1997.01.08 22:36 "Re: TIFF Class F", by Helge Blischke
Can anyone please let me know where I can find the specifications for TIFF Class F. The class is a sub-class of baseline TIFF and is used for facsimiles.
TIF Class F has, as far as I know, never become part of an official TIFF specification. It was discussed as an appendix to TIFF 5.0 but then incorporated in TIFF 6.0 specification.
As the TIFF-F draft may be of some general interest nevertheless, I looked it up in my dusty paper archive and OCRed it; it is attached to this message. Hope this will help to solve your problems.
H. Blischke <H.Blischke@srz-berlin.de>
October 1, 1991
TIFF Class F was defined in late 1989 by Joe Campbell of Everex
Systems, Inc. from the results of a poll of the facsimile industry.
The goal was to define a file format that is simultaneously
suitable for native use in Group 3 computer facsimile products, and
as a file interchange medium with the outside world. Since that
time, there have been only three minor revisions, mostly editorial in
nature. The revision history of the specification is at the end of
TIFF Class F defines a subset (a "Class") of existing TIFF
tags, necessary to support Group 3 facsimile data. In many cases,
the values and sizes of these tags are also defined. Three new
optional tags are also defined.
TIFF Classes reduce the information burden on TIFF readers and
writers that wish to support narrow applications. For example,
Appendix G-l of TIFF 5.0 states that classes enable TIFF readers
"to know when they can stop adding TIFF features." In other
words, defining a Class enables applications interested only in
reading that Class to give up if the characteristic tags and values
are not present. Therefore, TIFF Class F insists on a rather narrow
definition of tags. In a general TIFF file, for example, the writer
would be free to create single-page documents without the
NewSubFileType and PageNumber tags. Not so for a Class F file, where
the multi-page tag is required even for a single page.
TIFF Class B (Bilevel) is a sub-class of TIFF. That is, all tags
that are required in TIFF are also required in Class B. TIFF Class F
(Facsimile) is a sub-class of Class B tBilevel). That is, all
tags that are required in Class B are also required in Class F. For
some common tags, however, Class F limits the range of acceptable
values. The YResolution tag, for example, is a Class B tag, but its
Class F value is limited to either 98 or 196 dpi. Such tags are
listed in "Required Class F Tags."
Other Class B tags have a slightly eccentric meaning when applied to
facsimile images. These are discussed in the section "Bilevel
Required." There are also tags that may be helpful but are not
required. These are listed in the "Recommended Tags" section.
A brief list of all the tags required by TIFF Class F, grouped by
class, is in the section "Required Facsimile Tags Grouped By
Class." Finally, technical topics are discussed in the sections
"Technical Points" and "Warnings."
A machine-readable copy of this document can be downloaded from
the Aldus Forum on Compuserve. Type GO ALDUS and look through the
"Libraries" menu. Substantive questions about TIFF Class F can
be faxed to its author: Joe Campbell, Everex Systems, Inc:
(510) 540-5835 or (510) 841-5441, or via Compuserve Mail
TIFF CLASS F Revision 3: October 1, 1991 Page 1
TIFF Class F is a parallel but unrelated effort to EIA Project Number
2388, an industry standards group working to standardize facsimile
hardware. For information about this standard, contact the EIA.
Those wishing to participate in the revision and upkeep of TIFF Class
F should read the section "Revising the TIFF Class F
Specification," at the end of this document.
Group 3 facsimile is described in the "Blue Book," Volume VII,
Fascicle VII.3, Terminal Equipment and Protocols for Telematic
Services, The International Telegraph and Telephone Consultative
Committee (CCITT), Melbourne, 1988.
CLASS F REQUIRED
Compression = 3 or 4. SHORT.
3 Group 3, one-dimensional encoding with "byte-aligned"
EOL's. 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 l byte preceded by a zero nibble:
xxxx0000 00000001. The data in a Class F image is not
terminated with an RTC. Please see items 4 and 5 in
the "Warnings" section. For Group 3 two-dimensional
encoding, set bit 1 in Group30ptions. Please see item 2
in the "Warnings" section.
4 Group 4 two-dimensional encoding. MMR (Modified-
Modified READ) compression, formerly found only in
Group 4 facsimile, are now available in Group 3
devices. When this option is used, bits zero and one
in the Group30ptions tag are ignored.
FillOrder = 1, 2. SHORT. TIFF Class 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.
Group30ptions = 4,5. LONG. Data may be one- or two-
dimensional, but EOL's must be byte-aligned. Uncompressed
data is not allowed. When Group 4 compression is used,
bits zero and one in the Group30ptions tag should be
bit O O for 1-Dimensional, 1 for 2-Dimensional
bit 1 Must be O (uncompressed data not allowed)
TIFF CLASS F Revision 3: October 1, l991 Page 2
bit 2 1 for byte-aligned EOL's
ImageWidth = 864, 1216, 1728, 2048, 2432. SHORT or LONG. LONG
recommended. These are the fixed page widths in pixels
defined in CCITT Group 3.
NewSubFileType = 2. LONG. The value 2 identifies a single
page of a multi-page image, even if the document contains
only one page.
PageNumber. SHORT/SHORT. This tag specifies the page numbers
in the fax document. The tag comprises two SHORT values:
the first value is the page number, the second is the
total number of pages. The number of the first page is
zero. Single-page documents therefore use 00000001 hex.
The total number of pages is required only in the
PageNumber tag associated with the first IFD, and is
optional in subsequent IFD's. Writers utilizing this
option should set the page count portion of the subsequent
PageNumber tags to zero. (Please remember that the first
IFD is not necessarily page one.)
ResolutionUnit = 2,3. SHORT. The units of measure for
XResolution = 204, 300, 400 (inches). RATIONAL. The horizontal
resolution of the image expressed in pixels per
resolution unit. See "Technical Point #6," below.
YResolution = 98, 196, 300, 400 (inches). RATIONAL. The
vertical resolution of the image expressed in pixels
per resolution unit. See "Technical Point #6," below.
Although these tags are already required in Class B (Bi-Level)
files, an explanation of their usage for facsimile images may be
BitsPerSample = 1. SHORT. Since facsimile is a black-and-
white medium, this must be 1 (the default) for all files.
ImageLength. SHORT or LONG. LONG recommended. The total
number of scan lines in the image.
PhotometricInterp = O,1. SHORT. This tag allows notation of
an inverted ("negative") image:
TIFF CLASS F Revision 3: October 1, 1991 Page 3
RowsPerStrip. SHORT or LONG. LONG recommended. The number of
scan lines per strip. When a page is expressed as one
large strip, this is the same as the ImageLength tag.
SamplesPerPixel = 1. SHORT. The value of 1 denotes a bi-
level, gray scale, or palette color image.
StripByteCounts. SHORT or LONG. SHORT 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.
Stripoffsets. SHORT or LONG. For each strip, the offset of
that strip. The offset is measured from the beginning
of the file. If a page is expressed as one large strip,
there is one such entry per page.
There are only three new tags for Class F. All three tags describe
page quality. The information contained in these tags is usually
obtained from the receiving facsimile hardware, but since not all
devices are capable of reporting this information, the tags are
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.
Tag = 326 (146 hex)
Type = SHORT or LONG
This tag 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
TIFF CLASS F Revision 3: October 1, 1991 Page 4
Tag = 327 (147 hex)
Type = SHORT
O 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
the receiving device
2 Lines with an incorrect pixel count existed, but were
not regenerated by receiving device
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 tag describes the error content of the data. That is,
when the BadFaxLines and ImageLength tags indicate that the facsimile
device encountered lines with an incorrect number of pixels during
reception, the CleanFaxData tag indicates whether these lines are
actually in the data or if the receiving facsimile device replaced
them with regenerated lines.
Tag = 328 (148 hex)
Type = LONG or SHORT
This tag 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 tag is an indicator of their
distribution and may therefore be a better general indicator of
perceived image quality.
BadFaxLines. LONG. The number of
"bad" scan lines encountered by the facsimile during
CleanFaxData = O, 1, 2. BYTE. This tag
indicates whether lines with incorrect pixel count are actually
in the data or if the receiving facsimile device replaced them
with regenerated lines.
0 Data contains no lines with incorrect pixel counts or
regenerated lines (i.e., computer generated)
TIFF CLASS F Revision 3: October 1, 1991 Page 5
1 Lines with an incorrect pixel count were regenerated by
2 Lines with an incorrect pixel count existed, but were
not regenerated by receiving device
ConsecutiveBadFaxLines. LONG or SHORT. The maximum number of
consecutive scan lines with incorrect pixel count
encountered by the facsimile device reception.
DateTime. ASCII. Date and time in the format YYYY:MM:DD
HH:MM:SS, in 24-hour format. String length including
NUL byte is 20 bytes. Space between DD and HH.
DocumentName. ASCII. This is the name of the document from
which the document was scanned.
ImageDescription. ASCII. This is an ASCII string describing
the contents of the image.
orientation. SHORT. This tag might be useful for displayers
that always want to show the same orientation,
regardless of the image. The default value of l is
"Oth row is visual top of image, and 0th column is the
visual left." An 180-degree rotation is 3. See TIFF
5.0 for an explanation of other values.
Software. ASCII. The optional name and release number of the
software package that created the image.
REQUIRED TAGS GROUPED CLASS
Required Tags, all TIFF: NewSubFileType, ImageWidth, ImageLength,
StripByteCounts, XResolution, YResolution,
Required Tags, Class B: BitsPerSample, Compression,
Required Tags, Class F: FillOrder, Group30ptions, PageNumber
File portability among various TIFF F applications, regardless of
platform or operating system, is a primary goal of TIFF Class F.
The following tag values should be used to assure maximum
1. FillOrder is 2 (least-significant bit first).
TIFF CLASS F Revision 3: October 1, 1991 Page 6
2. Group30ptions = 4 (one-dimensional encoding).
3. ImageWidth is 1728 (that is, an A4 page).
4. ImageLength must not exceed 1084 for 98 dpi documents and 2167
for 196 dpi documents (that is, an A4 page). See Note 2, below.
5. PhotometricInterp is 0 (normal).
6. ResolutionUnit = 2 (inches).
7. XResolution is 204.
8. YResolution tag is 98 or 196.
Note 1: In order for a document to print at a one-to-one scale
on U.S. plain-paper fax machines, its ImageLength must not
exceed 1025 for 98 dpi documents and 2050 for 196 dpi
Those new to TIFF may not be familiar with the concept of
"strips" embodied in the three tags RowsPerStrip,
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 tags. The location in the
TIFF file of each strip is contained in the StripOffsets
The TIFF documentation suggests using strips of an
arbitrary size of about 8K. Although various application
programs assert that they "prefer" banded images, research
failed to uncover a single existing application that could
not read a single-strip page where they could read the same
file in a multi- strip format. Indeed, applications seem to
be more sensitive to the total size of the decoded image
and are not particularly fussy about banding. This result
is not surprising, considering that most desktop publishing
programs are prepared to deal with massively larger images
than those one finds in facsimile. In short, each page may
be represented as a single strip of any length.
In fact, there may be a compelling reason to employ a
strip size equal to the length of one A4 page (297 mm).
TIFF CLASS F Revision 3: October 1, 1991 Page 7
When a document is imaged, it may be of any length. Not
all fax machines, however, can accept unlimited length
documents. Worse,the remote machine's page-length
capability is not known until the fax connection has been
established. The solution is for the transmitting fax
device to image long documents into A4-size strips, then
seam them together at transmission, after the capabilities
of the remote fax machine is known.
2. Bit Order
Although the TIFF 5.0 documentation lists the FillOrder tag
in the category "No Longer Recommended," Class F resurrects
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 Class F readers must be able to
read data in both bit orders.
Many existing applications already read Class F-like files,
but do not support the multi- page tag. Since a multi-page
format greatly simplifies file management in fax
application software, Class F specifies multi- page
documents (NewSubfileType = 2). A "multi-page document" may
contain only one page.
4. Two-dimensional Encoding
PC Fax applications that wish to support two-dimensional
encoding may do so by setting Bit O in the Group30ptions
tag. Please see item 2 in the "Warnings" section.
5. Example Use of Page-Quality Tags
Here are examples for writing the CleanFaxData,
BadFaxLines, and ConsecutiveBadFaxLines tags:
A. Facsimile hardware does not provide page quality
information: write no tags.
B. Facsimile hardware provides page quality
information, but reports no bad lines. Write
only BadFaxLines = O.
C. 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.
D. Computer generated file: write CleanFaxData = 0.
6. Although 300 and 400 dpi are, strictly speaking, Group 4
resolutions, it is virtually certain that they will soon be
added to Group 3 and, more important, are already in common
use today capabilities through Group 3's NSF mechanism.
Only the following resolutions are valid (horizontal x
TIFF CLASS F Revision 3: October 1, 1991 Page 8
vertical): 204 x 98, 204 x 196, 300 x 300, 400 x 400. Those
who choose to store images at the #Group 4" resolutions
risk incompatibility with other fax applications.
WHAT CONSTITUTES TIFF CLASS F SUPPORT
Fax applications that do not wish to embrace TIFF Class F as a
native format may elect to support it as import/export medium.
Export The simplest form of support is a Class F writer
that produces individual single-page Class F files
with the proper NewSubFile and PageNumber tags.
Import A Class F reader must be able to handle a Class F
file containing multiple pages.
1. Class F requires the ability to read and write at least one-
dimensional T.4 Huffman ("compressed") data. Due to the
disruptive effect to application software of line-length errors
and because such errors are likely in everyday facsimile
transmissions, uncompressed data is not allowed. In other words,
"Uncompressed" bit in Group30ptions must be 0.
2. Since two-dimensional encoding is not required for Group 3
compatibility, Class F readers may decline to read such files.
Therefore, for maximum portability write only one- dimensional
files. Although the same argument technically holds for "fine#
(196 dpi) vertical resolution, only a tiny fraction of facsimile
products support only 98 dpi. Therefore, 196-dpi files are quite
portable in the real world.
3. In the spirit of TIFF, all EOL's in data must be byte- aligned.
An EOL is said to be byte-aligned when Fill bits have been added
as necessary before EOL codes such that EOL always ends on a byte
boundary, thus ensuring an EOL-sequence of a one byte preceded by
a zero nibble: xxxx0000 00000001.
Recall that 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).
4. As illustrated in FIGURE l/T.4 in Recommendation T.4 (the "Blue
Book," page 20), facsimile documents begin with an EOL (which in
Class F is byte-aligned). The last line of the image is not
terminated by an EOL.
5. Aside from EOL's, TIFF Class F files contain only image data.
This means that the Return To Control sequence (RTC) is
TIFF CLASS F Revision 3: October 1, 1991 Page 9
specifically prohibited. Exclusion of RTC's not only makes
possible the simple concatenation of images, it eliminates the
mischief failed communications and unreadable images that their
mistreatment inevitably produces. (This view is reflected in the
work of the EIA PN2388 committee, where the modem device attaches
the RTC outbound and removes it inbound.)
REVISING THE TIFF CLASS F SPECIFICATION
Changes in the specification that reflect changes in the underlying
CCITT specifications as well as non-technical changes are incorporated
by editorial fiat. Before substantial modifications are allowed,
however, the author will consult members of the facsimile industry.
The main goal in revision is to retain a specification that fulfills
the original goals and serves the facsimile industry. It is
especially important that Class F not be modified to accommodate
unrelated goals. For example, there have been proposals to relax
Class F tag requirements to make it more compatible with other
flavors of TIFF. In particular, non-facsimile users seem to be vexed
by the necessity to byte-align EOL's and to support the multi-page
format. Such proposals inevitably originate with users outside the
mainstream of facsimile vendors, in whose applications both of these
features are vitally important. Non-facsimile users who find Class F
too restrictive might better be served by designing a new TIFF classes
to accomplish their ends.
Manuscript of Proposed Revision
Any person or group that wishes to propose an amendment to TIFF Class
F should prepare the following manuscript:
1. Name of the person or group making the request and their
affiliation (business, academic, etc.).
2. The revision date from which you are working.
3. The reason for the request.
4. A list of changes exactly as you propose that they appear in the
specification. Do not submit an edited version of the entire
specification. Use inserts, callouts, or other obvious editorial
techniques to indicate areas of change and number each change.
5. Referring to each change number, discuss its potential impact on
the installed base. In particular address the effect on other
standards that incorporate TIFF Class F (e.g., FaxBios).
6. A list of phone numbers of persons outside your company who may
support your position. Include their affiliation (business,
TIFF CLASS F Revision 3: October 1, 1991 Page 10
This manuscript may be submitted to Joe Campbell, Everex Systems,
Inc: (510) 540-5835 or (510) 841-5441, or via Compuserve Mail
11/17/89: Initial Publication
4/20/90: First Revision
PageNumber tag was incorrectly illustrated as page one. The
correct number for the first page i9 zero.
5/1/91: Second Revision
1. Added 300 and 400 valid values to to the XResolution and
2. Software tag moved from Bi-level Required (not true), to
3. ImageWidth tag values of 2482 was corrected to 2432.
4. New ImageWidth tag values added to conform to the "Blue Book,":
5. Corrected miscellaneous typographical errors.
6. Added summary of required tags.
10/l/91: Third #evision,
1. Total page count needed only in first tag after IFD.
2. Added MMR compression, modification procedure.
TIFF CLASS F Revision 3: October 1, 1991 Page 11