Network Working Group
Request for Comments: 2306
Category: Informational

G. Parsons
Northern Telecom
J. Rafferty
Human Communications
March 1998

Tag Image File Format (TIFF) - F Profile for Facsimile

Status of this Memo

This memo provides information for the Internet community. It does not specify an Internet standard of any kind. Distribution of this memo is unlimited.

Copyright Notice

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>.

1. Abstract

This document references the Tag Image File Format (TIFF) to define the F profile of TIFF for facsimile (TIFF-F) as a file format that may be used for the storage and interchange of 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 implementations can exchange image data. [TIFF] characterizes Baseline TIFF as being the core of TIFF, the essentials that all mainstream TIFF developers should support in their products. 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 defines the F application of [TIFF]. For ease of reference, the term TIFF-F will be used throughout this document as a shorthand for "F Profile of TIFF for Facsimile". TIFF-F files are intended for use with the image/tiff MIME media content-type which includes support for the "application" parameter (e.g., application=faxbw).

   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].

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 profile of TIFF for
   facsimile, 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 to document:

   1)  The fields and values which are applicable for this F profile
       of TIFF for facsimile.
   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.  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 section 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 implementations in environments
   such as facsimile terminals where a complex file structure is
   difficult to support.

3.1.4 Practical Guidelines for Writing/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. However, for interchange robustness, TIFF-F readers SHOULD be prepared to read TIFF files whose structure is consistent with [TIFF], which supports a more flexible file structure than is recommended here.

   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 section 3.1.3, 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 implementations 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 implementations 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. If any long field values are present (see section 3.1.3) then these SHOULD be placed after their referencing IFD and before the image data they describe.

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.

TIFF-F writers MAY make a fifth simplifying assumption, in which the IFD, the value data and the image data for which the IFD has offsets precede the next image IFD. These elements MUST precede the next image IFD in the minimum set TIFF-F files (see section 3.6.2). However, this principle has been relaxed in the case of TIFF-F to reflect past practices.

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 implementor).

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, PhotometricInterpretation, 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
      --------------------------------------------|------------------
       204x98, 204x196, 204x391, 200x100, 200x200 | 1728, 2048, 2432
       300x300                                    | 2592, 3072, 3648
       408x391, 400x400                           | 3456, 4096, 4864
      --------------------------------------------|------------------
   
       Historical TIFF-F did not include support for the following
       widths related to higher resolutions:  2592, 3072, 3648, 3456,
       4096 and 4864.   Historical TIFF-F documents also included the
       following values related to A5 and A6 widths:  816 and 1216.  Per
       the most recent version of [T.4], A5 and A6 documents are no

longer supported in Group 3 facsimile, so the related width values are now obsolete. See section 3.8.2 for more information on inch/metric equivalencies and other implementation details.

ImageLength (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 does
       not contain EOLs. 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.

XResolution (282) = 204, 200, 300, 400, 408 (inches). RATIONAL.: The horizontal resolution of the TIFF-F image expressed in pixels per resolution unit. The values of 200 and 408 have been added to the historical TIFF-F values, for consistency with [T.30]. Some existing TIFF-F implementations may also support values of 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 (297). 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 the second value is 0, the total number of pages in the document is not available.

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 MAY
       be byte aligned (see section 3.8.3).
               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

This field is made up of a set of 32 flag bits. Unused bits must be set to 0. 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 section 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 section 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.

       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 implementations 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.

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

hese are fields that MAY be used in encoding TIFF-F files, but are ptional in nature and may be ignored by many TIFF readers. These ields are called recommended consistent with historical TIFF-F ractice.

   BadFaxLines (326) [defined in section 3.4]
   
   CleanFaxData (327) [defined in section 3.4]
   
   ConsecutiveBadFaxLines (328) [defined in section 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 is designated as "Recommended" for consistency with
       historical TIFF-F, but is also a Baseline TIFF field with a
       default value of 1 per [TIFF]. The default value of 1 applies
       if the field is omitted, but for clarity, TIFF-F writers SHOULD
       include this field.  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.

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 implementations. 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 implementations which need to write minimum subset TIFF-F files, the file structure shown in Figure 3.1 MUST be used:

                   +-----------------------+
                   |         Header        |------------+
                   +-----------------------+            | First IFD
                   |      IFD (page 0)     | <----------+ Offset
               +---|                       |------------+
               |   |                       |--+         |
         Value |   +-----------------------+  |         |
        Offset +-->|      Long Values      |  |         |
                   +-----------------------|  | Strip   |
                   |  Image Data (page 0)  |<-+ Offset  |
                   +-----------------------+            | Next IFD
                   |      IFD (page 1)     | <----------+ Offset
               +---|                       |------------+
               |   |                       |--+         |
         Value |   +-----------------------+  |         |
        Offset +-->|      Long Values      |  |         |
                   +-----------------------|  | Strip   |
                   |  Image Data (page 1)  |<-+ Offset  |
                   +-----------------------+            | Next IFD
                   |      IFD (page 2)     | <----------+ Offset
                   +-----------------------+
                   |          :            |
                   |          :            |
       
       Figure 3.1     TIFF-F Minimum Subset File Structure

As depicted in Figure 3.1, the IFD of each page precedes the related Image Data for that page. If present, any long field values appear between the IFD and the 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

 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 implementations 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

The default bit order in Baseline TIFF per [TIFF] is indicated by FillOrder=1, where bits are not reversed before being stored. However, TIFF-F typically utilizes the setting of FillOrder=2, where the bit order within bytes is reversed before storage (i.e., bits are stored with the Least Significant Bit first).

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 implementations 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 implementations 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 Element Address Designate). In 1984, a somewhat more efficient compression method known as Modified Modified READ (MMR) was defined in the T.6 recommendation. It was originally defined for use with Group 4 facsimile, so that this compression method has been commonly called Group 4 compression. In 1991, the MMR method was approved for use in Group 3 facsimile and has since been widely utilized.

TIFF-F permits three different compression methods. In the most common practice, the one-dimensional compression method (Modified Huffman) is used. This is specified by setting the value of the Compression field to 3 and then setting bit 0 of the T4Options field to 0. 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 implementation requirements, 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 implementations 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.

3.7.7 TIFF-F Export and Import

Fax implementations 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 implementations 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 204h and 391v x 408h.  Therefore, the full set of
   inch-based equivalents of the new resolutions are supported in the
   TIFF-F writer, since they may appear in some image data streams
   received from Group 3 facsimile devices.  However, many facsimile
   terminals and older versions of  TIFF-F readers are likely to not
   support the use of these higher resolutions.

Per [T.4], it is permissible for implementations to treat the following XResolution values as being equivalent: <204,200> and <400,408>. 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 implementations, 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 may be 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 implementations 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, implementations which need to support "transparency" of [T.4] image data MAY include RTCs when writing TIFF-F files 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. Therefore, TIFF-F readers MUST be able to process files which do not include RTCs and SHOULD be able to process files which do include RTCs.

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 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

Recommended Fields in the table are shown with an asterisk (*).

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.

  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 1 = 0   |
                    | Bit 2 = 0,1 | Bit 2 = 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
                    | 400,408,    |             |
                    | 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
  ------------------|-------------|-------------|----------------------

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. MIME sub-type image/tiff

   [TIFFREG] describes the registration of the MIME content-type image/
   tiff to refer to TIFF 6.0 encoded image data.   When transported by
   MIME, the TIFF content defined by this document must be encoded
   within an image/tiff content type. In addition, an optional
   "application" parameter is defined for image/tiff to identify a
   particular application's subset of TIFF and TIFF extensions for the

encoded image data, if it is known. Typically, this would be used to assist the recipient in dispatching a suitable rendering package to handle the display or processing of the image file.

4.1 Refinement of MIME sub-type image/tiff for Application F

Since this document defines a facsimile specific profile of TIFF, it is useful to note an appropriate application parameter for the image/tiff MIME content-type.

The "faxbw" application parameter is defined for black and white facsimile. It is suitable for use by applications that can process one or more TIFF for facsimile profiles or subsets used for the encoding of black and white facsimile data.

Since this document defines a profile of TIFF for facsimile which is suitable for use with black and white facsimile image data, applications which use this profile or its minimum subset should set the value of the application parameter to "faxbw".

An example of the use of the image/tiff MIME Content-type with the application parameter set with the value "faxbw" follows:

Example:

          Content-type: image/tiff; application=faxbw

In this example, use of this parameter value will enable applications to identify the content as being within a profile or subset of TIFF for Facsimile that is suitable for encoding black and white image data, before attempting to process the image data.

5. Implementation Usage

5.1 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 services and their use of TIFF-F will be specified in other documents.

5.2 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 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.

6. Security Considerations

This document describes the encoding for TIFF-F, which is a profile of the TIFF encoding for facsimile. As such, it does not create any security issues not already identified in [TIFFREG], in its use of fields as defined in [TIFF]. There are also new TIFF fields defined within this specification, but they are of a purely descriptive nature, so that no new security risks are incurred.

Further, the encoding specified in this document does not in any way preclude the use of any Internet security protocol to encrypt, authenticate, or non-repudiate TIFF-F encoded facsimile messages.

7. Authors' Addresses

   Glenn W. Parsons
   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

8. References

   [MIME1] Freed, N. and N. Borenstein,  "Multipurpose Internet Mail
        Extensions (MIME) Part One: Format of Internet Message Bodies",
        RFC 2045, November 1996.
   [MIME4] Freed, N. and N. Borenstein,  "Multipurpose Internet Mail
        Extensions (MIME) Part Four: Registration Procedures", RFC 2048,
        November 1996.
   [REQ] Bradner, S., "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.
   [TIFFREG] Parsons, G., Rafferty, J. and S. Zilles, "Tag Image File
        Format (TIFF) - image/tiff:  MIME Sub-type Registration ", RFC
        2302, March 1998.
   [VPIM2] G. Vaudreuil and G. Parsons, "Voice Profile for Internet
        Mail - version 2", Work In Progress, <draft-ema-vpim-06.txt>,
        November 1997.

9. Full Copyright Statement

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