rfc1314.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

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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RFC 1314                 Image Exchange Format                April 1992


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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RFC 1314                 Image Exchange Format                April 1992


   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.





Katz & Cohen                                                    [Page 3]

RFC 1314                 Image Exchange Format                April 1992


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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RFC 1314                 Image Exchange Format                April 1992


   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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RFC 1314                 Image Exchange Format                April 1992


   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