rfc2531.txt

<VC++网络游戏建摸与实现>源代码

Network Working Group                                         G. Klyne
Request for Comments: 2531                    5GM/Content Technologies
Category: Standards Track                                  L. McIntyre
                                                     Xerox Corporation
                                                            March 1999


                Content Feature Schema for Internet Fax

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1999).  All Rights Reserved.

Abstract

   This document defines a content feature schema that is a profile of
   the media feature registration mechanisms [1,2,3] for use in
   performing capability identification between extended Internet fax
   systems [5].

   This document does not describe any specific mechanisms for
   communicating capability information, but does presume that any such
   mechanisms will transfer textual values.  It specifies a textual
   format to be used for describing Internet fax capability information.



















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Table of Contents

   1. Introduction .............................................3
   1.1 Organization of this document............................3
   1.2 Terminology and document conventions.....................3
   2. Fax feature schema syntax ................................4
   3. Internet fax feature tags ................................4
   3.1 Image size...............................................5
   3.2 Resolution...............................................5
   3.3 Media type...............................................6
   3.4 Paper Size...............................................6
   3.5 Color capability.........................................6
   3.6 Color model..............................................8
   3.7 Image coding............................................10
   4. Examples ................................................12
   4.1 Simple mode Internet fax system.........................12
   4.2 High-end black-and-white Internet fax system............12
   4.3 Grey-scale Internet fax system..........................13
   4.4 Full-color Internet fax system..........................13
   4.5 Full-color Internet fax system (MRC)....................14
   4.6 Sender and receiver feature matching....................15
   5. IANA Considerations .....................................17
   6. Security Considerations .................................17
   6.1 Capability descriptions and mechanisms..................17
   6.2 Specific threats........................................18
   7. Acknowledgements ........................................18
   8. References ..............................................18
   9. Authors' Addresses ......................................21
   Appendix A: Feature registrations ..........................22
   A.1 Image size..............................................22
   A.2 Resolution aspect ratio.................................24
   A.3 Color levels............................................25
   A.4 Color space.............................................27
   A.5 CIELAB color depth......................................30
   A.6 CIELAB color gamut......................................32
   A.7 Image file structure....................................34
   A.8 Image data coding.......................................36
   A.9 Image coding constraint.................................38
   A.10 JBIG stripe size.......................................39
   A.11 Image interleave.......................................41
   A.12 Color subsampling......................................42
   A.13 MRC availability and mode..............................43
   A.14 MRC maximum stripe size................................45
   Appendix B: TIFF mode descriptions .........................47
   Appendix C: Revision history ...............................49
   Full Copyright Statement ...................................51





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1. Introduction

   This document defines a content feature schema that is a profile of
   the media feature registration mechanisms [1,2,3] for use in
   performing capability identification between extended Internet fax
   systems [5].

   This document does not describe any specific mechanisms for
   communicating capability information, but does presume that any such
   mechanisms will transfer textual values.  It specifies a textual
   format to be used for describing Internet fax capability information.

   The range of capabilities that can be indicated are based on those
   covered by the TIFF file format for Internet fax [7] and Group 3
   facsimile [6].  A companion document [4] describes the relationship
   and mapping between this schema and Group 3 fax capabilities.

1.1 Organization of this document

   Section 2 specifies the overall syntax for fax feature descriptions
   by reference to the media feature registration and syntax documents
   [1,2].

   Section 3 enumerates the feature tags that are to be recognized and
   processed by extended Internet fax systems, according to their
   capabilities.

   Appendix A contains additional feature tag registrations for media
   features that are specific to fax and for which no applicable
   registration already exists.  These are presented in the form
   prescribed by the media feature registration procedure [1].

1.2 Terminology and document conventions

   The term "extended Internet fax system" is used to describe any
   software, device or combination of these that conforms to the
   specification "Extended Facsimile Using Internet Mail" [5].

   "capability exchange" describes any transfer of information between
   communicating systems that is used to indicate system capabilities
   and hence determine the form of data transferred.  This term covers
   both one-way and two-way transfers of capability information.

   "capability identification" is a particular form of capability
   exchange in which a receiving system provides capability information
   to a sending system.





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   "capability description" is a collection of data presented in some
   specific format that describes the capabilities of some communicating
   entity.  It may exist separately from any specific capability
   exchange mechanism.

      NOTE: Comments like this provide additional nonessential
      information about the rationale behind this document.  Such
      information is not needed for building a conformant
      implementation, but may help those who wish to understand the
      design in greater depth.

2. Fax feature schema syntax

   The syntax for the fax feature schema is described by "A syntax for
   describing media feature sets" [2].  This in turn calls upon media
   feature tags that may be registered according to the procedure
   described in "Media Feature Tag Registration Procedure" [1].

      NOTE: Media feature registration provides a base vocabulary of
      features that correspond to media handling capabilities.  The
      feature set syntax provides a mechanism and format for combining
      these to describe combinations of features.  This memo indicates
      those features that may be associated with extended Internet fax
      systems.

3. Internet fax feature tags

   This section enumerates and briefly describes a number of feature
   tags that are defined for use with extended Internet fax systems and
   applications.  These tags may be used also by other systems and
   applications that support corresponding capabilities.

   The feature tags presented below are those that an extended Internet
   fax system is expected to recognize its ability or non-ability to
   handle.

   Definitive descriptions of feature tags are indicated by reference to
   their registration per the media feature registration procedure [1]
   (some of which are appended to this document)

      NOTE: The presence of a feature tag in this list does not mean
      that an extended Internet fax system must have that capability;
      rather, it must recognize the feature tag and deal with it
      according to the capabilities that it does have.







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      Further, an extended Internet fax system is not prevented from
      recognizing and offering additional feature tags.  The list below
      is intended to provide a basic vocabulary that all extended
      Internet fax systems can use in a consistent fashion.

      If an unrecognized or unused feature tag is received, the feature
      set matching rule (described in RFC2533 [2]) operates so that tag
      is effectively ignored.

3.1 Image size

      Feature tag name    Legal values
      ----------------    ------------
      size-x              <Rational> (>0)
      size-y              <Rational> (>0)

   Reference:  this document, Appendix A.

   These feature values indicate a rendered document size in inches.

   Where the actual size is measured in millimetres, a conversion
   factor of 10/254 may be applied to yield an exact inch-based value.

3.2 Resolution

      Feature tag name    Legal values
      ----------------    ------------
      dpi                 <Integer> (>0)
      dpi-xyratio         <Rational> (>0)

   Reference: "Media Features for Display, Print, and Fax" [3], and this
   document appendix A.

   If 'dpi-xyratio' is present and not equal to 1 then the horizontal
   resolution (x-axis) is indicated by the 'dpi' feature value, and the
   vertical resolution (y-axis) is the value of 'dpi' divided by 'dpi-
   xyratio'.

   For example, the basic Group 3 fax resolution of 200*100dpi might be
   indicated as:

         (& (dpi=200) (dpi-xyratio=200/100) )

   When describing resolutions for an MRC format document, the complete
   set of usable resolutions is listed.  However, there are some
   restrictions on their use: (a) 100dpi resolution can be used only





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   with multi-level images, and (b) any multi-level image resolution is
   required to be an integral sub-multiple of the applicable mask
   resolution.

3.3 Media type

      Feature tag name    Legal values
      ----------------    ------------
      ua-media            screen
                          screen-paged
                          stationery
                          transparency
                          envelope
                          envelope-plain
                          continuous

   Reference: "Media Features for Display, Print, and Fax" [3].

      NOTE: Where the recipient indicates specific support for hard copy
      or soft copy media type, a sender of color image data may wish to
      adjust the color components (e.g.  per the related rules of ITU
      recommendation T.42 [9]) to improve rendered image quality on that
      medium.

3.4 Paper Size

      Feature tag name    Legal values
      ----------------    ------------
      paper-size          A4
                          A3
                          B4
                          letter
                          legal

   Reference: "Media Features for Display, Print, and Fax" [3].

3.5 Color capability

   Feature tag name    Legal values
   ----------------    ------------
   color               Binary  (bi-level only)
                       Limited (a limited number of colors)
                       Mapped  (palette or otherwise mapped color)
                       Grey    (grey-scale only)
                       Full    (full continuous-tone color)


   Reference: "Media Features for Display, Print, and Fax" [3].



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   The intention here is to give a broad indication of color handling
   capabilities that might be used, for example, to select among a small
   number of available data resources.

   The value of this feature also gives an indication of the more
   detailed color handling features that might be applicable (see next
   section).

   'Binary' indicates black-and-white, or other bi-level capability.  No
   further qualifying feature tags are required.

   'Limited' indicates a small number of distinct fixed colors, such as
   might be provided by a highlight printer, pen plotter or limited
   color display.  The 'color-levels' tag should be used to indicate the
   number of distinct colors available.

      NOTE: No ability to indicate any specific or named color is
      implied by this option.

   Some devices might use different intensity levels rather than
   different hues for distinction.

   'Mapped' indicates that pixel color values are mapped in some
   specifiable way to a multi-component color space.  The 'color-levels'
   tag may be used to indicate the number of distinct colors available;
   in its absence, sufficient levels to display a photographic image
   should be assumed.

   'Grey' indicates a continuous tone grey-scale capability.

   'Full' indicates full continuous tone color capability.

   For 'Mapped', 'Grey' and 'Full' color, additional feature tags
   (section 3.6) may be used to further qualify the color reproduction.

















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3.6 Color model

      Feature tag name    Legal values
      ----------------    ------------
      color-levels        <integer>   (>2)
      color-space         Device-RGB  (device RGB)
                          Device-CMY  (device CMY)