Network Working Group Glenn Parsons
Internet Draft James Rafferty
Expires in six months May 30, 1997
Tag Image File Format (TIFF) - Application F
<draft-ietf-fax-tiff-02.txt>
Status of this Memo
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Overview
This document describes in detail the definition of TIFF-F that is
used to store facsimile images and also describes the registration
refinement of the MIME sub-type image/tiff for Application F. The
TIFF-F encoding has been folklore with no standard reference
definition before this document.
Internet Fax Working Group
This document is a product of the IETF Internet Fax Working Group.
All comments on this document should be forwarded to the email
distribution list at <ietf-fax@imc.org>.
Internet Draft TIFF-F May 30, 1997
1. Abstract
This document formalizes the reference for the Tag Image File
Format (TIFF) and formally defines TIFF-F that is used to store
facsimile images. As well, the original registration for
image/TIFF from RFC 1528 is refined by this document.
2. TIFF Definition
TIFF (Tag Image File Format) Revision 6.0 is defined in detail by
Adobe in [TIFF]. The documentation can be obtained from Adobe at:
Adobe Developers Association
Adobe Systems Incorporated
345 Park Avenue
San Jose, CA 95110-2704
Phone: +1-408-536-6000
Fax: +1-408-537-6000
A copy of this specification can also be found in:
ftp://ftp.adobe.com/pub/adobe/devrelations/devtechnotes/pdffiles/ti
ff6.pdf
Only an introduction to the baseline TIFF is included in this
document. The reader is directed to the original TIFF
specification [TIFF] to obtain specific technical details.
2.1 TIFF Scope
TIFF describes image data that typically comes from scanners, frame
grabbers, and paint- and photo-retouching programs. TIFF is not a
printer language or page description language. The purpose of TIFF
is to describe and store raster image data. A primary goal of TIFF
is to provide a rich environment within which applications can
exchange image data. This richness is required to take advantage of
the varying capabilities of scanners and other imaging devices.
Though TIFF is a rich format, it can easily be used for simple
scanners and applications as well because the number of required
fields is small.
2.2 TIFF Features
- 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.
- TIFF is not tied to specific scanners, printers, or computer
display hardware.
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- TIFF is portable. It does not favor particular operating
systems, file systems, compilers, or processors.
- TIFF is designed to be extensible and to evolve gracefully as
new needs arise.
- TIFF allows the inclusion of an unlimited amount of private or
special-purpose information.
3. TIFF-F Definition
Though it has been in common usage for many years, TIFF-F has
previously never been documented in the form of a standard. An
informal TIFF-F document was originally created by a small group of
fax experts led by Joe Campbell. The existence of TIFF-F is noted
in [TIFF] but it is not defined. This document serves as the
formal definition for TIFF-F for Internet applications.
3.1 The Spirit of TIFF-F
Up until TIFF 6.0, TIFF supported various "Classes" which defined
the use of TIFF for various applications. The intent of Classes
had been to reduce the information burden on TIFF readers and
writers that wished to support narrow applications. In this
spirit, TIFF-F has been known historically as "TIFF Class F" and
previous informal TIFF-F documents used this terminology.
In the context of TIFF Classes, TIFF Class F had been a sub-class
of TIFF Class B (Bilevel). That is, all fields that were required
in Class B were also required in Class F. For some common fields,
however, TIFF-F has historically limited the range of acceptable
values, based on requirements for the facsimile application and the
need to stay aligned with ITU-T recommendations.
As of TIFF 6.0 [TIFF], the TIFF Class concept has been eliminated
in favor of the concept of Baseline TIFF. Therefore, this
document will update the definition of TIFF-F based on the
conventions used in TIFF 6.0. In almost all cases, the resulting
definition of TIFF-F fields and values will be consistent with
those used historically in earlier definitions of TIFF Class F.
Where some of the values for fields have been updated to provide
more precise conformance with the ITU-T [T.30] fax protocol, these
differences will be noted. The intent of this specification will
be to clearly document:
1) A minimum set of TIFF-F fields and values which should be able
to interwork with virtually all historic TIFF-F readers.
2) A broader range of values for the traditional TIFF-F fields
that will provide support for the most widely used facsimile
compressions, page sizes and resolutions, consistent with the
ITU-T [T.30] recommendation.
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The structure of the TIFF-F definition will be as follows. A
review of the structure of TIFF files and practical guidelines for
TIFF-F files in particular is provided in sections 3.1.1 and 3.1.2
The review of TIFF-F fields follows. Section 3.2 reviews
the fields from Baseline TIFF that are applicable for black and
white (bi-level) images and are also used by TIFF-F.
Section 3.3 reviews the other required TIFF-F fields. Several
fields that are specific extensions for TIFF-F are reviewed in
section 3.4. There are also fields that may be helpful, but are
not required. These recommended fields are listed in the section
3.5.
Several technical topics, including implementation issues, warnings
and conformance are discussed in subsequent sections. Finally,
section 3.9 introduces the TIFF-F Reader and Writer. A table of
the required and recommended fields for a TIFF-F Reader is
provided, along with details on the minimum and permitted set of
values for TIFF-F purposes.
3.1.1 Structure of TIFF Files
The structure of TIFF files is specified within [TIFF]. In this
section, a short summary of the TIFF structure is included for the
informational purposes. In addition, some practical guidelines
for the use of this structure in writing multi-page TIFF-F files
are also addressed here.
A TIFF file begins with an 8-byte image file header that defines
the byte order used within a file (see 3.9.1), includes a magic
number sequence that identifies the content as a TIFF file, and
then uses an offset to point to the first Image File Directory
(IFD). An IFD is a sequence of tagged fields, sorted in ascending
order (by tag value), that contains attributes of an image and
pointers to the image data. TIFF fields (also called entries)
contain a tag, its type (e.g. short, long, rational, etc.), a count
(which indicates the number of values/offsets) and a value/offset.
However, the actual value for the field will only be present if it
fits into 4 bytes; otherwise, an offset will be used to point to
the location of the data associated with the field. In turn, this
offset may itself be used to point to an array of offsets.
For the case of facsimile data, many documents consist of a series
of multiple pages. Within TIFF, these may be represented using
more than one IFD within the TIFF file. Each IFD defines a
subfile whose type is given in the NewSubfileType field. For the
case of facsimile data that is placed in a TIFF-F file, each
facsimile page in a multi-page document has its own IFD. For bi-
level facsimile files, multiple IFDs are organized as a linked
list, with the last entry in each IFD pointing to the next IFD (the
pointer in the last IFD is 0). (There is also another technique for
organizing multiple IFDs as a tree, that uses the SubIFDs field,
but this technique is not applicable for TIFF-F images.) Within
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each IFD, the location of the related image data is defined by
using fields that are associated with strips. These fields
identify the size of strips (in rows), the number of bytes per
strip after compression and a strip offset, which is used to point
to the actual location of the image strip.
TIFF has a very flexible file structure, but the use of some
practical guidelines on structuring a multi-page TIFF-F file may
yield benefits for implementers, especially for applications in
environments such as facsimile terminals where a complex file
structure is not practical.
3.1.2 Practical Guidelines for Structuring TIFF-F Files
The implementation of TIFF-F can be greatly simplified if certain
practical guidelines are kept in mind when writing TIFF-F files.
In general, the structure for a multi-page TIFF-F file will include
one IFD per page of the document. Therefore, each IFD will define
the attributes for a single page. For simplicity, it is advisable
that the writer of TIFF-F files present IFDs in the same order as
the actual sequence of pages. (The pages are numbered within TIFF-
F beginning with page 0 as the first page and then ascending (i.e.
0, 1, 2, ...). However, as noted in 3.1.1, any field values over
4 bytes will be stored separately from the IFD.
Per [TIFF], the exact placement of image data is not specified.
However, the strip offsets for each strip of image are defined from
within each IFD. Where possible, a second simplifying assumption
for the writing of TIFF-F files is to specify that the image data
for each page of a multi-page document will be contained within a
single strip (i.e. one image strip per fax page). In the event a
different assumption has been used (e.g. constant size for image
strips which may be less than the page size), this will immediately
be evident from the values/offsets of the fields that are related
to strips. From an implementor's standpoint, one image strip per
page will permit the image data to be found through reference via a
single offset, resulting in a much simplified image structure.
In addition, a third simplifying assumption for TIFF-F writers will
be to place the actual image data in a physical order within the
TIFF file structure which is consistent with the logical page
order. So, a TIFF-F file which is structured using the guidelines
of this section will essentially be composed of a linked list of
IFDs, presented in ascending page order, which in turn each point
to a single page of image data (one strip per page), where the
pages of image data are also placed in a logical page order within
the TIFF-F file structure. (The pages of image data may themselves
be stored in a contiguous manner, at the option of the
implementer).
3.2 Baseline TIFF Required Fields for BiLevel Images
Baseline TIFF per [TIFF] requires that the following fields be
present for all BiLevel Images: ImageWidth, ImageLength,
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Compression, Photometric Interpretation, StripOffsets,
RowsPerStrip, StripByteCounts, Xresolution, YResolution and
ResolutionUnit. TIFF-F uses all of these fields, but in some
cases specifies a different range of acceptable values than
Baseline TIFF. Per [TIFF], if fields are omitted, the Baseline
TIFF default value(if specified) will apply.
A brief summary of these fields and their use in TIFF-F follows:
ImageWidth = 1728, 2048, 2432, 2592, 3072, 3648, 3456, 4096, 4864.
SHORT or LONG. These are the fixed page widths in pixels. The
permissible values are dependent upon X and Y resolutions as
shown in sections 2 and 3 of [T.4] and reproduced here for
convenience:
XResolution x Yresolution | ImageWidth
-------------------------------------------|------------------
204 x 98, 204 x 196, 200 x 100, 200 x 200 | 1728, 2048, 2432
300 x 300 | 2592, 3072, 3648
406 x 392, 400 x 400 | 3456, 4096, 4864
-------------------------------------------|------------------
Historical TIFF-F did not include support for the following
widths related to higher resolutions: 2592, 3072, 3648, 3456,
4096 and 4864. 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. See section 3.8.2 for more
information on inch/metric equivalencies and other
implementation details.
ImageLength. SHORT or LONG. LONG recommended.
The total number of scan lines in the image.
Compression = 3,4. SHORT.
This is a required TIFF-F field. The permitted values for TIFF-
F purposes are 3 and 4 as shown. The default value per
Baseline TIFF is 1 (Uncompressed), but this value is invalid
for facsimile images. Baseline TIFF also permits use of
value 2 (Modified Huffman encoding), but the data is presented
in a form which is not byte aligned. Instead, TIFF-F specifies
the value 3 for encoding one-dimensional T.4 Modified Huffman
or 2-dimensional Modified READ data. The detailed settings
which apply for T.4 encoded data are specified using the
T4Options field. TIFF-F also permits use of the value 4 for
the compression field, which indicates that the data is coded
using a [T.6] compression method (i.e the Modified Modified
READ two-dimensional method). The detailed settings which apply
for T.6 encoded data are specified using the T6Options field.
Please refer to the definitions of the T4Options and T6Options
fields in section 3.3, and section 3.8 for more information on
the encoding of images and conventions used within TIFF-F.
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PhotometricInterpretation = 0,1. SHORT.
This field allows notation of an inverted ("negative") image:
0 = normal
1 = inverted
StripOffsets. SHORT or LONG.
For each strip, the offset of that strip. The offset is
measured from the beginning of the file. If a page is expressed
as one large strip, there is one such entry per page.
RowsPerStrip. SHORT or LONG. LONG recommended.
The number of scan lines per strip. When a page is expressed
as one large strip, this is the same as the ImageLength field.
StripByteCounts. LONG or SHORT. LONG recommended.
For each strip, the number of bytes in that strip. If a page is
expressed as one large strip, this is the total number of bytes
in the page after compression. Note that the choice of LONG or
SHORT depends upon the size of the strip.
ResolutionUnit = 2,3. SHORT.
The units of measure for resolution:
2 = Inch
3 = Centimeter
TIFF-F has traditionally used inch based measures.
XResolution = 204, 200, 300, 400, 406 (inches). RATIONAL.
The horizontal resolution of the TIFF-F image expressed in
pixels per resolution unit. The values of 200 and 406 have been
added to the historical TIFF-F values, for consistency with
[T.30]. Some existing TIFF-F implementations may also support
values of 77 (cm). See section 3.8.2 for more information on
inch/metric equivalencies and other implementation details.
YResolution = 98, 196, 100, 200, 300, 392, 400 (inches). RATIONAL.
The vertical resolution of the TIFF-F image expressed in pixels
per resolution unit. The values of 100, 200, and 392 have been
added to the historical TIFF-F values, for consistency with
[T.30]. Some existing TIFF-F implementations may also support
values of 77, 38.5 (cm). See section 3.8.2 for more information
on inch/metric equivalencies and other implementation details.
3.3 TIFF-F Required Fields
In addition to the Baseline TIFF fields, there are additional
required fields for TIFF-F. There is also a requirement to
include either the T4Options or the T6Options field, depending upon
the setting of the compression field. A review of the additional
required fields for TIFF-F follows:
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BitsPerSample = 1. SHORT.
Since TIFF-F is only used for black-and-white facsimile
images, the value is 1 (the default) for all files.
FillOrder = 1, 2. SHORT.
TIFF F readers must be able to read data in both bit orders,
but the vast majority of facsimile products store data LSB
first, exactly as it appears on the telephone line.
1 = Most Significant Bit first.
2 = Least Significant Bit first.
NewSubFileType = 2. LONG.
This field is made up of 32 flag bits. Unused bits are
expected to be 0 and bit 0 is the low order bit. Bit 0 is set
to 0 for TIFF-F. Bit 1 is always set to 1 for TIFF-F,
indicating a single page of a multi-page image. See sections
3.1.1 and 3.1.2 for more details on the structure of multi-page
TIFF-F image files.
PageNumber. SHORT/SHORT.
This field specifies the page numbers in the fax document. The
field comprises two SHORT values: the first value is the page
number, the second is the total number of pages. Single-page
documents therefore use 00000001 hex.
SamplesPerPixel = 1. SHORT.
The value of 1 denotes a bi-level, grayscale, or palette color
image.
T4Options = 4,5. LONG.
This field is required if the value for the compression field
has been set to 3. The values are set as shown below for TIFF-
F. For TIFF-F, uncompressed data is not allowed and EOLs are
byte aligned.
bit 0 = 0 for 1-Dimensional, 1 for 2-Dimensional (MR)
bit 1 = must be 0 (uncompressed data not allowed)
bit 2 = 1 for byte-aligned EOLs
Bit 0 is the low order bit. Please note that T4Options was
known as G3Options in earlier versions of TIFF and TIFF-F.
The data in a TIFF-F image encoded using one of the T.4 methods
is not terminated with an RTC (see 3.8.5).
T6Options = 0 LONG.
This field is required for TIFF F if value of the compression
field has been set to 4. The value for this field is made up of
a set of 32 flag bits. Setting bit 0 to 0 indicates that the
data is compressed using the Modified Modified READ (MMR) two-
dimensional compression method. MMR compressed Data is two-
dimensional and does not use EOLs. Each MMR encoded image
should include an "end-of-facsimile-block" (EOFB) code at the
end of each coded strip (see 3.8.6). Uncompressed data is not
applicable for bi-level facsimile images, so that bit 1 must be
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set to 0. Unused bits must be set to 0. Bit 0 is the low-order
bit. The default value is 0 (all bits 0).
bit 0 = 0 for 2-Dimensional
bit 1 = must be 0 (uncompressed data not allowed)
In earlier versions of TIFF, this field was named
Group4Options. The significance has not changed and the present
definition is compatible.
3.4 TIFF-F Extensions
There are only three new fields defined as TIFF-F extensions. All
three fields describe page quality. The information contained in
these fields is usually obtained from receiving facsimile hardware
(if applicable). These fields are optional. They should not be
used for facsimile image data that is error corrected or otherwise
guaranteed not to have coding errors.
Some applications need to understand exactly the error content of
the data. For example, a CAD program might wish to verify that a
file has a low error level before importing it into a high-accuracy
document. Because Group 3 facsimile devices do not necessarily
perform error correction on the image data, the quality of a
received page must be inferred from the pixel count of decoded scan
lines. A "good" scan line is defined as a line that, when decoded,
contains the correct number of pixels. Conversely, a "bad" scan
line is defined as a line that, when decoded, comprises an
incorrect number of pixels.
BadFaxLines
Tag = 326 (146 hex)
Type = SHORT or LONG
This field reports the number of scan lines with an incorrect
number of pixels encountered by the facsimile during reception
(but not necessarily in the file).
Note: PercentBad = (BadFaxLines/ImageLength) * 100
CleanFaxData
Tag = 327 (147 hex)
Type = SHORT
N = 0
0 = Data contains no lines with incorrect pixel counts or
regenerated lines (i.e., computer generated)
1 = Lines with an incorrect pixel count were regenerated by
receiving device
2 = Lines with an incorrect pixel count are in the data and
were not regenerated by receiving device (i.e. data
contains bad scan lines)
Many facsimile devices do not actually output bad lines.
Instead, the previous good line is repeated in place of a bad
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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 bad
lines are actually still in the data or if the receiving
facsimile device replaced them with regenerated lines.
ConsecutiveBadFaxLines
Tag = 328 (148 hex)
Type = LONG or SHORT
This field reports the maximum number of consecutive lines
containing an incorrect number of pixels encountered by the
facsimile device during reception (but not necessarily in the
file).
The BadFaxLines and ImageLength data indicate only the quantity
of such lines. The ConsecutiveBadFaxLines field is an
indicator of their distribution and may therefore be a better
general indicator of perceived image quality.
3.5 Recommended Fields
These are fields that may be used in encoding TIFF-F files, but are
optional in nature and may be ignored by many TIFF readers. These
fields are called recommended consistent with historical TIFF-F
practice.
BadFaxLines. LONG.
The number of "bad" scan lines encountered by the facsimile
during reception.
CleanFaxData = 0, 1, 2. BYTE.
This field indicates whether lines with incorrect pixel count
are actually in the data or if the receiving facsimile device
replaced them with regenerated lines.
0 = Data contains no lines with incorrect pixel counts or
regenerated lines (i.e., computer generated)
1 = Lines with an incorrect pixel count were regenerated
by receiving device
2 = Lines with an incorrect pixel count are in the data
and were not regenerated by receiving device (i.e. data
contains bad scan lines)
ConsecutiveBadFaxLines. LONG or SHORT.
The maximum number of consecutive scan lines with incorrect
pixel count encountered by the facsimile device reception.
DateTime. ASCII.
Date and time in the format YYYY:MM:DD HH:MM:SS, in 24-hour
format. String length including NUL byte is 20 bytes. Space
between DD and HH.
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DocumentName. ASCII.
This is the name of the document from which the document was
scanned.
ImageDescription. ASCII.
This is an ASCII string describing the contents of the image.
Orientation. SHORT.
This field might be useful for displayers that always want to
show the same orientation, regardless of the image. The
default value of 1 is "0th row is visual top of image, and 0th
column is the visual left." An 180-degree rotation is 3. See
[TIFF] for an explanation of other values.
Software. ASCII.
The optional name and release number of the software package
that created the image.
3.6 Technical Implementation Issues
3.6.1 Strips
Those new to TIFF may not be familiar with the concept of "strips"
embodied in the three fields RowsPerStrip, StripByteCount,
StripOffsets.
In general, third-party applications that read and write TIFF files
expect the image to be divided into "strips," also known as
"bands." Each strip contains a few lines 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 dimensions of a strip are described by the RowsPerStrip and
StripByteCount fields. The location in the TIFF file of each strip
is contained in the StripOffsets field.
The size of TIFF-F strips is application dependent. However, a
practical approach for multi-page TIFF-F images is to represent
each page as a single strip.
3.6.2 Bit Order
Although the TIFF 6.0 documentation lists the FillOrder field in
the category "No Longer Recommended," TIFF-F utilizes it.
Facsimile data appears on the phone line in bit-reversed order
relative to its description in CCITT Recommendation T.4.
Therefore, a wide majority of facsimile applications choose this
natural order for storage. Nevertheless, TIFF-F readers must be
able to read data in both bit orders.
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3.6.3. Multi-Page
Many existing applications already read TIFF-F like files, but do
not support the multi- page field. Since a multi-page format
greatly simplifies file management in fax application software,
TIFF-F specifies multi-page documents (NewSubfileType = 2).
3.6.4. Compression
In Group 3 facsimile, there are three compression methods which had
been standardized as of 1994 and are in common use. The ITU-T T.4
recommendation 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 Addressing). In
1984, a somewhat more efficient compression method known as
Modified Modified READ (MMR) was defined in the T.6 recommendation.
It 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 permits three different compression methods. In the most
common practice, the one-dimensional compression method (Modified
Huffman) has used. This is specified by setting the value of the
Compression field to 3 and then setting bit 0 of the T4Options
field. Alternatively, the two dimensional Modified READ method
(which is much less frequently used in historical TIFF-F
implementations) may be selected by setting bit 0 to a value of 1.
Optionally, depending upon the application, the more efficient two-
dimensional compression method from T.6 (i.e. MMR or "Group 4
compression") may be selected. This method is selected by setting
the value of the Compression field to 4 and then setting the value
of the first two bits (and all unused bits) of T6options to 0.
More information to aid the implementer in making a compression
selection is contained in section 3.8 on Implementation Warnings.
3.6.5. 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.
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3.7 TIFF-F Conformance
Fax applications that do not wish to support TIFF-F as a native
format may elect to support it as import/export medium.
Export
It is recommended that applications export multiple page TIFF-F
files without manipulating fields and values. Historically, some
TIFF-F writers have attempted to produce individual single-page
TIFF-F files with modified NewSubFile and PageNumber (page one-of-
one) values for export purposes. However, there is no way to link
such multiple single page files together into a logical multiple
page document, so that this practice is not recommended.
Import
A TIFF-F reader must be able to handle a TIFF-F file containing
multiple pages.
3.8 Implementation Warnings
3.8.1 Uncompressed data
TIFF-F requires the ability to read and write at least one-
dimensional T.4 Huffman ("compressed") data. Uncompressed data is
not allowed. This means that the "Uncompressed" bit in T4Options
or T6Options must be set to 0.
3.8.2 Encoding and Resolution
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.
For the case of resolution, almost all facsimile products support
both standard (98 dpi) vertical resolution and "fine" (196 dpi)
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 writer, 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.
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Per [T.4], 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.4] 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.
3.8.3 EOL byte-aligned
In the spirit of TIFF-F, all EOLs in Modified Huffman or Modified
READ 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.
Recall that Modified 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).
For Modified READ encoding, each line is terminated by an EOL and a
one bit tag bit. Per [T.4], The value of the tag bit is 0 if the
next line contains two dimensional data and 1 if the next line is a
reference line. To maintain byte alignment, fill bits are added
before the EOL/tag bit, so that the first bit of data following an
MR tag bit begins on a byte boundary.
3.8.4. 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.
3.8.5 RTC Exclusion
Aside from EOLs, TIFF-F files contain only image data. This means
that the Return To Control sequence (RTC) is specifically excluded.
3.8.6 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"
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(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.
3.9 TIFF-F Fields Summary
Implementations may choose to implement a TIFF-F Reader, TIFF-F
Writer or both, depending upon application requirements. The TIFF-
F Reader is typically used to read an existing TIFF-F file which
resides on a computer or peripheral device. The TIFF-F Writer is
typically used to convert a bi-level image bit stream into a TIFF-F
compliant file. For many Internet applications, only the Reader
needs to be implemented. The specific field support required for
TIFF-F Readers and Writers is summarized below.
3.9.1 TIFF Reader
The fields in following table are specified for a TIFF-F Reader.
The range of values for required and recommended fields are as
shown. A minimum subset of values is also shown, denoting those
values which should provide maximum portability with historical
TIFF-F readers. If required fields are omitted in a TIFF-F file,
the Baseline TIFF default value will apply. Image data must not
have any coding errors.
As noted within [TIFF], a TIFF file begins with an 8-byte image
file header, of which the first two bytes (0-1) contain the byte
order within the file. The permissible values are:
II- Byte order from least significant byte to the most
significant byte (little-endian)
MM - byte order is always from most significant to least
significant (big-endian)
For a TIFF-F Reader, the legal values are:
ByteOrder: MM,II (Either byte order is allowed)
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3.9.1.1 Fields for TIFF-F Reader
Field | Values | Minimum | Comment
------------------|-------------|--------------|----------------------
BitsPerSample | 1 | 1 |one bit per sample
Compression | 3,4 | 3 |3 for T.4 (MH, MR)
| | |4 for T.6 - MMR
FillOrder | 2,1 | 2 |LSB first or MSB first
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 |
| or other | |
SamplesPerPixel | 1 | 1 |one sample per pixel
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 | | If unit is per inch
| 77 | | If unit is per cm
Yresolution | 196,98,100, | 196,98 | If unit is per inch
| 200,300,392,| | If unit is per inch
| 400 | | If unit is per inch
| 77,38.5 | | If unit is per cm
------------------|-------------|--------------|--------------------
Recommended Fields are shown with an *
Other fields may be present, but they should be of an
informational nature, so that a reader can elect to ignore them.
Informational fields which are often present in TIFF-F images
are:
Software, Datetime, BadFaxLines, CleanFaxData and
ConsecutiveBadFaxLines.
3.9.2 TIFF-F Writer
For the case of writing (creating) a TIFF-F file format from an
image data stream or other raster data, implementations should
write files which can be read by a TIFF-F Reader as defined in
3.9.1. It is recommended that all fields from the table in 3.9.1.1
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should be included when writing TIFF-F files in order to minimize
dependencies on default values. Image data must not have any coding
errors.
Other fields may be present, but they should be of an informational
nature, so that a Reader may elect to ignore them.
Informational fields that may be useful are:
Software, Datetime, BadFaxLines, ConsecutiveBadFaxLines
TIFF Writers should only generate the fields that describe
facsimile image quality when the image has been generated from a
fax image data stream where error correction (e.g. Group 3 Error
Correction Mode) was not used. These fields are: CleanFaxData,
BadFaxLines and ConsecutiveBadFaxLines.
4. MIME sub-type image/TIFF
The image/TIFF sub-type was originally defined in RFC 1528 as
containing TIFF 5.0 encoded image data. [TIFFREG] re-defines the
original image/TIFF definition to refer to TIFF 6.0 encoded image
data. The TIFF 6.0 specification is a cleaner document and is
compatible with previous TIFF 5.0 encoded image data. In addition,
an optional "application" parameter is defined for image/TIFF to
identify the TIFF application of the encoded image data, if it is
known.
Example:
Content-type: image/TIFF; application=F
5. Implementation Usage
5.1 Internet Fax Usage
The usage of TIFF-F is envisaged to be a component of Internet Fax.
It is anticipated that Internet Fax will make use of both a TIFF-F
Reader and TIFF-F Writer. The details of these applications will be
specified in other documents.
5.2 VPIM Usage
The image/TIFF sub-type with the application F parameter (i.e. TIFF-
F content) is a secondary component of the VPIM Message as defined
in [VPIM2]. Voice messaging systems can often handle fax store-and-
forward capabilities in addition to traditional voice message store-
and-forward functions. As a result, this sub-type is used to hold
fax messages within the multipart/voice-message content that is
sent between compliant VPIM systems. In this context, the fax
content is optional and may be rejected if the recipient system
cannot deal with fax. VPIM implementations must at least implement
and support the TIFF-F Reader.
Refer to the VPIM Specification for proper usage of this content.
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6. Security Consideration
This document describes the encoding for TIFF-F, which is an
application of the TIFF encoding. As such, it does not create any
security issues not already existing in TIFF (though, none have
been identified).
Further, the encoding specified in this document does not in any
way preclude the use of any Internet security protocol to encrypt,
authenticate, or non-repudiate TIFF-F encoded facsimile messages.
7. Authors' Addresses
Glenn W. Parsons
Nortel Technology
P.O. Box 3511, Station C
Ottawa, ON K1Y 4H7
Canada
Phone: +1-613-763-7582
Fax: +1-613-763-8385
Email: Glenn.Parsons@Nortel.ca
James Rafferty
Human Communications
12 Kevin Drive
Danbury, CT 06811-2901
USA
Phone: +1-203-746-4367
Fax: +1-203-746-4367
Email: Jrafferty@worldnet.att.net
8. References
[MIME4] N. Freed and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Four: Registration Procedures", RFC
2048, Innosoft, First Virtual, Nov 1996.
[T.30] ITU-T Recommendation T.30 - "Procedures for Document
Facsimile Transmission in the General Switched Telephone
Network", June, 1996
[T.4] ITU-T Recommendation T.4 - "Standardization of Group 3
Facsimile Apparatus for Document Transmission", June, 1996
[T.6] ITU-T Recommendation T.6 - "Facsimile Coding Schemes and
Coding Control Functions for Group 4 Facsimile Apparatus",
March, 1993
[TIFF] Adobe Developers Association, TIFF (TM) Revision 6.0 -
Final, June 3, 1992.
[TIFFREG] G. Parsons and J. Rafferty, "Tag Image File Format (TIFF)
- image/TIFF: MIME Sub-type Registration ", Work In Progress,
<draft-ietf-fax-tiff-reg-00.txt>, May 1997.
[TPC.INT] C. Malamud, M. Rose, "Principles of Operation for the
TPC.INT Subdomain: Remote Printing -- Technical Procedures",
RFC 1528, 10/06/1993
[VPIM2] G. Vaudreuil and G. Parsons, "Voice Profile for Internet
Mail - version 2", Work In Progress, <draft-ema-vpim-05.txt>,
May 1997.
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