1. Introduction

This document describes the use of TIFF (Tag Image File Format) to represent the data content and structure generated by the current suite of ITU-T Recommendations for Group 3 facsimile. These Recommendations and the TIFF fields described here support the following facsimile modes or profiles:

      S:  minimal black-and-white mode, using binary MH compression
             [T.4]
      F:  extended black-and-white mode, using binary MH, MR and MMR
             compression [T.4, T.6]
      J:  lossless JBIG black-and-white mode, with JBIG compression
             [T.85, T.82]
      C:  lossy color and grayscale mode, using JPEG compression
             [T.42, T.81]
      L:  lossless color and grayscale mode, using JBIG compression
             [T.43, T.82]
      M:  mixed raster content mode [T.44], using a combination of
             existing compression methods

Each profile corresponds to the content of ITU-T Recommendations shown and is a subset of the full TIFF for facsimile specification.

Profile S describes a minimal interchange set of fields, which will guarantee that, at least, binary black-and-white images will be supported. Implementations are required to support this minimal interchange set of fields.

With the intent of specifying a file format for Internet Fax, this draft:

1. specifies the structure of TIFF files for facsimile data,
2. defines ITU fax-compatible values for existing TIFF fields,
3. defines new TIFF fields and values required for compatibility with ITU color fax.

This specification of TIFF for facsimile is known as TIFF-FX.

1.1. Scope

This document defines a TIFF-based file format specification for enabling standardized messaging-based fax over the Internet. It specifies the TIFF fields and field values required for compatibility with the existing ITU-T Recommendations for Group 3 black-and-white, grayscale and color facsimile. TIFF has historically been used for handling fax image files in applications such as store-and-forward messaging. Implementations that support this file format specification for import/export may elect to support it as a native format. This document recommends a TIFF file structure that is compatible with low-memory and page-level streaming implementations.

Unless otherwise noted, the current TIFF specification [TIFF] and selected TIFF Technical Notes [TTN1, TTN2] are the primary references for describing TIFF and defining TIFF fields. This document is the primary reference for defining TIFF field values for fax applications.

1.2. Approach

The basic approach to using TIFF for facsimile data is to insert the compressed fax image data in a TIFF file and use TIFF fields to encode the parameters that describe the image data. These fields will have values that comply with the ITU-T Recommendations. The MIME content type of the resulting file will be image/tiff, with an optional Application parameter [TIFF-REG]; see Section 9.

This approach takes advantage of TIFF features and structures that bridge the data formats and performance requirements of both legacy fax machines and host-based fax applications. TIFF constructs for pages, images, and strips allow a TIFF file to preserve the fax data stream structure and the performance advantages that come with it. A TIFF-based approach also builds on an established base of users and implementors and ensures backward compatibility with existing TIFF- based IETF proposals and work in progress for Internet fax.

1.3. Overview of this draft

Section 2 gives an overview of TIFF. Section 2.1 describes the structure of TIFF files, including general guidelines for structuring multi-page TIFF files. Section 2.2 lists the TIFF fields that are required or recommended for all fax modes. The TIFF fields used only by specific fax modes are described in Sections 3-8, which describe the individual fax modes. These sections also specify the ITU- compatible field values (image parameters) for each mode.

The full set of permitted fields of TIFF for facsimile are included in the current TIFF specification, Section 2 of this document and the sections on specific modes of facsimile operation. This document defines profiles of TIFF for facsimile, where a profile is a subset of the full set of permitted fields and field values of TIFF for facsimile.

Section 3 defines the minimal black-and-white facsimile mode (Profile S), which is required in all implementations. Section 4 defines the extended black-and-white fax mode (Profile F), which provides a standard definition of TIFF-F. Section 5 describes the lossless black-and-white mode using JBIG compression (Profile J). Section 6 defines the base color mode, required in all color implementations, for the lossy JPEG representation of color and grayscale facsimile data (Profile C). Section 7 defines the lossless JBIG color and grayscale facsimile mode (Profile L) and Section 8 defines the Mixed Raster Content facsimile mode (Profile M). Each of these sections concludes with a table summarizing the required and recommended fields for each mode and the values they can have.

Section 9 describes the MIME content type image/tiff and the use of the optional Application parameter in connection with TIFF for facsimile. Sections 10, 11, 12 and 13 give Security Considerations, the ISOC Copyright Notice, References and Authors' Addresses. Annex A gives a summary of the TIFF fields used or defined in this document and provides a convenient reference for implementors.

To implement only the minimal interchange black-and-white set of fields and values (Profile S), one need read only Sections 1, 2, 3, 9 and 10.

The following tree diagram shows the relationship among profiles and between profiles and coding methods.

                                S (MH)
                               / \
                       B&W    /   \   Color
                  ------------     ----------
                 /      \                    \
                /        F (MMR, MR)          C (JPEG)
               /                             / \
              J (JBIG)                   ----   \
                                        /        \
                                       L (JBIG)   \
                                                   \
                                                    M (MRC)

A profile is based on a collection of ITU-T facsimile coding methods.

For example, Profile S, the minimal mode, is based on Modified Huffman (MH) compression, which are defined in ITU-T Rec. T.4. Profile F specifies Modified Read (MR) and Modified Modified Read (MMR) compressions, which are defined in ITU-T Rec. T.4 and T.6.

All implementations of TIFF for facsimile MUST implement Profile S, which is the root node of the tree. All color implementations of TIFF for facsimile MUST implement Profile C. The implementation of a particular profile MUST also implement those profiles on the path that connect it to the root node, and MAY optionally implement profiles not on the path connecting it to the root node. For example, an implementation of Profile M must also implement Profiles C and S, and may optionally implement Profile F, J or L. For another example, an implementation of Profile C must also implement Profile S, and may optionally implement Profile F or J.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", " NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [REQ].