Network Working Group Glenn Parsons
Internet Draft James Rafferty
Expires in six months September 22, 1997
Tag Image File Format (TIFF) - Application F
<draft-ietf-fax-tiff-04.txt>
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Overview
This document describes in detail the definition of TIFF-F that is
used to store facsimile images. 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 September 22, 1997
1. Abstract
This document references the Tag Image File Format (TIFF) to
formally define the Application F of TIFF (TIFF-F) as a file format
that may be used to store facsimile images.
2. TIFF Definition
TIFF (Tag Image File Format) Revision 6.0 is defined in detail
within [TIFF].
A brief review of concepts used in TIFF is included in this
document as background information, but the reader is directed to
the original TIFF specification [TIFF] to obtain specific technical
details.
2.1 Baseline TIFF and Applications
TIFF provides a method 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. [TIFF] also defines a
commonly used, default subset of TIFF that is known as Baseline
TIFF. Applications of TIFF are defined by using Baseline TIFF as
a starting point and then defining "extensions" to TIFF that are
used for the specific "application", as well as specifying any
other differences from Baseline TIFF.
3. TIFF-F Definition
3.1 Introduction
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 "Application F of
TIFF".
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[REQ].
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3.1.1 TIFF-F Historical Background
Up until TIFF 6.0, TIFF supported various "Classes" which defined
the use of TIFF for various applications. Classes were used to
support specific applications and in this spirit, TIFF-F has been
known historically as "TIFF Class F". Previous informal TIFF-F
documents 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 application of TIFF, by
using Baseline TIFF as defined in [TIFF] as the starting point and
then defining the differences from 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.
3.1.2 Overview
The intent of this specification is be to document:
1) The fields and values which are applicable for the application
F of TIFF.
2) A minimum set of TIFF-F fields and values which should be able
to interwork with virtually all historic TIFF-F readers.
3) 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.4] and [T.30] recommendations.
The structure of the TIFF-F definition will be as follows. A brief
review of the structure of TIFF files and practical guidelines for
the writing and reading of multi-page TIFF-F files is provided in
sections 3.1.3 and 3.1.4
A 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. Section 3.6 defines the requirements for the minimum subset
of TIFF-F fields and values to maximize interoperability.
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Several technical topics, including implementation issues and
warnings 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 permitted set of values.
3.1.3 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 reading and writing TIFF-F files
are addressed in the following section 3.1.4. The structure for
writing "minimum subset" TIFF-F files is defined in section 3.6.2.
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
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 for implementors when writing multi-page TIFF-
F files can produce TIFF structures which are easier for readers to
process. This is especially for applications in environments such
as facsimile terminals where a complex file structure is difficult
to support.
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3.1.4 Practical Guidelines for Writing and Reading Multi-Page TIFF-F
Files
Traditionally, historical TIFF-F has required readers and writers
to be able to handle multi-page TIFF-F files. Based on the
experience of various TIFF-F implementors, it has been seen that
the implementation of TIFF-F can be greatly simplified if certain
practical guidelines are followed when writing multi-page TIFF-F
files.
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, the writer of TIFF-
F files SHOULD 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. TIFF-F readers SHOULD
expect IFDs to be presented in page order, but be able to handle
exceptions.
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 SHOULD be contained within a
single strip (i.e. one image strip per fax page). The use of a
single image strip per page is very useful for applications such as
store and forward messaging, where the file is usually prepared in
advance of the transmission, but other assumptions may apply for
the size of the image strip for applications which require the use
of "streaming" techniques. (see section 3.7.6) In the event a
different image strip size 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 the TIFF-F reader standpoint,
one image strip per page permits the image data to be found through
reference via a single offset, resulting in a much simplified image
structure and faster processing.
A third simplifying assumption is that each IFD SHOULD be placed in
the TIFF-F file structure at a point which precedes the image which
the IFD describes.
In addition, a fourth simplifying assumption for TIFF-F writers and
readers is to place the actual image data in a physical order
within the TIFF file structure which is consistent with the logical
page order. In practice, TIFF-F readers will need to use the strip
offsets to find the exact physical location of the image data,
whether or not it is presented in logical page order.
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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,
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.
In the field definitions which follow in this section and
subsequent sections, the fields will be presented in the following
form:
Fieldname (tag-number) = values (if applicable). TYPE
A brief summary of the Baseline TIFF fields and their use in TIFF-F
follows:
ImageWidth(256) = 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 391, 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.
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ImageLength (257). SHORT or LONG. LONG recommended.
The total number of scan lines in the image.
Compression (259) = 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.
PhotometricInterpretation (260) = 0,1. SHORT.
This field allows notation of an inverted ("negative") image:
0 = normal
1 = inverted
StripOffsets (273). 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 (278). 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 (279). 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 (296) = 2,3. SHORT.
The units of measure for resolution:
2 = Inch
3 = Centimeter
TIFF-F has traditionally used inch based measures.
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XResolution (282) = 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 (283) = 98, 196, 100, 200, 300, 391, 400 (inches).
RATIONAL.
The vertical resolution of the TIFF-F image expressed in pixels
per resolution unit. 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, 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. A review of the additional required
fields for TIFF-F follows:
BitsPerSample (258) = 1. SHORT.
Since TIFF-F is only used for black-and-white facsimile
images, the value is 1 (the default) for all files.
FillOrder (266) = 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 (254)= (Bit 1 = 1). 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. The same bit
settings are used when TIFF-F is used for a one page fax image.
See sections 3.1.1 and 3.1.2 for more details on the structure
of multi-page TIFF-F image files.
PageNumber (285). 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 0000/0001 hex. If PageNumber[1] is 0,
the total number of pages in the document is not available.
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SamplesPerPixel (277) = 1. SHORT.
The value of 1 denotes a bi-level, grayscale, or palette color
image.
There is also a requirement to include either the T4Options or the
T6Options field in a TIFF-F IFD, depending upon the setting of the
Compression field. These fields are defined in the next section on
TIFF extensions.
3.4 TIFF-F Extensions
These are fields which are extensions beyond the required TIFF-F
fields. The following fields have been defined as extensions in
[TIFF].
T4Options (292) (Bit 0 = 0 or 1, Bit 1 = 0, Bit 2 = 0 or 1). 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 = 0 for non-byte-aligned EOLs or 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 (293) = (Bit 0 = 0, Bit 1 = 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 MUST
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
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.
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In addition, three new fields, defined as TIFF-F extensions,
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
in writing TIFF-F files 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 (326). 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 (327). SHORT
N =
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
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.
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ConsecutiveBadFaxLines (328). 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 (326)[defined in 3.4]
CleanFaxData (327) [defined in 3.4]
ConsecutiveBadFaxLines (328) [defined in 3.4]
DateTime (306). 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.
DocumentName (269). ASCII.
This is the name of the document from which the document was
scanned.
ImageDescription (270). ASCII.
This is an ASCII string describing the contents of the image.
Orientation (274). 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 (305). ASCII.
The optional name and release number of the software package
that created the image.
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3.6 Requirements for TIFF-F Minimum Subset
This section defines the requirements for a minimum subset of TIFF-F
fields and values that all TIFF-F readers SHOULD support to maximize
interoperability with current and historical TIFF-F applications.
The TIFF-F structure for writing minimum subset files is also
defined.
3.6.1 Summary of Minimum Subset Fields and Values
A summary of the minimum subset TIFF-F fields and values is provided
in the following table. The required fields for the minimum subset
are shown under the column labeled "Field". The values for these
fields in the minimum subset are shown under the column labeled
"Minimum".
Field | Minimum | Comment
------------------|--------------|------------------------------
BitsPerSample | 1 |one bit per sample
Compression | 3 |3 for T.4 (MH)
FillOrder | 2 |LSB first
ImageWidth | 1728 |
ImageLength | |required
NewSubFileType | Bit 1 = 1 |single page of multipage file
PageNumber | X/X |pg/tot, 0 base, tot in 1st IFD
PhotometricInterp | 0 |0 is white
ResolutionUnit | 2 |inches (default)
RowsPerStrip |=ImageLength |
SamplesPerPixel | 1 |one sample per pixel
StripByteCounts | |required
StripOffsets | |required
T4Options | Bit 0 = 0 |MH
| Bit 1 = 0 |
| Bit 2 = 0,1 |Non-Byte-aligned,
| | Byte-Aligned EOLs
Xresolution | 204 |Units is per inch
Yresolution | 196,98 |Units is per inch
------------------|--------------|------------------------------
3.6.2 TIFF-F Minimum Subset File Structure
For applications which need to write minimum subset TIFF-F files,
the file structure shown in Figure 3.1 below SHOULD be used:
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+-----------------------+
| Header |
+-----------------------+
+---| IFD |----+
| +-----------------------+ |
+-> | Image Data | |
Data Offset | (page 1) | |
+-----------------------+ <--+
| IFD | Next IFD
+-----------------------+
| Image Data |
| (page 2) |
+-----------------------+
| : |
| : |
Figure 3.1 TIFF-F Minimum Subset File Structure
As depicted in Figure 3.1, the IFD of each page immediately precedes
the related Image Data for that page. For multiple page documents,
each IFD/image pair is immediately followed by the next IFD/image
pair in logical page order within the file structure, until all
pages have been defined.
The format for the TIFF Header is as defined in [TIFF]. When
writing TIFF-F minimum subset files, the value for the byte order in
the Header SHOULD be II (0x4949, denoting that the bytes in the TIFF
file are in LSB first(little-endian)order.
This results in a TIFF header whose content is as shown in Figure
3.2.
Offset | Description | Type | Value |
+--------+-------------------+--------+--------------------+
| 0 | Byte Order | Short | 0x4949 (II) |
+--------+-------------------+--------+--------------------+
| 2 | Version | Short | 42 |
+--------+-------------------+--------+--------------------+
| 4 | Offset of 0th IFD | Long | 0x 0000 0008 |
+--------+-------------------+--------+--------------------+
Figure 3.2: Image File Header for Minimum Subset TIFF-F Files
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3.7 Technical Implementation Issues
3.7.1 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. The
recommended approach for multi-page TIFF-F images is to represent
each page as a single strip.
3.7.2 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.
3.7.3. 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) as the
standard case.
3.7.4. 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
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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.
3.7.5. 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: MUST 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. Source image data stream is error-corrected or otherwise
guaranteed to be error-free such as for a computer generated
file: SHOULD NOT write page-quality fields.
3.7.6 Use of TIFF-F for Streaming Applications
TIFF-F has historically been used for handling fax image files in
applications such as store and forward messaging where the entire
size of the file is known in advance. While TIFF-F may also
possibly be used as a file format for cases such as streaming
applications, different assumptions may be required than those
provided in this document (e.g., the entire size and number of pages
within the image are not known in advance). As a result, a
definition for the streaming application of TIFF-F is beyond the
scope of this document.
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3.7.7 TIFF-F Export and Import
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 NewSubFileType and PageNumber (page one-
of-one) values for export purposes. However, there is no easy 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. The minimum subset of TIFF-F REQUIRES support for
one dimensional (Modified Huffman) files, so this choice maximizes
portability. However, implementers seeking greater efficiency
SHOULD use T.6 MMR compression when writing TIFF-F files. Some
TIFF-F readers will also support two-dimensional Modified READ
files. Implementers that wish to have the maximum flexibility in
reading TIFF-F files SHOULD support all three of these compression
methods (MH, MR and MMR).
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: 391v x 203h and 391v x 406h. Therefore, the full set
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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.
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
<391, 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
The historical convention for TIFF-F has been that all EOLs in
Modified Huffman or Modified READ data must be byte-aligned.
However, Baseline TIFF has permitted use of non-byte-aligned EOLs
by default, so that a large percentage of TIFF-F reader
implementations support both conventions. Therefore, the minimum
subset of TIFF-F as defined in this document includes support for
both byte-aligned and non-byte-aligned EOLs.
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.
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 sequence, 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
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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.
3.8.5 RTC Exclusion
Aside from EOLs, TIFF-F files have historically only contained
image data. This means that applications which wish to maintain
strict conformance with the rules in [TIFF] and compatibility with
historical TIFF-F, SHOULD NOT include the Return To Control
sequence (RTC) (consisting of 6 consecutive EOLs) when writing TIFF-
F files. However, applications which need to support
"transparency" of [T.4] image data MAY include RTCs if the flag
settings of the T4Options field are set for non-byte aligned MH or
MR image data. Implementors of TIFF readers should also be aware
that there are some existing TIFF-F implementations which include
the RTC sequence in MH/MR image data.
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 MUST include an "end-of-facsimile-block" (EOFB)
code at the end of each coded strip. Per [TIFF], the EOFB code is
followed by pad bits as needed to align on a byte boundary. TIFF
readers SHOULD ignore any bits other than pad bits beyond the EOFB.
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 the following table are specified for a TIFF-F
Reader. The range of values for required and recommended fields
are as shown. The minimum subset of values are also shown. 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. In the table, certain fields have a value that is a
sequence of flag bits (e.g. T4Options). An implementation should
test the setting of the relevant flag bits individually to allow
extensions to the sequence of flag bits to be appropriately
ignored.
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
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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)
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 |depends on XResolution
| 2432, 2592, | |
| 3072, 3648, | |
| 3456, 4096, | |
| 4864 | |
ImageLength | >0 | |required
NewSubFileType | Bit 1 = 1 | Bit 1 = 1 |single page of
| | | multipage file
Orientation * | 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 | Bit 0 = 0,1 | Bit 0 = 0 |MH,MR(incl if not MMR)
| Bit 1 = 0 | Bit 0 = 0 |
| Bit 0 = 0,1 | Bit 0 = 0,1 |Non-Byte-aligned and
| | | Byte-Aligned EOLs
T6Options | 0 | |MMR (incl only if MMR)
Xresolution | 204,200,300,| 204 |If unit is per inch
| 400,406, | |
| 77 | |If unit is per cm
Yresolution | 196,98,100, | 196,98 |If unit is per inch
| 200,300,391,| |
| 400, | |
| 77,38.5 | |If unit is per cm
------------------|-------------|-------------|----------------------
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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
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.
For the case of writing "minimum subset" TIFF-F files, the rules
defined in section 3.6 apply.
Informational fields that may be useful for TIFF-F files 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. Implementation Usage
4.1 Internet Fax Usage
The usage of TIFF-F is envisioned as a component of Internet Fax.
It is anticipated that Internet Fax may use both a TIFF-F Reader
and TIFF-F Writer. The details of the Internet Fax applications and
their use of TIFF-F will be specified in other documents.
4.2 VPIM Usage
The application F of TIFF (i.e. TIFF-F content) is a secondary
component of the VPIM Message as defined in [VPIM2]. Voice
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messaging systems can often handle fax store-and-forward
capabilities in addition to traditional voice message store-and-
forward functions. As a result, TIFF-F fax messages can optionally
be sent between compliant VPIM systems, and may be rejected if the
recipient system cannot deal with fax.
Refer to the VPIM Specification for proper usage of this content.
5. Security Considerations
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.
6. Authors' Addresses
Glenn W. Parsons
Northern Telecom
P.O. Box 3511, Station C
Ottawa, ON K1Y 4H7
Canada
Phone: +1-613-763-7582
Fax: +1-613-763-2697
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
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7. References
[MIME1] N. Freed and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, Innosoft, First Virtual, Nov 1996
[MIME4] N. Freed and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Four: Registration Procedures", RFC
2048, Innosoft, First Virtual, Nov 1996.
[REQ] S. Bradner, "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.
[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.
[VPIM2] G. Vaudreuil and G. Parsons, "Voice Profile for Internet
Mail - version 2", Work In Progress, <draft-ema-vpim-06.txt>,
July 1997.
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