossless) | Content |
+----------+-----------+----------+------------+-------------+
| BitsPer | | | | |
| Sample | 1 | 8, 12 | 1, 8, 9-16 | 1, 8, 9-16 |
+----------+-----------+----------+------------|-------------|
| ColorMap | | | <n> | <n> |
+----------+-----------+----------+------------|-------------|
| Compres- | | | | |
| sion | 2, 3, 4 | 7 | 9 |2, 3, 4, 7, 9|
+----------+-----------+----------+------------+-------------+
| DateTime | {ASCII} | {ASCII} | {ASCII} |{ASCII} |
+----------+-----------+--------- +------------|-------------|
| FillOrder| 1, 2 | | 1, 2 | 1, 2 |
+----------+-----------+----------+------------+-------------+
| ImageDes-| (ASCII} | {ASCII} | {ASCII} | {ASCII} |
| cription | | | | |
+----------+-----------+----------+------------+-------------|
| Image- |1728, 2048 |864, 1024 |864, 1024 |864, 1024 |
| Width |2432, 2592 |1216, 1728|1216, 1728 |1216, 1728 |
| |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 |
+----------+-----------+----------+------------+-------------+
| NewSub- | | | | |
| FileType | 2 | 2 | 2 | 16, 18 |
+----------+-----------+----------+------------+-------------+
| Orien- | | | | |
| tation | 1 | 1 |1 | 1 |
+----------+-----------+----------+------------+-------------+
| Photo- | | | | |
| metric- | | | | |
| Interp- | | | | |
| retation | 0, 1 | 10 | 2, 5, 10 | 0,1,2,5,10 |
+----------+-----------+----------+------------+-------------+
| Resolu- | | | | |
| tionUnit | 2, 3 | 2, 3 | 2, 3 | 2, 3 |
+----------+-----------+----------+------------+-------------+
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+----------+-----------+----------+------------+-------------+
| RowsPer- | | | | |
| Strip |n | n | n | |
+----------+-----------+----------+------------+-------------+
| Samples- | | | | |
| PerPixel | 1 | 1, 3 | 1, 3, 4 | 1, 3, 4 |
+----------+-----------+----------+------------+-------------+
| Software | {ASCII} | {ASCII} | {ASCII} | {ASCII} |
+----------+-----------+----------+------------+-------------+
| Strip- | | | | |
| Byte- | | | | |
| Counts | <n> | <n> | <n> | <n> |
+----------+-----------+----------+------------+-------------+
| Strip- | | | | |
| Offsets | <n> | <n> | <n> | <n> |
+----------+-----------+----------+------------+-------------+
| XResolu- | 200, 204, | 100, 200,| 100, 200, | 100, 200, |
| tion | 300, 400, | 300, 400 | 300, 400 | 300, 400 |
| | 406 | | | |
+----------+-----------+----------+------------+-------------+
| YResolu- | 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 Modes |
+-----------+------------------------------------------------|
| Field | Bi-level | Color | Color | Mixed Raster|
| | | (Lossy) | (Lossless) | Content |
+-----------+-----------+----------+----- ------+------------+
| T4Options | 0, 1, 4, 5| | | 0, 1, 4, 5 |
+-----------+-----------+----------+------------+------------+
| T6Options | 0 | | | 0 |
+-----------+-----------+----------+------------+------------+
| Document- | | | | |
| Name | {ASCII} | {ASCII} | {ASCII} | {ASCII} |
+-----------+-----------+----------+------------+------------+
| Page- | | | | |
| Number | n, m | n, m | n, m | n, m |
+-----------+-----------+----------+------------+------------+
| XPosition | | | | r |
+-----------+-----------+----------+------------+------------+
| YPosition | | | | r |
+-----------+-----------+----------+------------+------------+
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+-----------+-----------+----------+------------+------------+
| SubIFDs | | | | <n> |
+-----------+-----------+----------+------------+------------+
| Indexed | | | 0, 1 | 0, 1 |
+-----------+-----------+----------+------------+------------+
| 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 Modes |
+-----------+------------------------------------------------|
| Field | Bi-level | Color | Color | Mixed Raster|
| | | (Lossy | (Lossless) | Content |
+-----------+-----------+---------+------------+-------------+
| Global- | | | | |
| Parame- | | | | |
| tersIFD | <IFD> | <IFD> | <IFD> | <IFD> |
+-----------+-----------+---------+------------+-------------+
| Profile- | | | | |
| Type | n | n | n | n |
+-----------+-----------+---------+------------+-------------+
| ITUFax- | | | | |
| Mode | n | n | n | n |
+-----------+-----------+---------+------------+-------------+
| Coding- | | | | |
| Methods | n | n | n | n |
+-----------+-----------+---------+------------+-------------+
| Version- | | | | |
| Number | n | n |n | n |
+-----------+-----------+---------+------------+-------------+
| Version- | | | | |
| Year | <b> | <b> | <b> | <b> |
+-----------+-----------+---------+------------+-------------+
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+-----------+-----------+---------+------------+-------------+
| BadFax- | | | | |
| Lines | n | | | |
+-----------+----------------------------------+-------------+
| CleanFax- | | | | |
| Data | 0, 1, 2 | | | |
+-----------+-----------+---------+------------+-------------+
| Consecu- | | | | |
| tiveBad- | | | | |
| FaxLines | n | | | |
+-----------+-----------+---------+------------+-------------+
| ImageLayer| | | | n, m |
+-----------+-----------+---------+------------+-------------+
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