Network Working Group A. Katz

Network Working Group A. Katz
Request for Comments: 1314 D. Cohen
ISI
April 1992

A File Format for the Exchange of Images in the Internet

Status of This Memo

This document specifies an IAB standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "IAB
Official Protocol Standards" for the standardization state and status
of this protocol. Distribution of this memo is unlimited.

Abstract

This document defines a standard file format for the exchange of
fax-like black and white images within the Internet. It is a product
of the Network Fax Working Group of the Internet Engineering Task
Force (IETF).

The standard is:

** The file format should be TIFF-B with multi-page files
supported. Images should be encoded as one TIFF strip
per page.

** Images should be compressed using MMR when possible. Images
may also be MH or MR compressed or uncompressed. If MH or MR
compression is used, scan lines should be "byte-aligned".

** For maximum interoperability, image resolutions should
either be 600, 400, or 300 dpi; or else be one of the
standard Group 3 fax resolutions (98 or 196 dpi
vertically and 204 dpi horizontally).

Note that this specification is self contained and an implementation
should be possible without recourse to the TIFF references, and that
only the specific TIFF documents cited are relevant to this
specification. Updates to the TIFF documents do not change this
specification.

Experimentation with this file format specified here is encouraged.

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1. Introduction

The purpose of this document is to define a standard file format for
exchange of black and white images using the Internet. Since many
organizations have already started to accumulate and exchange scanned
documents it is important to reach agreement about an interchange
file format in order to promote and facilitate the exchange and
distribution of such documents. These images may originate from
scanners, software, or facsimile (fax) machines. They may be
manipulated by software, communicated, shared, duplicated, displayed,
printed by laser printers, or faxed.

This file format provides for the uniform transfer of high quality
images at a reasonable cost and with reasonable speed whether these
files are generated by scanners, totally by software (e.g., text-to-
fax, bitmap-to-fax, OCR, etc), or by fax. Also the intent of this
document is to remain compatible with future moves to multi-level
(i.e., gray-scale), higher resolution, or color images. The format
proposed here is supported by both commercially available hardware
and commercial and public domain software for most popular platforms
in current use.

The file format for images is a totally separate issue from how such
files are to be communicated. For example, FTP or SMTP could be used
to move an image file from one host to another, although there are
complications in the use of SMTP as currently implemented due to file
size and the need to move binary data. (There is currently a
proposal for removing these limitations from SMTP and in particular
extending it to allow binary data. See reference [1].)

One major potential application of the communications format defined
here is to allow images to be sent to fax machines using the
Internet. It is intended that one or more separate companion
documents will be formulated to address the issues of standardization
in the areas of protocols for transmitting images through the
Internet and the issues of addressing fax machines and routing faxes.
Just as the exchange format is separate from the transmission
mechanism, it is also separate from how hosts store images.

This document specifies a common exchange format; it does not require
a host to store images in the format specified here, only to convert
between the host's local image storage formats and the exchange
format defined here for the purpose of exchanging images with other
hosts across the network.

This standard specifies the use of TIFF (Tagged Image File Format,
see below) as a format for exchange of image files. This is not a
specific image encoding, but a framework for many encoding

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techniques, that can be used within the TIFF framework. For example,
within TIFF it is possible to use MMR (the data encoding of CCITT
Group 4 fax, see below), MH or MR (the data encodings of CCITT Group
3 fax), or other encoding methods.

Which encoding technique to use is not specified here. Instead, with
time the encoding schemes used by most document providers will emerge
as the de-facto standard. Therefore, we do not declare any as "the
standard data encoding scheme," just as we do not declare that
English is the standard publication language. (However, we expect
that most document providers will use MMR in the immediate future
because it offers much better compression ratios than MH or MR.)

Similarly, TIFF does not require that an image be communicated at a
specific resolution. Resolution is a parameter in the TIFF
descriptive header. We do suggest that images now be sent using one
of a set of common resolutions in the interests of interoperability,
but the format accommodates other resolutions that may be required by
specialized applications or changing technologies.

Occasionally, image files will have to be converted, such as in the
case where a document that was scanned at 400 dpi is to be printed on
a 300 dpi printer. This conversion could be performed by the
document provider, by the consumer, or by a third party. This
document specifies neither who performs the conversion, nor which
algorithms should be used to accomplish it.

Note that this standard does not attempt to define an exchange format
for all image types that may be transmitted in the Internet. Nothing
in this standard precludes it from being used for other image type
such as gray-scale (e.g., JPEG) or color images but, for the purposes
of standardization, the scope of this document is restricted to
monochromatic bitmapped images.

The developers of this standard recognize that it may have a limited
lifespan as Office Document Architecture (ODA) matures and comes into
use in the Internet; ultimately the class of images covered by this
standard will likely be subsumed by the more general class of images
supported by the ODA standards. However, at present, there does not
appear to be a sufficient installed base of ODA compliant software
and the ODA standards are not fully mature. This standard is
intended to fill the need for a common image transfer format until
ODA is ready. Finally, we believe that it should be possible to
automatically map images encoded in the format specified here into a
future ODA-based image interchange format, thus providing a
reasonable transition path to these future standards.

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2. Relationship to Fax

Transmission of facsimile (fax) images over phone lines is becoming
increasingly widespread. The standard of most fax machines in the
U.S. is CCITT Group 3 (G3), specified in Recommendations T.4 and
T.30 [2] and in EIA Standards EIA-465 and EIA-466. G3 faxes are 204
dots per inch (dpi) horizontally and 98 dpi (196 dpi optionally, in
fine-detail mode) vertically. Since G3 neither assumes error free
transmission nor retransmits when errors occur, the encoding scheme
used is differential only over small segments never exceeding 2 lines
at standard resolution or 4 lines for fine-detail. (The incremental
G3 encoding scheme is called two-dimensional and the number of lines
so encoded is specified by a parameter called k.)

CCITT Group 4 fax (G4) is defined by the T.400 and T.500 series of
Recommendations as well as Recommendation T.6 [2]. It provides for
400 dpi (both vertical and horizontal) and is a fully two-dimensional
encoding scheme (k is infinite) called MMR (Modified Modified READ,
where READ stands for: Relative Element Address Designate). G4
assumes an error free transmission medium (generally an X.25 Public
Data Network, or PDN). Because of this, G4 is not in widespread use
in the U.S. today.

The traditional fax bundles together four independent issues:

(1) Data presentation and compression;
(2) Data transmission;
(3) Image input from paper ("scanning"); and
(4) Image output to paper ("printing").

This bundling supports, for example, the high quality CCITT Group 4
(G4) images (400x400 dpi) but only over X.25 public data networks
with error correction, and similarly it supports the mid-quality
CCITT Group 3 (204x98 and 204x196 dpi) but only over phone voice
circuits (the Switched Telephone Network, or STN) without error
correction. This bundling does not support the use of any other data
transmission capabilities (e.g., FTP over LANs and WANs), nor
asynchrony between the scanning and the printing, nor image storage,
nor the use of the popular laser printers for output (even though
they are perfectly capable of doing so).

In conventional fax, images are never stored. In today's computer
network environment, a better model is:

(1) Images are scanned into files or created by software;
(2) These image files are stored, manipulated, or communicated;
(3) Images in a file are printed or displayed.

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The only feature of the CCITT fax that should be used is the encoding
technique (preferably MMR, but with MR or MH allowed) which may be
implemented with a variety of fax-oriented chips at low cost due to
the popularity of fax.

"Sending a fax" means both encoding (and decoding) the fax images as
well as transmitting the data. Since the Internet ALREADY provides
several mechanisms for data transmission (in particular, FTP for
general file transmission), it is unnecessary to use the data
transmission methods specified in the CCITT standard. Within the
Internet, each fax image should be stored in a file and these files
could be transferred (e.g., using FTP, SMTP, RPC-based methods,
etc.).

Fax machines should be considered just as scanners and printers are,
as I/O devices between paper and files; but not as a transmission
means. Higher quality Group 4 images are thus supported at low cost,
while enjoying the freedom to use any computerized file transfer and
duplication mechanism, standard laser printers, multiple printing
(possibly at multiple remote sites) of the same image without having
to rescan it physically, and a variety of software for various
processing of these images, such as OCR and various drawing programs.
We should be able to interoperate with files created by fax machines,
scanners, or software and to be able to print all of them on fax
machines or on laser printers.

The CCITT Recommendations assume realtime communications between fax
machines and do not therefore specify any kind of fax file format.
We propose using TIFF [3] which seems to be emerging as a standard,
for encapsulation of encoded images. Because they assume realtime
communications, the CCITT fax protocols require negotiations to take
place between the sender and receiver. For example, they negotiate
whether to use two-dimensional coding (and with what k parameter) and
what (if any) padding there is between scan lines.

In our approach, the image in the file is already compressed in a
particular manner. If it is to be sent to an ordinary fax machine
using a fax board/modem, that board will perform the negotiations
with the receiving fax machine. In the cases where the receiver
cannot handle the type of compression used in the file, it will be
necessary to convert the image to another compression scheme before
transmission. (Most fax cards seem to either store images using the
default values of the parameters which are negotiated or in a format
which can quickly be converted to this. With currently available
hardware and software, any necessary format conversion should be easy
to accomplish.)

In conventional fax, if the compression used for a particular image

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is "negative" (i.e., the compressed form is larger than the
uncompressed form, something that happens quite frequently with
dithered photographic images), the larger compressed form of the
image is still sent. If the images are first scanned into files,
this problem could be recognized and the smaller, uncompressed file
sent instead. (Also, Recommendations T.4 and T.6 [2] allow for an
"uncompressed mode." Thus, lines which have negative compression may
each be sent uncompressed. However, very few G3 fax machines support
this mode.)

3. Image File Format

Image files should be in the TIFF-B format which is the bi-level
subclass of TIFF. TIFF and TIFF-B are described in reference [3],
cited at the end of this document. Images should be compressed using
MMR (the G4 compression scheme) because it offers superior
compression ratios. However, images may also be compressed using MH
or MR (the G3 methods). MMR offers much better compression ratios
than these (which are used in G3 fax because of the lack of an
error-free communications path).

TIFF-F, described in [4], is the proposed subclass of TIFF-B for fax
images. However, since TIFF-F was intended for use with G3, it
recommends against certain features we recommend. Specifically, it
suggests not using MMR or MR compression (we recommend MMR and allow
MR) and prohibits uncompressed mode (which we allow and suggest for
some photographic images). Apart from these, the TIFF-F restrictions
should be followed. (Complete compatibility between the format
specified here and TIFF-F can only be guaranteed for MH compressed
images.)

[NOTE: Aldus Corp., the TIFF Developer, considers fax
applications to be outside the scope of mainstream TIFF
since it is not a part of general publishing which is
what TIFF was originally designed for. They specify the
LZW [5] compression scheme rather than MMR. We, however,
are concerned with the transmission and storage of images
rather than publishing. Therefore, we are more concerned
with compression ratios and compatibility with CCITT fax
than Aldus is.]

TIFF itself allows for gray-scale and color images. Image files
should be restricted to TIFF-B for now because most of the currently
available hardware is bi-level (1 bit per pixel). In the future,
when gray-scale or color scanners, printers, and fax becomes
available, the file format suggested here can already accommodate it.
(For example, though JPEG is not currently a TIFF defined compression
type, work is currently underway for including it as such.)

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[NOTE: In this document, we will use the term "reader"
or "TIFF reader" to refer to the process or device
which reads and parses a TIFF file.]

3.A. TIFF File Format

Figure 1 below (reproduced here from Figure 1 of reference [3])
depicts the structure of a TIFF file.

TIFF files start with a file header which specifies the byte order
used in the file (i.e., Big or Little Endian), the TIFF version
number, and points to the first "Image File Directory" (IFD). If the
first two bytes are hex 4D4D, the byte order is from most to least
significant for both 16 and 32 bit integers (Big Endian). If the
first two bytes are hex 4949, the byte order is from least to most
significant (Little Endian). In both formats, character strings are
stored into sequential bytes and are null terminated.

The next two bytes (called the TIFF Version) must be 42 (hex 002A).
This does not refer to the current TIFF revision number. The
following 4 bytes contain the offset (in bytes from the beginning of
the file) to the first IFD.

An IFD contains a 2 byte count of the number of entries in the IFD, a
sequence of 12 byte directory entries, and a 4 byte pointer to the
next IFD. One of these fields (StripOffsets) points to (parts of) an
image in the file. There may be more than one image in the file
(e.g., a "multi-page" TIFF file) and therefore more then one IFD.
IFD field entries may appear in any order.

Each directory entry is 12 bytes and consists of a tag, its type, a
length, and an offset to its value. If the value can fit into 4
bytes (i.e., if the type is BYTE, SHORT, or LONG), the actual value
rather than an offset is given. If the value is less than 4 bytes
(i.e., if the type is BYTE or SHORT), it is left-justified within the
4 byte value offset. More details about directory entries and the
possible tags will be given in Section 3.C.

All pointers (called offsets in the TIFF reference [3]) are the
number of bytes from the beginning of the file and are 4 bytes long.
The first byte of the file has an offset of 0. In the case of only
one image per file, there should therefore be only one IFD. The last
IFD's pointer to the next IFD is set to hex 00000000 (32 bits).

The entries in an IFD must be sorted in ascending order by Tag.

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Header
+--------+--------+ Directory Entry
0 | | | Byte Order +--------+--------+
+--------+--------| X | | | Tag
2 | | | Version(42) +--------+--------|
+--------+--------| X+2 | | | Type
4 | | | Offset of +--------+--------|
+- - A - -+ 0th IFD X+4 | | |
6 | | | +- -+ Length
+--------+--------+ | | |
| +--------+--------+
| X+8 | | | Value
| +- - Y - -+ or
V | | | Value
+--------+--------+ Offset
IFD
+--------+--------+ |
A | - B - | Entry Count |
+--------+--------| |
| | | V
A+2 Entry 0
| | | +--------+--------+
+--------+--------+ | | |
| | | Y Value
A+14 Entry 1 | | |
| | | +--------+--------|
+--------+--------+
| | |
A+26 Entry 2
| | |
+--------+--------+
| | | .
.
| | | .
+--------+--------+
| | |
Entry B-1
| | |
+--------+--------+
| | | Offset of
A+2+B*12 - C - + Next IFD
| | |
+--------+--------+
|
V
(next IFD)

Figure 1: The Structure of a TIFF File

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3.B. Image Format and Encoding Issues

Images in TIFF files are organized as horizontal strips for fast
access to individual rows. One can specify how many rows there are
in each strip and all of the strips are the same size (except
possibly the last one). Each strip must begin on a byte boundary but
successive rows are not so required. For two-dimensional G3
compression (MR), each strip must begin with an "absolute" one-
dimensional line. For MMR (G4) compression, each strip must be
encoded as if it were a separate image.

For a variety of reasons, each page must be a single strip (e.g., not
broken up into multiple strips).

One problem with multiple strips per page is that images which come
from G4 fax machines as well as most scanned images will be generated
as a single strip per page. These would have to be decoded and re-
encoded as multiple strips (remember that for MMR compression, each
strip must be start with a one-dimensionally encoded line).

Another problem with multiple strips per page arises in MR
compression. Here, there MAY be at most k-1 two-dimensionally
encoded lines following a one-dimensionally encoded line, but this is
not required. It is possible to have one-dimensional lines more
frequently than every k lines. However, since each strip (except
possibly the last one) is required to be the same size, it may be
necessary to re-encode the image to insure that each strip starts
with a one-dimensional line. This is not a problem if each page is a
single strip.

[NOTE: The TIFF document [3] suggests using strips which
are about 8K bytes long. However, TIFF-F [4] recommends
that each page be a single strip regardless of its size.
The format specified in this document follows the TIFF-F
recommendation.]

Also, as TIFF-F recommends, all G3 encoded images (MH and MR) should
be "byte-aligned." This means that extra zero bits (fill bits) are
added before each EOL (end-of-line) so that every line starts on a
byte boundary.

In addition, as in the TIFF-F specification, the RTC (Return to
Control signal which consists of 6 continuous EOL's) of G3 shall not
be included at the end of G3 encoded documents. RTC is to be
considered part of the G3 transmission protocol and not part of the
encoding. Most, if not all, G3 fax modems attach RTC to outgoing
images and remove it from incoming ones.

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For MMR (G4) encoded files, readers should be able to read images
with only one EOFB (End Of Facsimile Block) at the end of the page
and should not assume that Facsimile Blocks are of any particular
size. (It has been reported that some MMR readers assume that all
Facsimile Blocks are the maximum size.)

Systems may optionally choose to store the entire image uncompressed
if the compression increases the size of the image file. Also,
uncompressed mode (specified in Group3Options or Group4Options, see
below) allows portions of the image to be uncompressed.

The multi-page capability of TIFF is supported and should be used for
multi-page documents. TIFF files which have multiple pages have an
IFD for each page of the document each of which describes and points
to a single page image. (Note: though the current TIFF specification
does not specifically prohibit having a single IFD point to an image
which is actually multiple pages, with one strip for each page, most
if not all TIFF readers would probably not be able to read such a
file. Therefore, this should not be done.)

[A NOTE ON TIFF AND MULTI-PAGE DOCUMENTS:

Since most publications (e.g., reports, books, and
magazine articles) are composed of more than a single
page, multi-page TIFF files should be used where
appropriate. However, many current TIFF implementations
now only handle single-page files.

It is hoped that in the future, more TIFF implementations
will handle multi-page files correctly. In the meantime,
it would be useful to develop a utility program which
could join several single-page TIFF files into a single
multi-page file and also separate a multi-page TIFF file
into several single page files.

For example, the utility could take a single TIFF file
with N pages, called doc.tif, and create the files
doc.000, doc.001, doc.002, ..., doc.N. doc.000 would be
an ASCII listing of the files created. This naming
scheme is compatible with that used by the image systems
we have seen which only handle single page files.

In going the other way, the N+1 single page files could
be combined into a single multi-page TIFF file. In this
case, if the file doc.000 exists but contains information
contrary to what is found in looking for the files
doc.001, doc.002, ..., the program would notify the user.]

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3.C. TIFF Fields

TIFF is tag or field based. The various fields and their format are
listed in [3]. There are Basic Fields (common to all TIFF files),
Informational Fields (which provide useful information to a user),
Facsimile Fields (used here), and Private Fields.

Each directory entry contains:

The Tag for the field (2 bytes)

The field Type (2 bytes)

The field Length (4 bytes)
(This is in terms of the data type, not in bytes. For
example, a single 16-bit word or SHORT has a Length
of 1, not 2)

The Value Offset (4 bytes)
(Pointer to the actual value, which must begin on a
word boundary. Therefore, this offset will always
be an even number. If the Value fits into 4 bytes, the
Value Offset contains the Value instead. If the Value
takes less than 4 bytes, it is left justified)

The allowed types and their codes are:

1 = BYTE 8-bit unsigned integer (1 byte)

2 = ASCII 8-bit ASCII terminated with a null (variable
length)

3 = SHORT 16-bit unsigned integer (2 bytes)

4 = LONG 32-bit unsigned integer (4 bytes)

5 = RATIONAL Two LONGs (64 bits) representing the
numerator and denominator of a fraction.
In this document, RATIONAL's will be written
as numerator/denominator. (8 bytes)

For ASCII, the Length specifies the number of characters and includes
the null. It does not, however, include padding if such is
necessary.

(Note that ASCII strings of length 3 or less may be stored in the
Value Offset field instead of being pointed to.)

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The following fields should be used in a TIFF image file. Only the
Basic Fields are mandatory; the others are optional (except that for
MH and MR encoded files, the Group3Options Facsimile Field is
mandatory). The optional fields have default values which are given
in the TIFF specification. (Note that the TIFF reference [3]
recommends not relying on the default values.)

Some fields contain one or more flag bits all stored as one value.
In these cases, the bit labeled 0 is the least significant bit (i.e.,
Little Endian order). Where there is more than one suggested value
for a tag, the possible values are separated by |.

Note that some fields (such as ImageLength or ImageWidth) can be of
more than one type.

It would be useful to develop a TIFF viewer and editor which would
allow one to read, add, and edit the fields in a TIFF file. Such an
editor would display fields in sorted order and force the inclusion
of all mandatory fields. Also, resolution and position should always
be displayed or specified together with their units.

3.C.1. Basic Fields (Mandatory)

Basic Fields are those which are fundamental to the pixel
architecture or visual characteristics of an image. The following
Basic Fields should be included in a TIFF image file:

FIELD NAME
(TAG in hex, TYPE) VALUE DESCRIPTION
------------------ ----- -----------

BitsPerSample 1 Number of bits
(0102, SHORT) per pixel (bi-level for
now, but may allow
more later)

Compression 4 Type of Compression
(0103, SHORT) (could also be 1 = Uncompressed
1 or 3) 3 = G3 (MH or MR)
4 = G4 (MMR)
Use 4 if possible

ImageLength <image's length> Length of the Image
(0101, SHORT in scan lines
or LONG)

ImageWidth <image's width> Width of the Image
(0100, SHORT in pixels

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or LONG)

NewSubFileType 0 usually Flag bits indicating
(00FE, LONG) bit 0: 1 if the kind of image.
reduced (see the TIFF
resolution of reference [3])
another image
bit 1: 1 if
single page of a
multi-page image
bit 2: 1 if
image defines a
transparency
mask

Photometric- 0 for positive
Interpretation image (0 imaged
(0106, SHORT) as white, 1 as
black)
1 means reverse
black and white

RowsPerStrip <Number of Rows> Number of Rows in
(0116, SHORT Each Strip. Each
or LONG) page should be a
single strip.

SamplesPerPixel 1 (since are Bi-level
(0115, SHORT) images)

StripByteCounts count1, count2... Number of Bytes in
(0117, SHORTs each strip of the
or LONGs) images. (The Value
is an offset which
points to a series
of counts, each of
which is the same
Type, LONG or SHORT.
The Length is the
same as the number
of strips.)

StripOffsets off1, off2,... Pointers to the strips
(0111, SHORTs of the image (remember,
or LONGs) one strip per page).
(The Value is an offset
which points to a
series of offsets,

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each of which points
to the actual image
data for the strip.)

ResolutionUnit 2 | 3 Units of Resolution
(0128, SHORT) See Below, 3.C.6 2: Inches
3: Centimeters

XResolution See Below, 3.C.6 Resolution in the X
(011A, RATIONAL) direction in pixels
per ResolutionUnit
(we suggest 400 dots
per inch when possible)

YResolution See Below, 3.C.6 Resolution in the Y
(011B, RATIONAL) direction in pixels
per ResolutionUnit
(we suggest 400 dots
per inch when possible)

3.C.2. Informational Fields (Optional)

The following Informational Fields are optional. They provide
useful information to a user. All Field values are ASCII strings.

NAME (TAG in hex) DESCRIPTION
---------------- -----------

Artist (013B) Person Who Created the Image

DateTime (0132) Date and Time of Image Creation

HostComputer (013C) Name of Computer Image was Created On

ImageDescription A Short Text Description
(010E)

Make (010F) Manufacturer of Hardware (Scanner) Used

Model (0110) Model Number of Hardware (Scanner) Used

Software (0131) Software Package that Created the Image

3.C.3. Facsimile Fields (Optional, Mandatory for G3 Compression)

In addition to the above, the Facsimile Fields below should be
used. The TIFF document recommends that they not be used for
interchange between applications, but they are now in wide enough

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use for just that. These fields are optional and default to 0
(all bits off).

FIELD NAME
(TAG in hex, TYPE) VALUE DESCRIPTION
------------------ ----- -----------

Group3Options bit 0: 1 for Flag bits indicating
(0124, LONG) 2-dimensional Options for G3
coding
(i.e., MR with
k > 1)
bit 1: 1 if
uncompressed
mode MAY be used,
0 if uncompressed
mode IS NOT used.
bit 2: 1 if fill (As allowed by the G3
bits have been protocol, fill bits
added may be added between
each line of data
and the EOL. Since
fill bits are used to
"byte-align" G3 image
files, bit 2 should be
set to 1 for these
images.)

Group4Options bit 0: unused Flag bits indicating
(0125, LONG) bit 1: 1 if Options for G4
uncompressed
mode MAY be used,
if this bit is 0
it means that
uncompressed mode
IS NOT used.

3.C.4. Storage and Retrieval Fields (Optional)

The following fields are optional and may be useful for document
storage and retrieval.

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FIELD NAME
(TAG in hex, TYPE) DESCRIPTION
------------------ -----------

DocumentName Name of the Document
(010D, ASCII)

PageName Name of the Page
(011D, ASCII)

PageNumber Page Number in a Multi-Page Document
(0129, SHORTs) Two SHORT Values are specified, the
first is the page number and the
second is the total number of pages
in the document. The first page
is page 0. (NOTE: This does not
necessarily correspond to page
numbers which may be printed
in the image.)

XPosition X Offset of the Left Side of
(011E, RATIONAL) the Image, in ResolutionUnits

YPosition Y Offset of the Top of
(011F, RATIONAL) the Image, in ResolutionUnits

3.C.5. TIFF-F Fields (NOT Recommended)

TIFF-F defines the following new fields for G3 (MH) encoded
images. Since these fields are not defined in TIFF-B itself,
their use is not recommended. However, since TIFF-F files may
include these tags for image data which came from a G3 fax
machine, readers should be prepared for them.

These three fields deal with corrupted image data which is due to
the fact that G3 devices may not perform error correction on bad
data.

FIELD NAME
(TAG in hex, TYPE) DESCRIPTION
------------------ -----------

BadFaxLines Number of Bad fax scan lines
(0146, SHORT or LONG) encountered during fax reception
(but not necessarily in the file)

CleanFaxData 0 means no bad lines received
(0147, SHORT) 1 means bad lines were regenerated

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by the receiving device
2 means bad lines were detected
but not regenerated

ConsecutiveBadFaxLines The maximum number of consecutive
(0148, SHORT or LONG) bad fax lines (but not necessarily
in the file)

3.C.6. More on Representing Resolutions

The tags XResolution and YResolution are both RATIONALs, i.e., the
ratio of two LONGS. G3 fax resolutions are actually specified in
dots (or lines) per mm while G4 is in dots per inch (actually,
dots per 25.4 mm).

For example, G3 horizontal resolution is defined to be 1728 dots
per 215 mm which comes out to 80.4 dots per cm or about 203 dots
per inch. It is frequently referred to as just 200 dpi. To avoid
any possibility of problems due to round off error, this should be
represented by having XResolution = 17280/215 and ResolutionUnit =
3 (cm). However when reading, 204/1 or even 200/1 with
ResolutionUnit = 2 (inches) should be recognized as representing
the same resolution.

For G4, on the other hand, the resolution 400 dots/inch should be
represented by an XResolution of 400/1 and ResolutionUnit = 2.

The following table shows various ways of representing the
standard resolutions in order of preference:

ResolutionUnit XResolution YResolution
-------------- ----------- -----------

G3 normal 3 17280/215 3850/100
3 80/1 3850/100
3 17280/215 385/10
3 80/1 385/10
2 2042/10 9779/100
2 204/1 98/1
2 200/1 100/1

G3 fine 3 17280/215 77/1
3 80/1 77/1
2 2042/10 19558/100
2 204/1 196/1
2 200/1 200/1

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G4 200 dpi 2 200/1 200/1

G4 300 dpi 2 300/1 300/1

Other 300 dpi 2 300/1 300/1

G4 400 dpi 2 400/1 400/1

600 dpi 2 600/1 600/1

It is suggested that Image readers be able to handle all of the
above representations.

4. A Sample TIFF Image File

Below is a sample of what might be in a TIFF file for an MMR (G4)
encoded single image which is about 100K bytes compressed at 400 dpi.
A generic outline is given first, followed by a more detailed hex
listing.

4.A. Sample File

Comments are to the right and are preceded by a semicolon. Note that
tags must be sorted in order of the tag codes.

0:, IFDADDR:, and STRIP0: are addresses within the file and denote
the number of bytes from the beginning of the file.

Header:

0: Byte Order= hex 4D4D ;first bytes of the file, from
;most significant bit to least
;significant (big endian)
Version= 42 (hex 002A) ;Must be 42
First IFD= IFDADDR ;Address of first (and only) IFD

Image File Directory (the only one in this example):

IFDADDR:

IFD Entry Count= 24 ;(NOT A TAG) Count of
; Number of IFD Entries

NewSubFileType= 0
ImageWidth= 3400 ;8.5 inches at 400 dpi
ImageLength= 4400 ;11 inches at 400 dpi
BitsPerSample= 1 ;Bi-Level

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Compression= 4 ;MMR
Photometric-
Interpretation= 0
DocumentName= "LAMap1"
ImageDescription= "A map of Los Angeles"
Make= "Fujitsu"
Model= "M3093E"
StripOffsets= <STRIP0> ;There is only one strip in
;this example. However, note
;that strips can be in any
;order. (Offsets are from the
;beginning of the TIFF file.)

SamplesPerPixel= 1 ;Bi-Level
RowsPerStrip= 4400 ;Entire image in 1 strip
StripByteCounts= <COUNT0> ;Byte count of entire
;compressed image

XResolution= 400/1
YResolution= 400/1
XPosition= 0/1 ;position of left side of image
YPosition= 0/1 ;position of top of image
Group4Options= hex 00000002 ;bit 1 on means uncompressed
;mode MAY be used

ResolutionUnit= 2 ;Inches
Software= "Xionics"
DateTime= "1990:10:05 15:00:00"
Artist= "Joe Pro"
HostComputer= "Tardis.Isi.Edu"

Next IFD Pointer= hex 00000000 ;(NOT A TAG) Indicates no
; more IFDs in this file

Image Data:

<STRIP0>: <actual compressed image data>

[end of TIFF file]

In this example there is only one strip. Note that if there were
more than one, the TIFF specification does not require them to be in
any particular order. Strips may be given in any order and TIFF
readers must use the StripOffsets to locate them.

Also, the TIFF document recommends not relying on the default values
of the tags.

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RFC 1314 Image Exchange Format April 1992

4.B. Detailed Hex Listing

All offsets and values are represented by hex except for ASCII
strings which are double quoted. Remember that Value Offsets must
always be an even number since the value it points to must always be
on a 16-bit word boundary.

Entries in the Name column are for reference and are not actually a
part of the TIFF file.

Offset Name Value
---- ------------------- -------------------------------------
Header (first byte is Offset 0):
0000 Byte Order 4D4D
0002 Version 002A
0004 1st. IFD pointer 00000010

IFD (IFDADDR from above is 0010 here):
0010 Entry Count 0018
0012 NewSubFileType 00FE 0004 00000001 00000000
001E ImageWidth 0100 0004 00000001 00000D48
002A ImageLength 0101 0004 00000001 00001130
0036 BitsPerSample 0102 0003 00000001 00010000
0042 Compression 0103 0003 00000001 00040000
004E Photometric Interp. 0106 0003 00000001 00000000
005A DocumentName 010D 0002 00000007 00000136
0066 ImageDescription 010E 0002 00000015 0000013E
0072 Make 010F 0002 00000008 00000154
007E Model 0110 0002 00000007 0000015C
008A StripOffsets 0111 0004 00000001 000001A8
0096 SamplesPerPixel 0115 0003 00000001 00010000
00A2 RowsPerStrip 0116 0004 00000001 00001130
00AE StripByteCounts 0117 0004 00000001 <COUNT0>
00BA XResolution 011A 0005 00000001 00000164
00C6 YResolution 011B 0005 00000001 00000164
00D2 XPosition 011E 0005 00000001 0000016C
00DE YPosition 011F 0005 00000001 0000016C
00EA Group4Options 0125 0004 00000001 00000002
00F6 ResolutionUnit 0128 0003 00000001 00020000
0102 Software 0131 0002 00000008 00000174
010E DateTime 0132 0002 00000014 0000017C
011A Artist 013B 0002 00000008 00000190
0126 HostComputer 013C 0002 0000000F 00000198
0132 Next IFD Pointer 00000000

Fields Offsets Point to:
0136 DocumentName "LAMap1"
013E ImageDescription "A map of Los Angeles"

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0154 Make "Fujitsu"
015C Model "M3093E"
0164 X,Y Resolution 00000190 00000001
016C X,Y Position 00000000 00000001
0174 Software "Xionics"
017C DateTime "1990:10:05 15:00:00"
0190 Artist "Joe Pro"
0198 HostComputer "Tardis.Isi.Edu"

Image Data (<STRIP0> from above is here 01A8)
01A8 Compressed Data for single strip, of length <COUNT0> bytes

[end of TIFF file]

NOTE: Since in this example there is only a single strip, there is only
one count for StripByteCounts and one offset for StripOffsets.
Thus, each of these only takes 4 bytes and will fit in the
Value Offset instead of being pointed to.

5. Conclusions

Bitmapped images transferred within the Internet should be in the
following format:

1. The file format should be TIFF-B with multi-page files
supported. Images should be encoded as one TIFF strip
per page.

2. Images should be compressed using MMR when possible. Images
may also be MH or MR compressed or uncompressed. If MH or MR
compression is used, scan lines should be "byte-aligned".

3. For maximum interoperability, image resolutions should
either be 600, 400, or 300 dpi; or else be one of the
standard Group 3 fax resolutions (98 or 196 dpi
vertically and 204 dpi horizontally).

Note that this specification is self contained and an implementation
should be possible without recourse to the TIFF references, and that
only the specific TIFF documents cited are relevant to this
specification. Updates to the TIFF documents do not change this
specification.

Existing commercial off-the-shelf products are available which can
handle images in the above format. ISI would be delighted to help
those interested in assembling a system.

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RFC 1314 Image Exchange Format April 1992

6. Acknowledgments

Many contributions to this work were made by members of the IETF
Network Fax Working Group especially by its chairman, Mark Needleman
and by Clifford Lynch of the University of California Office of the
President, Library Automation. Also, Kiyo Inaba of Ricoh Co. Ltd.
made a number of helpful suggestions.

7. References

[1] Borenstein, N., and N. Freed, "Mechanisms for Specifying and
Describing the Format of Internet Message Bodies", RFC in
preparation.

[2] International Telegraph and Telephone Consultative Committee
(CCITT), Red Book, October, 1984.

[3] Aldus Corp., Microsoft Corp., "Tag Image File Format
Specification", Revision 5.0, Final, 1988.

[4] Cygnet Corporation, "The Spirit of TIFF Class F, 1990", available
from Cygnet Technologies, 2560 9th., Suite 220, Berkeley, CA
94710, FAX: (415) 540-5835.

[5] Welch, T., "A Technique for High Performance Data Compression",
IEEE Computer, Vol. 17, No. 6, Page 8, June 1984.

8. Security Considerations

While security issues are not directly addressed by this document, it
is important to note that the file format described in this document
is intended for the communications of files between systems and
across networks. Thus the same precautions and cares should be
applied to these files as would be to any files received from remote
and possibly unknown systems.

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9. Authors' Addresses

Alan Katz
USC Information Sciences Institute
4676 Admiralty Way #1100
Marina Del Rey, CA 90292-6695

Phone: 310-822-1511
Fax: 310-823-6714
EMail: [email protected]

Danny Cohen
USC Information Sciences Institute
4676 Admiralty Way #1100
Marina Del Rey, CA 90292-6695

Phone: 310-822-1511
Fax: 310-823-6714
EMail: [email protected]

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