RFC2301 - File Format for Internet Fax

Network Working Group L. McIntyre
Request for Comments: 2301 Xerox Corporation
Category: Standards Track S. Zilles
Adobe Systems, Inc.
R. BUCkley
Xerox Corporation
D. Venable
Xerox Corporation
G. Parsons
Northern Telecom
J. Rafferty
Human Communications
March 1998
File Format for Internet Fax
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (1998). All Rights Reserved.
Abstract
This document describes the TIFF (Tag Image File Format)
representation of image data specified by the ITU-T Recommendations
for black-and-white and color facsimile. This file format
specification is commonly known as TIFF-FX. It formally defines
minimal, extended and lossless JBIG modes (Profiles S, F, J) for
black-and-white fax, and base JPEG, lossless JBIG and Mixed Raster
Content modes (Profiles C, L, M) for color and grayscale fax. These
modes or profiles correspond to the content of the applicable ITU-T
Recommendations. Files formatted according to this specification use
the image/tiff MIME Content Type.
Table of Contents
1. INTRODUCTION........................................................4
1.1. Scope..........................................................5
1.2. Approach.......................................................5
1.3. Overview of this draft.........................................5
2. TIFF and Fax........................................................7
2.1. TIFF Overview..................................................7
2.1.1. File Structure.............................................7
2.1.2. Image Structure............................................9
2.1.3. TIFF File Structure for Fax Applications..................10
2.2 TIFF Fields for All Fax Applications...........................11
2.2.1. TIFF Fields required for all fax modes....................12
2.2.2. Additional TIFF Fields required for all fax modes.........13
2.2.3. TIFF Fields recommended for all fax modes.................15
2.2.4. New TIFF Fields recommended for fax modes.................16
3. Minimal Black-and-White Fax Mode...................................18
3.1. Overview......................................................18
3.2. Required TIFF Fields..........................................18
3.2.1 Baseline Fields............................................18
3.2.2 Extension Fields...........................................20
3.2.3 New Fields.................................................20
3.3. Recommended TIFF Fields.......................................20
3.4. End of Line (EOL) and Return to Control (RTC).................20
3.4.1 RTC Exclusion..............................................21
3.5. File Structure................................................22
3.6. Minimal Black-and-White Mode Summary..........................23
4. Extended Black-and-White Fax Mode..................................24
4.1. TIFF-F Overview...............................................25
4.2. Required TIFF Fields..........................................26
4.2.1. Baseline Fields...........................................26
4.2.2. Extension Fields..........................................28
4.2.3. New Fields................................................29
4.3. Recommended TIFF Fields.......................................29
4.3.1. Baseline Fields...........................................29
4.3.2. Extension Fields..........................................29
4.3.3. New Fields................................................29
4.4. Technical Implementation Issues...............................30
4.4.1. Strips....................................................30
4.4.2. Bit Order.................................................31
4.4.3. Multi-Page................................................31
4.4.4. Compression...............................................31
4.4.5. Example Use of Page-quality Fields........................32
4.4.6. Practical Guidelines for Writing and Reading Multi-Page
TIFF-F Files..............................................33
4.4.7. Use of TIFF-F for Streaming Applications..................34
4.5. Implementation Warnings.......................................34
4.5.1. Uncompressed Data.........................................34
4.5.2. Encoding and Resolution...................................35
4.5.3. EOL byte-aligned..........................................35
4.5.4. EOL.......................................................36
4.5.5. RTC Exclusion.............................................36
4.5.6. Use of EOFB for T.6 Compressed Images.....................37
4.6. Example Use of TIFF-F.........................................37
4.7. Extended Black-and-white Fax Mode Summary.....................37
5. Lossless JBIG Black-and-White Fax Mode.............................39
5.1. Overview......................................................40
5.2. Required TIFF Fields..........................................40
5.2.1. Baseline Fields...........................................40
5.2.2. Extension Fields..........................................40
5.2.3. New Fields................................................41
5.3. Recommended TIFF Fields.......................................41
5.4. Lossless JBIG Black-and-White Mode Summary....................41
6. Base Color Fax Mode................................................43
6.1. Overview......................................................43
6.2. Required TIFF Fields..........................................43
6.2.1. Baseline Fields...........................................43
6.2.2. Extension Fields..........................................45
6.2.3. New Fields................................................46
6.3. Recommended TIFF Fields.......................................47
6.4. Base Color Fax Mode Summary...................................47
7. Lossless Color Mode................................................49
7.1. Overview......................................................50
7.1.1. Color Encoding............................................50
7.1.2. JBIG Encoding.............................................50
7.2. Required TIFF Fields..........................................51
7.2.1. Baseline Fields...........................................51
7.2.2. Extension Fields..........................................52
7.2.3. New Fields................................................53
7.3. Recommended TIFF Fields.......................................53
7.4. Lossless Color Fax Mode Summary...............................53
8. Mixed Raster Content Mode..........................................55
8.1 Overview.......................................................55
8.1.1. MRC 3-layer model.........................................55
8.1.2. A TIFF Representation for the MRC 3-layer model...........56
8.2. Required TIFF Fields..........................................58
8.2.1. Baseline Fields...........................................58
8.2.2. Extension Fields..........................................59
8.2.3. New Fields................................................60
8.3. Recommended TIFF Fields.......................................62
8.4. Rules and Requirements for Images.............................62
8.5. MRC Fax Mode Summary..........................................63
9. MIME content-type image/tiff.......................................66
9.1 Refinement of MIME content-type image/tiff for Facsimile
Applications...................................................66
10. Security Considerations...........................................67
11. References........................................................67
12. Authors" Addresses................................................69
Annex A: Summary of TIFF Fields for Internet Fax .....................70
Annex B. IANA Registration for image/tiff Application Parameter
Values used for facsimile....................................75
Full Copyright Statement..............................................77
1. Introduction
This document describes the use of TIFF (Tag Image File Format) to
represent the data content and structure generated by the current
suite of ITU-T Recommendations for Group 3 facsimile. These
Recommendations and the TIFF fields described here support the
following facsimile modes or profiles:
S: minimal black-and-white mode, using binary MH compression
[T.4]
F: extended black-and-white mode, using binary MH, MR and MMR
compression [T.4, T.6]
J: lossless JBIG black-and-white mode, with JBIG compression
[T.85, T.82]
C: lossy color and grayscale mode, using JPEG compression
[T.42, T.81]
L: lossless color and grayscale mode, using JBIG compression
[T.43, T.82]
M: mixed raster content mode [T.44], using a combination of
existing compression methods
Each profile corresponds to the content of ITU-T Recommendations
shown and is a subset of the full TIFF for facsimile specification.
Profile S describes a minimal interchange set of fields, which will
guarantee that, at least, binary black-and-white images will be
supported. Implementations are required to support this minimal
interchange set of fields.
With the intent of specifying a file format for Internet Fax, this
draft:
1. specifies the structure of TIFF files 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 color fax.
This specification of TIFF for facsimile is known as TIFF-FX.
1.1 Scope
This document defines a TIFF-based file format specification for
enabling standardized messaging-based fax over the Internet. It
specifies the TIFF fields and field values required for compatibility
with the existing ITU-T Recommendations for Group 3 black-and-white,
grayscale and color facsimile. TIFF has historically been used for
handling fax image files in applications such as store-and-forward
messaging. Implementations that support this file format
specification for import/eXPort may elect to support it as a native
format. This document recommends a TIFF file structure that is
compatible with low-memory and page-level streaming implementations.
Unless otherwise noted, the current TIFF specification [TIFF] and
selected TIFF Technical Notes [TTN1, TTN2] are the primary references
for describing TIFF and defining TIFF fields. This document is the
primary reference for defining TIFF field values for fax
applications.
1.2 Approach
The basic approach to using TIFF for facsimile data is to insert the
compressed fax image data in a TIFF file and use TIFF fields to
encode the parameters that describe the image data. These fields will
have values that comply with the ITU-T Recommendations. The MIME
content type of the resulting file will be image/tiff, with an
optional Application parameter [TIFF-REG]; see Section 9.
This approach 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. 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. 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.
1.3 Overview of this draft
Section 2 gives an overview of TIFF. Section 2.1 describes the
structure of TIFF files, including general guidelines for structuring
multi-page TIFF files. Section 2.2 lists the TIFF fields that are
required or recommended for all fax modes. The TIFF fields used only
by specific fax modes are described in Sections 3-8, which describe
the individual fax modes. These sections also specify the ITU-
compatible field values (image parameters) for each mode.
The full set of permitted fields of TIFF for facsimile are included
in the current TIFF specification, Section 2 of this document and the
sections on specific modes of facsimile operation. This document
defines profiles of TIFF for facsimile, where a profile is a subset
of the full set of permitted fields and field values of TIFF for
facsimile.
Section 3 defines the minimal black-and-white facsimile mode (Profile
S), which is required in all implementations. Section 4 defines the
extended black-and-white fax mode (Profile F), which provides a
standard definition of TIFF-F. Section 5 describes the lossless
black-and-white mode using JBIG compression (Profile J). Section 6
defines the base color mode, required in all color implementations,
for the lossy JPEG representation of color and grayscale facsimile
data (Profile C). Section 7 defines the lossless JBIG color and
grayscale facsimile mode (Profile L) and Section 8 defines the Mixed
Raster Content facsimile mode (Profile M). Each of these sections
concludes with a table summarizing the required and recommended
fields for each mode and the values they can have.
Section 9 describes the MIME content type image/tiff and the use of
the optional Application parameter in connection with TIFF for
facsimile. Sections 10, 11, 12 and 13 give Security Considerations,
the ISOC Copyright Notice, References and Authors" Addresses. Annex A
gives a summary of the TIFF fields used or defined in this document
and provides a convenient reference for implementors.
To implement only the minimal interchange black-and-white set of
fields and values (Profile S), one need read only Sections 1, 2, 3, 9
and 10.
The following tree diagram shows the relationship among profiles and
between profiles and coding methods.
S (MH)
/
B&W / Color
------------ ----------
/
/ F (MMR, MR) C (JPEG)
/ /
J (JBIG) ----
/
L (JBIG)

M (MRC)
A profile is based on a collection of ITU-T facsimile coding methods.
For example, Profile S, the minimal mode, is based on Modified
Huffman (MH) compression, which are defined in ITU-T Rec. T.4.
Profile F specifies Modified Read (MR) and Modified Modified Read
(MMR) compressions, which are defined in ITU-T Rec. T.4 and T.6.
All implementations of TIFF for facsimile MUST implement Profile S,
which is the root node of the tree. All color implementations of TIFF
for facsimile MUST implement Profile C. The implementation of a
particular profile MUST also implement those profiles on the path
that connect it to the root node, and MAY optionally implement
profiles not on the path connecting it to the root node. For example,
an implementation of Profile M must also implement Profiles C and S,
and may optionally implement Profile F, J or L. For another example,
an implementation of Profile C must also implement Profile S, and may
optionally implement Profile F or J.
The key Words "MUST", "MUST NOT", "REQUIRED", "SHALL", " NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [REQ].
2. TIFF and Fax
2.1. TIFF Overview
TIFF provides a means for describing, storing and interchanging
raster image data. A primary goal of TIFF is to provide a rich
environment within which applications can exchange image data. The
current TIFF specification [TIFF] defines a commonly used, core set
of TIFF fields known as Baseline TIFF. The current specification and
TIFF Technical Notes 1 and 2 [TTN1, TTN2] define several TIFF
extensions. The TIFF- based specification for fax applications uses a
subset of Baseline TIFF fields, with selected extensions, as
described in this document. In a few cases, this document defines new
TIFF fields specifically for fax applications.
2.1.1. File Structure
TIFF is designed for raster images, which makes it a good match for
facsimile documents, which are multi-page raster images. Each raster
image consists of a number of rows or scanlines, each of which has
the same number of pixels, the unit of sampling. Each pixel has at
least one sample or component (exactly one for black-and-white
images).
A TIFF file begins with an 8-byte image file header. The first two
bytes describe the byte order used within the file. Legal values are
"II" (0x4949) when bytes are ordered from least to most significant
(little- endian), and "MM" (0x4D4D), when bytes are ordered from most
to least significant (big-endian) within a 16- or 32-bit integer.
Either byte order can be used, except in the case of the minimal
black-and-white mode, which SHALL use value "II". The next two bytes
contain the value 42 that identifies the file as a TIFF file and is
ordered according to the value in the first two bytes of the header.
The last four bytes give the offset that points to the first image
file Directory (IFD). This and all other offsets in a TIFF file are
with respect to the beginning of the TIFF file. An IFD can be at any
location in the file after the header but must begin on a word
boundary.
An IFD is a sequence of tagged fields, sorted in ascending order by
tag value. An IFD consists of a 2-byte count of the number of fields,
a sequence of field entries and a 4-byte offset to the next IFD. The
fields contain information about the image and pointers to the image
data. Each separate raster image in the file is represented by an
IFD.
Each field entry in an IFD has 12 bytes and consists of a 2-byte Tag,
2 bytes identifying the field type (e.g. short, long, rational,
ASCII), 4 bytes giving the count (number of values or offsets), and 4
bytes that either contain the offset to a field value stored outside
the IFD, or, based on the type and count, the field value itself.
Resolution and metadata such as dates, names and descriptions are
examples of "long" field values that do not fit in 4 bytes and
therefore use offsets in the field entry. Details are given in the
TIFF specification [TIFF].
A TIFF file can contain more than one IFD, where each IFD is a
subfile whose type is given in the NewSubfileType field. Multiple
IFDs can be organized either as a linked list, with the last entry in
each IFD pointing to the next IFD (the pointer in the last IFD is 0),
or as a tree, using the SubIFDs field in the primary IFD [TTN1]. The
SubIFDs field contains an array of pointers to child IFDs of the
primary IFD.
Child IFDs describe related images, such as reduced resolution
versions of the primary IFD image. The same IFD can point both to a
next IFD and to child IFDs, and child IFDs can themselves point to
other IFDs.
All fax modes represent a multi-page fax image as a linked list of
IFDs, with a NewSubfileType field containing a bit that identifies
the IFD as one page of a multi-page document. Each IFD has a
PageNumber field, identifying the page number in ascending order,
starting at 0 for the first page. While a Baseline TIFF reader is not
required to read any IFDs beyond the first, an implementation that
reads the files that comply with this specification SHALL read
multiple IFDs. Only the Mixed Raster Content fax mode, described in
Section 8, requires the use of child IFDs.
The following figure illustrates the structure of a multi-page TIFF
file.
+-----------------------+
Header ------------+
+-----------------------+ First IFD
IFD (page 0) <-----------+ Offset
+--- ------------+
Value +-----------------------+
Offset +--> Long Values --+
+----------------------- Strip
Image Data <-+ Offset
strip 1 page 0
+-----------------------+
: :

+-----------------------+ Next IFD
IFD (page 1) <-----------+ Offset
+--- ------------+
Value +-----------------------+
Offset +--> Long Values --+
+----------------------- Strip
Image Data <-+ Offset
strip 1 page 1
+-----------------------+
strip 2 page 1 <-+
+-----------------------+
: :

+-----------------------+ Next IFD
IFD (page 2) <-----------+ Offset
:
2.1.2 Image Structure
An IFD stores an image as one or more strips, as shown in the
preceding figure. A strip consists of 1 or more scanlines (rows) of
raster image data in compressed form. An image may be stored in a
single strip or may be divided into several strips, which would
require less memory to buffer. (Baseline TIFF recommends about 8k
bytes per strip, but existing fax usage is typically one strip per
image.)
Each IFD requires three strip-related fields: StripOffsets,
RowsPerStrip and StripByteCounts. The StripOffsets field is an array
of pointers to the strip or strips that contain the actual image
data. The StripByteCounts field gives the number of bytes in each
strip after compression. TIFF requires that each strip, except the
last, contain the same number of scanlines, which is given in the
RowsPerStrip field. This document introduces the new StripRowCounts
field that allows a variable number of scanlines per strip, which is
required by the Mixed Raster Content fax mode (Section 8).
Image data is stored as uninterpreted, compressed image data streams
within a strip. The formats of these streams follow the ITU-T
Recommendations. The Compression field in the IFD indicates the type
of compression, and other TIFF fields in the IFD describe image
attributes, such as color encoding and spatial resolution.
Compression parameters are stored in the compressed data stream,
rather than in TIFF fields. This makes the TIFF representation and
compressed data format specification independent of each another.
This approach, modeled on [TTN2], allows TIFF to gracefully add new
compression schemes as they become available.
Some attributes can be specified both in the compressed data stream
and within a TIFF field. It is possible that the two values will
differ. When this happens for values required to interpret the data
stream, then the values in the data stream take precedence. For
informational values that are not required to interpret the data
stream, such as author name, then the TIFF field value takes
precedence.
2.1.3 TIFF File Structure for Fax Applications
The TIFF specification has a very flexible file structure, which does
not specify the ordering of IFDs, field values and image data in a
file. Individual applications may require or recommend an ordering.
This specification recommends that when using a TIFF file for
facsimile, A multi-page fax document SHOULD be represented as a
linked list of IFDs. It also recommends that a TIFF file for
facsimile SHOULD order pages in a TIFF file in the same way that they
are ordered in a fax data stream. In a TIFF file, a page consists of
several elements: one or more IFDs (including subIFDs), long field
values that are stored outside the IFDs, and image data (in one or
more strips).
The minimal black-and-white mode (Profile S) specifies a required
ordering of pages and elements within a page (Section 3.5). The
extended black-and-white mode (Profile F) provides guidelines for
ordering pages and page elements (Section 4.4.6). Other profiles
SHOULD follow these guidelines. This recommendation is intended to
simplify the implementation of TIFF writers and readers in fax
applications and the conversion between TIFF file and fax data stream
representations. However, for interchange robustness, readers SHOULD
be prepared to read TIFF files whose structure is consistent with
[TIFF], which supports a more flexible file structure than is
recommended here.
This specification introduces an optional new GlobalParametersIFD
field, defined in Section 2.2.4. This field has type IFD and
indicates parameters describing the fax session. While it is often
possible to oBTain these parameters by scanning the file, it is
convenient to make them available together in one place for fast and
easy Access. If the GlobalParametersIFD occurs in a TIFF file, it
SHOULD be located in the first IFD, immediately following the 8-byte
image file header.
2.2 TIFF Fields for All Fax Applications
The TIFF specification [TIFF] is organized as a baseline set and
several extensions, including technical notes [TTN1, TTN2] that will
be incorporated in the next release of TIFF. The baseline and
extensions have required and optional fields.
Facsimile applications require (and recommend) a mixture of baseline
and extensions fields, as well as some new fields that are not part
of the TIFF specification and that are defined in this document. This
sub- section lists the fields that are required or recommended for
all modes. In particular, Section 2.2.1 lists the fields that are
required by all modes and that have values that do not depend on the
mode. Section 2.2.2 lists the fields that are required by all modes
and that have values which do depend on the mode. Section 2.2.3 lists
the fields that are recommended for all modes. Fields that are
required or recommended by some but not all modes are given in the
section (Section 3-8) that describes that mode. The sections for each
fax mode have sub-sections for required and recommended fields; each
sub-section organizes the fields according to whether they are
baseline, extension or new.
The fields required for facsimile have only a few legal values,
specified in the ITU-T Recommendations. Of these legal values, some
are required and some are optional, just as they are required
(mandatory) or optional in fax implementations that conform to the
ITU-T Recommendations. The required and optional values are noted in
the sections on the different fax modes.
This section describes the fields required or recommended by all fax
modes. The pattern for the description of TIFF fields in this draft
is:
FieldName(TagValueInDecimal) = allowable values. TYPE
WhetherRequiredByTIFForTIFFforFAX
Count = (omitted if =1) = (if not in current spec but available)
Explanation of the field, how it"s used, and the values it can have.
Default value, if any, as specified in [TIFF]
When a field"s default value is the desired value, that field may be
omitted from the relevant IFD unless specifically required by the
text of this specification.
2.2.1. TIFF fields required for all fax modes
The TIFF fields listed in this section SHALL be used by all fax
modes, but have field values that are not specified by the ITU
standards, i.e. the fields do not depend on the mode. The next sub-
section lists the fields that SHALL be used by all fax modes, but
which do have values specified by the ITU-specified or mode-specific
values. Fields that SHALL be used by some but not all modes are given
in the sections (3-8) which describe the modes that uses them.
ImageLength(257) SHORT or LONG
RequiredByTIFFBaseline
Total number of scanlines in image.
No default, must be specified.
PageNumber(297) SHORT
RequiredByTIFFforFAX, TIFFExtension
Count = 2
The first number represents the page number (0 for the first page);
the second number is the total number of pages in the document. If
the second value is 0, then the total page count is not available.
No default, must be specified
RowsPerStrip(278) SHORT or LONG
RequiredByTIFFBaseline
The number of scanlines per TIFF strip, except for the last strip.
For a single strip image, this is the same as the value of the
ImageLength field.
Default = 2**32 - 1 (meaning all scanlines in one strip)
StripByteCounts(279) SHORT or LONG
RequiredByTIFFBaseline
Count = number of strips
For each strip, the number of bytes in that strip after compression.
No default, must be specified.
StripOffsets(273) SHORT or LONG
RequiredByTIFFBaseline
Count = number of strips
For each strip, the byte offset from the beginning of the file to
the start of that strip.
No default, must be specified.
2.2.2 Additional TIFF fields required for all fax modes
The TIFF fields listed in this section SHALL be used by all fax
modes, but the values associated with them depend on the mode being
described and the associated ITU Recommendations. Therefore, only the
fields are defined here; the values applicable to a particular fax
mode are described in Sections 3-8. Fields that SHALL be used by some
but not all modes are given in the section (3-8) describing the mode
that uses them.
BitsPerSample(258) SHORT
RequiredByTIFFBaseline
Number of bits per image sample
Default = 1 (field may be omitted if this is the value)
Compression(259) SHORT
RequiredByTIFFBaseline
Compression method used for image data
Default = 1 (no compression, so may not be omitted for FAX)
FillOrder(266) SHORT
RequiredByTIFFforFax
The default bit order in Baseline TIFF per [TIFF] is indicated by
FillOrder=1, where bits are not reversed before being stored.
However, TIFF for Fax typically utilizes the setting of FillOrder=2,
where the bit order within bytes is reversed before storage (i.e.,
bits are stored with the Least Significant Bit first).
Default = 1 (field may be omitted if this is the value)
Facsimile data appears on the phone line in bit-reversed order
relative to its description in the relevant ITU compression
Recommendation. Therefore, a wide majority of facsimile
implementations choose this natural order for storage. Nevertheless,
all readers conforming to this specification must be able to read
data in both bit orders.
ImageWidth(256) SHORT or LONG
RequiredByTIFFBaseline
The number of pixels (columns) per scanline (row) of the image
No default, must be specified.
NewSubFileType(254) LONG
RequiredByTIFFforFAX
A general indication of the kind of data contained in this IFD
Bit 1 is 1 if the image is a single page of a multi-page document.
Default = 0 (no subfile bits on, so may not be omitted for FAX)
PhotometricInterpretation(262) SHORT
RequiredByTIFFBaseline
The color space of the image data
No default, must be specified
ResolutionUnit(296) SHORT
RequiredByTIFFBaseline
The unit of measure for resolution. 2 = inch, 3 = centimeter;
Default = 2 (field may be omitted if this is the value)
SamplesPerPixel(277) SHORT
RequiredByTIFFBaseline
The number of color components per pixel; SamplesPerPixel is 1 for a
black-and-white, grayscale or indexed (palette) image.
Default =1 (field may be omitted if this is the value)
XResolution(282) RATIONAL
RequiredByTIFFBaseline
The horizontal resolution of the image in pixels per resolution
unit. The ITU-T Recommendations for facsimile specify a small number
of horizontal resolutions: 100, 200, 300, 400 pixels per inch, and
80, 160 pixels per centimeter (or 204, 408 pixels per inch). The
allowed XResolution values for each mode are given in the section
defining that mode. Per [T.4], it is permissible for applications to
treat the following XResolution values as being equivalent: <204,
200> and <400,408> in pixels/inch. These equivalencies were allowed
by [T.4] to permit conversions between inch and metric based
facsimile terminals.
TIFF for Facsimile Writers SHOULD express XResolution in inch based
units, for consistency with historical practice and to maximize
interoperability. See the table below for information on how to
convert from an ITU-T metric value to its inch based equivalent
resolution.
No default, must be specified
YResolution(283) RATIONAL
RequiredByTIFFBaseline
The vertical resolution of the image in pixels per resolution unit.
The ITU-T Recommendations for facsimile specify a small number of
vertical resolutions: 100, 200, 300, 400 pixels per inch, and 38.5,
77, 154 pixels per centimeter (or 98, 196, 391 pixels per inch). The
allowed YResolution values for each mode are given in the section
defining that mode. Per [T.4], it is permissible for applications to
treat the following YResolution values as being equivalent: <98,
100>, <196, 200>, and <391, 400> in pixels/inch. These equivalencies
were allowed by [T.4] to permit conversions between inch and metric
based facsimile terminals. TIFF for Facsimile Writers SHOULD express
YResolution in inch based units, for consistency with historical
practice and to maximize interoperability. See the table below for
information on how to convert from an ITU-T metric value to its inch
based equivalent resolution. No default, must be specified
+-----------------------------+-----------------------------+
XResolution YResolution
+--------------+--------------+--------------+--------------+
ResolutionUnitResolutionUnitResolutionUnitResolutionUnit
=2 (inch) =3 (cm) =2 (inch) =3 (cm)
+--------------+--------------+--------------+--------------+
100 100
+--------------+--------------+--------------+--------------+
204 80 98 38.5
200 100
+--------------+--------------+--------------+--------------+
204 80 196 77
200 200
+--------------+--------------+--------------+--------------+
204 80 391 154
+--------------+--------------+--------------+--------------+
300 300
+--------------+--------------+--------------+--------------+
408 160 391 154
400 400
+--------------+--------------+--------------+--------------+
2.2.3 TIFF fields recommended for all fax modes
The TIFF fields listed in this section MAY be used by all fax modes.
However, Profile S writers (the minimal fax mode described in Section
3) SHOULD NOT use these fields. Recommended fields that are mode-
specific are described in Sections 3-8.
DateTime(306) ASCII
OptionalInTIFFBaseline
Date/time of image creation in 24-hour format "YYYY:MM:DD HH:MM:SS".
No default.
DocumentName(269) ASCII
OptionalInTIFFExtension(DocumentStorageAndRetrieval)
The name of the scanned document. This is a TIFF extension field,
not a Baseline TIFF field.
No default.
ImageDescription(270) ASCII
OptionalInTIFFBaseline
A string describing the contents of the image.
No default.
Orientation(274) = 1-8. SHORT
OptionalinTIFFBaseline
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
spec [TIFF] for further values; Baseline TIFF only requires value=1.
Default = 1.
Note: It is recommended that a writer that is aware of the
orientation will include this field to give a positive indication of
the orientation, even if the value is the default. If the
Orientation field is omitted, the reader SHALL assume a value of 1.
Software(305) ASCII
OptionalInTIFFBaseline
The optional name and release number of the software package that
created the image.
No default.
2.2.4 New TIFF fields recommended for fax modes
The new TIFF fields listed in this section MAY be used by all fax
modes, but their support is not expected for the minimal fax mode
described in Section 3. In addition, support for these new TIFF
fields has not been included in historical TIFF-F readers described
in Section 4 and [TIFF- F]. These fields describe "global" parameters
of the fax session that created the image data. They are optional,
not part of the current TIFF specification, and are defined in this
document.
The first new field, GlobalParametersIFD, is an IFD that contains
global parameters and is located in a Primary IFD.
GlobalParametersIFD (400) IFD
An IFD containing global parameters. It is recommended that a TIFF
writer place this field in the first IFD, where a TIFF reader would
find it quickly.
Each field in the GlobalParametersIFD is a TIFF field that is legal
in any IFD. Required baseline fields should not be located in the
GlobalParametersIFD, but should be in each image IFD. If a conflict
exists between fields in the GlobalParametersIFD and in the image
IFDs, then the data in the image IFD shall prevail.
Among the GlobalParametersIFD entries is a new ProfileType field
which generally describes information in this IFD and in the TIFF
file.
ProfileType(401) LONG
The type of image data stored in this IFD.
0 = Unspecified
1 = Group 3 fax
No default
The following new global fields are defined in this document as IFD
entries for use with fax applications.
FaxProfile(402) = 0 - 6. BYTE
The profile that applies to this file; a profile is subset of the
full set of permitted fields and field values of TIFF for facsimile.
The currently defined values are:
0: does not conform to a profile defined for TIFF for facsimile
1: minimal black & white lossless, Profile S
2: extended black & white lossless, Profile F
3: lossless JBIG black & white, Profile J
4: lossy color and grayscale, Profile C
5: lossless color and grayscale, Profile L
6: Mixed Raster Content, Profile M
CodingMethods(403) LONG
This field indicates which coding methods are used in the file. A
bit value of 1 indicates which of the following coding methods is
used:
Bit 0: unspecified compression,
Bit 1: 1-dimensional coding, ITU-T Rec. T.4 (MH - Modified Huffman),
Bit 2: 2-dimensional coding, ITU-T Rec. T.4 (MR - Modified Read),
Bit 3: 2-dimensional coding, ITU-T Rec. T.6 (MMR - Modified MR),
Bit 4: ITU-T Rec. T.82 coding, using ITU-T Rec. T.85 (JBIG),
Bit 5: ITU-T Rec. T.81 (Baseline JPEG),
Bit 6: ITU-T Rec. T.82 coding, using ITU-T Rec. T.43 (JBIG color),
Bits 7-31: reserved for future use
Note: There is a limit of 32 compression types to identify standard
compression methods.
VersionYear(404) BYTE
Count: 4
The year of the standard specified by the FaxProfile field, given as
4 characters, e.g. "1997"; used in lossy and lossless color modes.
ModeNumber (405) BYTE
The mode of the standard specified by the FaxProfile field. A
value of 0 indicates Mode 1.0; used in Mixed Raster Content mode.
3. Minimal Black-and-White Fax Mode
This section defines the minimal black-and-white subset of TIFF for
facsimile. This subset is designated Profile S. All implementations
of TIFF for facsimile SHALL support the minimal subset.
Black-and-white mode is the binary fax application most users are
familiar with today. This mode is appropriate for black-and-white
text and line art. Black-and-white mode is divided into two levels of
capability. This section describes the minimal interchange set of
TIFF fields that must be supported by all implementations in order to
assure that some form of image, albeit black-and-white, can be
interchanged. This minimum interchange set is a strict subset of the
fields and values defined for the extended black-and-white mode
(TIFF-F or Profile F) in Section 4, which describes extensions to the
minimal interchange set of fields that provide a richer set of
black-and-white capabilities.
3.1. Overview
The minimal interchange portion of the black-and-white facsimile mode
supports 1-dimensional Modified Huffman (MH) compression, with the
original Group 3 fax resolutions, commonly called "standard" and
"fine."
To assure interchange, this mode uses the minimal set of fields, with
a minimal set of values. There are no recommended fields in this
mode. Further, the TIFF file is required to be "little endian," which
means that the byte order value in the TIFF header is "II". This mode
defines a required ordering for the pages in a fax document and for
the IFDs and image data of a page. It also requires that a single
strip contain the image data for each page; see Section 3.5. The
image data may contain RTC sequences, as specified in Section 3.4.
3.2. Required TIFF Fields
Besides the fields listed in Section 2.2.1, the minimal black-and-
white fax mode requires the following fields. The fields listed in
Section 2.2.1 and the fields and fax-specific values specified in
this sub- section must be supported by all implementations.
3.2.1 Baseline fields
BitsPerSample(258) = 1. SHORT
RequiredByTIFFBaseline
Binary data only.
Default = 1 (field may be omitted if this is the value)
Compression(259) = 3. SHORT
RequiredByTIFFBaseline
3 = 1- or 2- dimensional coding.
The value 3 is a TIFF extension value [TIFF]. The T4Options field
must be specified and its value specifies that the data is encoded
using the Modified Huffman (MH) encoding of [T.4].
FillOrder(266) = 2. SHORT
RequiredByTIFFBaseline
2 = Least Significant Bit first
NOTE: Baseline TIFF readers are only required to support FillOrder =
1, where the lowest numbered pixel is stored in the MSB of the byte.
However, because many devices, such as modems, transmit the LSB first
when converting the data to serial form, it is common for black-and-
white fax products to use the second FillOrder =2, where the lowest
numbered pixel is stored in the LSB. Therefore, this value is
specified in the minimal black-and-white mode.
ImageWidth(256) = 1728. SHORT or LONG
RequiredByTIFFBaseline
This mode only supports a page width of 1728 pixels. This width
corresponds to North American Letter and Legal and to ISO A4 size
pages.
No default, must be specified.
NewSubFileType(254) = (Bit 1=1). LONG
RequiredByTIFFforFAX
Bit 1 is 1 if the image is a single page of a multi-page document.
Default = 0 (no subfile bits on, so may not be omitted for fax)
PhotometricInterpretation(262) = 0. SHORT
RequiredByTIFFBaseline
0 = pixel value 1 means black
No default, must be specified
ResolutionUnit(296) = 2. SHORT
RequiredByTIFFBaseline
The unit of measure for resolution. 2 = inch.
Default = 2 (field may be omitted if this is the value)
SamplesPerPixel(277) = 1. SHORT
RequiredByTIFFBaseline
The number of components per pixel; 1 for black-and-white
Default =1 (field may be omitted if this is the value)
XResolution(282) = 200, 204. RATIONAL
RequiredByTIFFBaseline
The horizontal resolution of the image is expressed in pixels per
resolution unit. In pixels/inch, the allowed values are 200 and 204,
which may be treated as equivalent. See Section 2.2.2 for inch-
metric equivalency.
No default, must be specified
YResolution(283) = 98, 100, 196, 200. RATIONAL
RequiredByTIFFBaseline
The vertical resolution of the image is expressed in pixels per
resolution unit. In pixels/inch, the allowed values are 98, 100,
196 and 200; 98 and 100 may be treated as equivalent, and 196 and
200 may be treated as equivalent. See Section 2.2.2 for inch-metric
equivalency.
No default, must be specified
3.2.2 Extension fields
T4Options(292) = (Bit 0 = 0, Bit 1 = 0, Bit 2 = 0, 1) LONG
RequiredTIFFExtension (when Compression = 3)
Bit 0 = 0 indicates MH encoding.
Bit 1 must be 0
Bit 2 = 1 indicates that EOLs are byte aligned, = 0 EOLs not byte
aligned
Default is all bits are 0 (applies when EOLs are not byte aligned)
Note: The T4Options field is required when the Compression field has
a value of 3. Bit 0 of this field specifies the encoding used (MH
only in this mode) and Bit 2 indicates whether the EOL codes are
byte-aligned or not. If they are byte aligned, then fill bits have
been added as necessary so that the End of Line (EOL) codes always
end on byte boundaries. See Section 3.4 for details.
3.2.3. New Fields
None.
3.3. Recommended TIFF Fields
None.
3.4. End of Line (EOL) and Return to Control (RTC)
The handling of End of Line (EOL) codes and Return to Control (RTC)
sequences illustrate the differences between conventional fax, which
is bit and stream oriented, and TIFF, which is byte and file
oriented. Conventional fax, Baseline TIFF and TIFF extensions for fax
all handle EOLs and RTCs differently.
In conventional fax, an MH-compressed fax data stream for a page
consists of the following sequence:
EOL, compressed data (first line), EOL, compressed data, ... ,
EOL, compressed data (last line), RTC (6 consecutive EOL codes)
Baseline TIFF does not use EOL codes or Return to Control (RTC)
sequences for MH-compressed data. However, the TIFF extension field
T4Options used in this specification for MH compression (Compression
= 3) requires EOLs.
Furthermore, Bit 2 in the T4Options field indicates whether or not
the EOL codes are byte aligned. If Bit 2 = 1, indicating the EOL
codes are byte aligned, then fill bits have been added as necessary
before EOL codes so that an EOL code always ends on a byte boundary,
and the first bit of data following an EOL begins on a byte boundary.
Without fill bits, an EOL code may end in the middle of a byte. 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). Not all TIFF readers
historically used for fax are able to deal with non-byte aligned
data.
While TIFF extension requires EOL codes, TIFF in fax applications has
traditionally prohibited RTC sequences. Implementations that want
common processing and interfaces for fax data streams and Internet
fax files would prefer that the TIFF data include RTC sequences.
To reconcile these differences, RTCs are allowed in cases where EOL
codes are not byte aligned and no fill bits have been added to the
data. This corresponds to situations where the fax data is simply
inserted in a strip without being processed or interpreted. RTCs
should not occur in the data when EOLs have been byte aligned. This
is formally specified in the next sub-section.
3.4.1. RTC Exclusion
Implementations which wish to maintain strict conformance with TIFF
and compatibility with the historical use of TIFF for fax SHOULD NOT
include the RTC sequence when writing TIFF files. However,
implementations which need to support "transparency" of T.4-generated
image data MAY include RTCs when writing TIFF files if the flag
settings of the T4Options field are set for non-byte aligned data,
i.e. Bit 2 is 0. Implementors of TIFF readers should be aware that
there are some existing TIFF implementations for fax that include the
RTC sequence in MH image data. Therefore, minimal set readers MUST be
able to process files which do not include RTCs and SHOULD be able to
process files which do include RTCs.
3.5. File Structure
The TIFF header, described in Section 2.1.1, contains two bytes which
describe the byte order used within the file. For the minimal black-
and- white mode, these bytes SHALL have the value "II" (0x4949),
denoting that the bytes in the TIFF file are in LSByte-first order
(little- endian). The first or 0th IFD immediately follows the
header, so that offset to the first IFD is 8. The headers values are
shown in the following table:
+--------+-------------------+--------+-----------+
Offset Description Value
+--------+-------------------+--------+-----------+
0 Byte Order 0x4949 (II)
+--------+-------------------+--------+-----------+
2 Identifier 42 decimal
+--------+-------------------+--------+-----------+
4 Offset of 0th IFD 0x 0000 0008
+--------+-------------------+--------+-----------+
The minimal black-and-white mode SHALL order IFDs and image data
within a file as follows: 1) there SHALL be an IFD for each page in a
multi- page fax document; (2) the IFDs SHALL occur in the same order
in the file as the pages occur in the document; (3) the IFD SHALL
precede the image data to which it has offsets; (4) the image data
SHALL occur in the same order in the file as the pages occur in the
document; (5) the IFD, the value data and the image data it has
offsets to SHALL precede the next image IFD; and (6) the image data
for each page SHALL be contained within a single strip.
As a result of (6), the StripOffsets field will contain the pointer
to the image data. With two exceptions, the field entries in the IFD
contain the field values instead of offsets to field values located
outside the IFD. The two exceptions are the values for the
XResolution and YResolution fields, both of which are type RATIONAL
and require 2 4- byte numbers. These "long" field values SHALL be
placed immediately after the IFD which contains the offsets to them,
and before the image data pointed to by that IFD.
The effect of these requirements is that the IFD for the first page
SHALL come first in the file after the TIFF header, followed by the
long field values for XResolution and YResolution, followed by the
image data for the first page, then the IFD for second page, etc.
This is shown in the following figure. Each IFD is required to have a
PageNumber field, which has value 0 for the first page, 1 for the
second page, and so on.
+-----------------------+
Header ------------+
+-----------------------+ First IFD
IFD (page 0) <----------+ Offset
+--- ------------+
--+
Value +-----------------------+
Offset +--> Long Values
+----------------------- Strip
Image Data (page 0) <-+ Offset
+-----------------------+ Next IFD
IFD (page 1) <----------+ Offset
+--- ------------+
--+
Value +-----------------------+
Offset +--> Long Values
+----------------------- Strip
Image Data (page 1) <-+ Offset
+-----------------------+ Next IFD
IFD (page 2) <----------+ Offset
+-----------------------+
:
Using this file structure may reduce the memory requirements in
implementations. It is also provides some support for streaming, in
which a file can be processed as it is received and before the entire
file is received.
3.6 Minimal Black-and-white Mode Summary
The table below summarizes the TIFF fields that comprise the minimal
interchange set for black-and-white facsimile. The Baseline and
Extension fields and field values MUST be supported by all
implementations. For convenience in the table, certain fields which
have a value that is a sequence of flag bits are shown taking integer
values that correspond to the flags that are set. An implementation
should test the setting of the relevant flag bits individually,
however, to allow extensions to the sequence of flag bits to be
appropriately ignored. (See, for example, T4Options below.)
+---------------------------+--------------------------------+
Baseline Fields Values
+---------------------------+--------------------------------+
BitsPerSample 1
+---------------------------+--------------------------------+
Compression 3: 1D Modified Huffman coding
set T4Options = 0 or 4
+------------------------------------------------------------+
+---------------------------+--------------------------------+
FillOrder 2: least significant bit first
+---------------------------+--------------------------------+
ImageWidth 1728
+---------------------------+--------------------------------+
ImageLength n: total number of scanlines
in image
+---------------------------+--------------------------------+
NewSubFileType 2: Bit 1 identifies single
page of a multi-page document
+---------------------------+--------------------------------+
PageNumber n,m: page number n followed by
total page count m
+---------------------------+--------------------------------+
PhotometricInterpretation 0: pixel value 1 means black
+---------------------------+--------------------------------+
ResolutionUnit 2: inch
+---------------------------+--------------------------------+
RowsPerStrip number of scanlines per strip
= ImageLength, with one strip
+---------------------------+--------------------------------+
SamplesPerPixel 1
+---------------------------+--------------------------------+
StripByteCounts number of bytes in TIFF strip
+---------------------------+--------------------------------+
StripOffsets offset from beginning of
file to single TIFF strip
+---------------------------+--------------------------------+
XResolution 204, 200 (pixels/inch)
+---------------------------+--------------------------------+
YResolution 98, 196, 100, 200 (pixels/inch)
+---------------------------+--------------------------------+
Extension Fields
+---------------------------+--------------------------------+
T4Options 0: MH coding, EOLs not byte
aligned
4: MH coding, EOLs byte aligned
+---------------------------+--------------------------------+
4. Extended Black-and-White fax mode
This section defines the extended black-and-white mode or Profile F
of TIFF for facsimile. It provides a standard definition of what has
historically been known as TIFF Class F and now TIFF-F. In doing so,
it aligns this mode with current ITU-T Recommendations for black-
and-white fax and with existing industry practice. Implementations of
this profile include implementations of Profile S.
This section describes extensions to the minimal interchange set of
fields (Profile S) that provide a richer set of black-and-white
capabilities. The fields and values described in this section are a
superset of the fields and values defined for the minimal interchange
set in Section 3. In addition to the MH encoding, Modified READ (MR)
and Modified Modified READ (MMR) encoding as described in [T.4] and
[T.6] are supported.
Section 4.1 gives an overview of TIFF-F. Section 4.2 describes the
TIFF fields that SHALL be used in this mode. Section 4.3 describes
the fields that MAY be used in this mode. In the spirit of the
original TIFF-F specification, Sections 4.4 and 4.5 discuss technical
implementation issues and warnings. Section 4.6 gives an example use
of TIFF-F. Section 4.7 gives a summary of the required and
recommended fields and their values.
4.1 TIFF-F Overview
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 of the F application of [TIFF] for Internet applications.
For ease of reference, the term TIFF-F will be used throughout this
document as a shorthand for the extended black-and-white mode or
profile of TIFF for facsimile.
Up until the TIFF 6.0 specification, TIFF supported various "Classes"
which defined the use of TIFF for various applications. Classes were
used to support specific applications. In this spirit, TIFF-F has
been known historically as "TIFF Class F". Previous informal TIFF-F
documents [TIFF-F0] used the "Class F" terminology. As of TIFF 6.0
[TIFF], the TIFF Class concept has been eliminated in favor of the
concept of Baseline TIFF. Therefore, this document updates the
definition of TIFF-F as the F profile of TIFF for facsimile, by using
Baseline TIFF as defined in [TIFF] as the starting point and then
adding the TIFF extensions to Baseline TIFF which apply for TIFF-F.
In almost all cases, the resulting definition of TIFF-F fields and
values remains 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.4] and [T.30] fax recommendations, these differences are noted.
4.2. Required TIFF Fields
This section lists the required fields and the values they must have
to be ITU-compatible. Besides the fields listed in Section 2.2.1, the
extended black-and-white fax mode SHALL use the following fields.
4.2.1. Baseline fields
BitsPerSample(258) = 1. SHORT
RequiredByTIFFBaseline
Binary data only.
Default = 1 (field may be omitted if this is the value)
Compression(259) = 3, 4. SHORT
RequiredByTIFFBaseline
3 = 1- or 2- dimensional coding, must have T4Options field This is
a TIFF Extension value [TIFF].
4 = 2-dimensional coding, ITU-T Rec. T.6 (MMR - Modified Modified
Read, must have T6Options field)) This is a TIFF Extension value.
Default = 1 (and is not applicable; field must be specified)
NOTE: Baseline TIFF permits use of value 2 for Modified Huffman
encoding, but data is presented in a form which does not use EOLs,
and so TIFF for facsimile uses Compression=3 instead. See Sections
4.4.4, 4.5.1 and 4.5.2 for more information on compression and
encoding.
FillOrder(266) = 1 , 2. SHORT
RequiredByTIFFBaseline
Profile 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
ImageWidth(256) SHORT or LONG
RequiredByTIFFBaseline
This mode supports the following fixed page widths: 1728, 2592, 3456
(corresponding to North American Letter and Legal, ISO A4 paper
sizes), 2048, 3072, 4096 (corresponding to ISO B4 paper size), and
2432, 3648, 4864 (corresponding to ISO A3 paper size).
No default; must be specified
NOTE: 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 4.5.2 for more information on inch/metric equivalencies and
other implementation details.
NewSubFileType(254) = (Bit 1=1). LONG
RequiredByTIFFforFAX
Bit 1 is 1 if the image is a single page of a multi-page document.
Default = 0 (no subfile bits on, so may not be omitted for fax)
NOTE: Bit 1 is always set to 1 for TIFF-F, indicating a single page
of a multi-page image. The same bit settings are used when TIFF-F is
used for a one page fax image. See Section 4.4.3 for details on
multi-page files.
PhotometricInterpretation(262) = 0, 1. SHORT
RequiredByTIFFBaseline
0 = pixel value 1 means black, 1 = pixel value 1 means white.
This field allows notation of an inverted or negative image.
No default, must be specified
ResolutionUnit(296) = 2, 3. SHORT
RequiredByTIFFBaseline
The unit of measure for resolution. 2 = inch, 3 = centimeter; TIFF-F
has traditionally used inch-based measures.
Default = 2 (field may be omitted if this is the value)
SamplesPerPixel(277) = 1. SHORT
RequiredByTIFFBaseline
1 = monochrome, bilevel in this case (see BitsPerSample)
Default =1 (field may be omitted if this is the value)
XResolution(282) = 200, 204, 300, 400, 408 RATIONAL
RequiredByTIFFBaseline
The horizontal resolution of the image is expressed in pixels per
resolution unit. In pixels/inch, the allowed values are: 200, 204,
300, 400, and 408. See Section 2.2.2 for inch-metric equivalency.
No default, must be specified
NOTE: The values of 200 and 408 have been added to the historical
TIFF-F values, for consistency with [T.30]. Some existing TIFF-F
implementations may also support values of 80 pixels/cm, which is
equivalent to 204 pixels per inch. See section 4.5.2 for information
on implementation details.
YResolution(283) = 98, 100, 196, 200, 300, 391, and 400 RATIONAL
RequiredByTIFFBaseline
The vertical resolution of the image is expressed in pixels per
resolution unit. In pixels/inch, the allowed values are: 98, 100,
196, 200, 300, 391, and 400 pixels/inch.
See Section 2.2.2 for inch-metric equivalency.
No default, must be specified
NOTE: The values of 100, 200, and 391 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 and 38.5 (cm),
which are equivalent to 196 and 98 pixels per inch respectively. See
section 4.5.2 for more information on implementation details.
NOTE: Not all combinations of XResolution, YResolution and ImageWidth
are legal. The following table gives the legal combinations and
corresponding paper size [T.30].
+--------------+-----------------+---------------------------+
XResolution x YResolution ImageW