rfc2531.txt

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Klyne & McIntyre            Standards Track                     [Page 7]

RFC 2531        Content Feature Schema for Internet Fax       March 1999


3.6 Color model

      Feature tag name    Legal values
      ----------------    ------------
      color-levels        <integer>   (>2)
      color-space         Device-RGB  (device RGB)
                          Device-CMY  (device CMY)
                          Device-CMYK (device CMYK)
                          CIELAB      (LAB per T.42 [9])
                          (may be extended by further registrations)

      CIELAB-L-depth      <integer>   (>0)
      CIELAB-a-depth
      CIELAB-b-depth
      CIELAB-L-min        <integer>
      CIELAB-L-max
      CIELAB-a-min
      CIELAB-a-max
      CIELAB-b-min
      CIELAB-b-max

   Reference: this document, appendix A.

   The general model for image handling (both color and non-color) is
   described here from a receiver's perspective; a similar model
   operates in the reverse direction for a scan/send perspective:

          raw bit        pixel         color         physical
          stream  -(A)-> values -(B)-> values -(C)-> rendition

      -   "raw bit stream" is a stream of coded bits

     (A)  indicates image coding/decoding (MH,MR,MMR,JPEG,JBIG,etc.)

      -   "pixel values" are a single numeric value per picture element
          that designates the color of that element.

     (B)  indicates pixel-to-color value mapping

      -   "color values" have a separate numeric value for each color
          component (i.e. L*, a*, b* in the case of CIELAB indicated
          above.)

     (C)  indicates how the color values are related to a physical
          color.  This involves interpretation of the color value with
          respect to a color model (e.g. RGB, L*a*b*, CMY, CMYK) and a
          color space (which is typically recipient-dependent).




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RFC 2531        Content Feature Schema for Internet Fax       March 1999


      -   "physical rendition" is a color value physically realized on a
          display, printer or other device.

   There are many variables that can be applied at each stage of the
   processing of a color image, and any may be critical to meaningful
   handling of that image in some circumstances.  In other circumstances
   many of the variables may be implied (to some level of approximation)
   in the application that uses them (e.g. color images published on a
   Web page).

   The color feature framework described here is intended to allow
   capability description at a range of granularity: feature tags which
   correspond to implied (or "don't care" or "unknown") feature values
   may simply be omitted from a capability description.

   Grey scale and bi-level images are handled within this framework as a
   special case, having a 1-component color model.  The following
   features are used for describing color capabilities:

   'color-levels' indicates the number of distinct values for each
   picture element, and applies to all but bi-level images.  For bi-
   level images, a value of 2 is implied.

   'color-space' is used mainly with 'Mapped' and 'Full', but could be
   used with other modes if the exact color used is significant.  Two
   kinds of color space can be distinguished: device-dependent and
   calibrated.  Device dependent spaces are named here as 'Device-xxx',
   and are used to indicate a color space that is defined by the
   receiving device.  Calibrated color spaces presume the existence of a
   rendering system that is calibrated with respect to an indicated
   definition, and is capable of processing the device-independent color
   information accordingly.

   A color-handling receiver should indicate any appropriate device
   color space capability in addition to any calibrated color spaces
   that it may support.  A calibrated color space should be used when
   precise color matching is required in the absence of specific
   knowledge of the receiving system.

      NOTE: In practice, although they appear to be separate concepts,
      the color model and color space cannot be separated.  In the final
      analysis, a color model (RGB, CMY, etc.) must be defined with
      respect to some color space.

   'CIELAB-L-depth', 'CIELAB-a-depth' and 'CIELAB-b-depth' indicate the
   number of different values that are possible for the L*, a* and b*
   color components respectively, and are significant only when colors




Klyne & McIntyre            Standards Track                     [Page 9]

RFC 2531        Content Feature Schema for Internet Fax       March 1999


   are represented in a CIELAB color space.  These features would be
   used with palettized color, or with full color where each color
   component has a different number of possible values.

   The 'CIELAB-x-min' and 'CIELAB-x-max' values indicate a color gamut
   (i.e. a range of color values that are used or may be rendered).  A
   gamut may be indicated in terms of the CIELAB color space even when
   colors are represented in some other space.

3.7 Image coding

      Feature tag name    Legal values
      ----------------    ------------
      image-file-         TIFF-S
      structure           TIFF-F
                          TIFF-J
                          TIFF-C
                          TIFF-L
                          TIFF-M
                          (may be extended by further registrations,
                          to cover non-TIFF image file structures)
      image-coding        MH
                          MR
                          MMR
                          JBIG
                          JPEG
                          (may be extended by further registrations)
      image-coding-       JBIG-T85    (bi-level, per ITU T.85)
      constraint          JBIG-T43    (multi-level, per ITU T.43)
                          JPEG-T4E    (per ITU T.4, Annex E)
                          (may be extended by further registrations)
      JBIG-stripe-size    <Integer>
      image-interleave    Stripe
                          Plane
      color-subsampling   "1:1:1"     (no color subsampling)
                          "4:1:1"     (4:1:1 color subsampling)
      MRC-mode            <Integer> (0..7)   (per ITU T.44 [15])
      MRC-max-stripe-size <Integer>

   Reference: this document, appendix A.

   'image-file-structure' defines how the coded image data is wrapped
   and formatted.  Options defined here are the various profiles of
   TIFF-FX, per RFC 2301 [7].  These options apply to overall formatting
   of the image data (TIFF file format, byte ordering, bit ordering,
   etc.) and do not define specific image coding issues that are covered
   by other aspects of the TIFF-FX profile specifications.




Klyne & McIntyre            Standards Track                    [Page 10]

RFC 2531        Content Feature Schema for Internet Fax       March 1999


   'image-coding' describes how the raw image data is compressed and
   coded as a sequence of bits.  These are generic tags that may apply
   to a range of file formats and usage environments.

   'image-coding-constraint' describes how the raw image data coding
   method is constrained to meet a particular operating environment.
   Options defined here are JBIG and JPEG coding constraints that apply
   in typical Group 3 fax environments.

   The 'JBIG-stripe-size' feature may be used with JBIG image coding,
   and indicates the number of scan lines in each stripe except the last
   in an image.  The legal constraints are:

      (JBIG-stripe-size=128)
      (JBIG-stripe-size>=0)

   The latter being equivalent to no restriction.

   The 'MRC-mode' feature is used to indicate the availability of MRC
   (mixed raster content) image format capability, and also the MRC mode
   available.  A zero value indicates MRC is not available, a non-zero
   value indicates the available MRC mode number.

   An MRC formatted document is actually a collection of several images,
   each of which is described by a separate feature collection.  An
   MRC-capable receiver is presumed to be capable of accepting any
   combination of contained images that conform to the MRC construction
   rules and declared image-coding capabilities.

   Within an MRC-formatted document, multi-level coders are used for
   foreground and background images (i.e. odd-numbered layers: 1, 3, 5,
   etc.) and bi-level coders are used for mask layers (i.e. even
   numbered layers 2, 4, 6, etc.).

      NOTE: an MRC formatted document may appear within a TIFF image
      file structure, so this separate feature is needed to capture the
      full range of possible capabilities.

   The 'MRC-max-stripe-size' feature may be used with MRC coding, and
   indicates the maximum number of scan lines in each MRC stripe.  The
   legal constraints are:

      (MRC-max-stripe-size=[0..256])
      (MRC-max-stripe-size>=0)

   These values indicate upper bounds on the stripe size.  The actual
   value may vary between stripes, and the actual size for each stripe
   is indicated in the image data.



Klyne & McIntyre            Standards Track                    [Page 11]

RFC 2531        Content Feature Schema for Internet Fax       March 1999


      NOTE: there are many image coding options here, and not all are
      required in all circumstances.

      Specification of the image-file-structure tag value alone is not
      normally sufficient to describe the capabilities of a recipient.
      A general rule is that sufficient detail should be provided to
      exclude any unsupported features.

      For extended Internet fax, image-file-structure and image-coding
      should always be specified, together with additional values
      described above as needed to clearly indicate which feature tag
      values are supported and which are not.  (See also the examples in
      section 4.)

4. Examples

   Some of the examples contain comments introduced by '--...'.  These
   are not part of the allowed capability description syntax.  They are
   included here to explain some of the constructs used.

   The level of detail captured here reflects that used for capability
   identification in Group 3 facsimile.

4.1 Simple mode Internet fax system

   This example describes the capabilities of a typical simple mode
   Internet fax system.  Note that TIFF application S is required to be
   supported by such a system.

      (& (color=Binary)
         (image-file-structure=TIFF-S)
         (dpi=200)
         (dpi-xyratio=[200/100,200/200])
         (paper-size=A4)
         (image-coding=MH) (MRC-mode=0)
         (ua-media=stationery) )

4.2 High-end black-and-white Internet fax system

   This would include support for B/W JBIG and be equivalent to what is
   sometimes called "Super G3", except that Internet fax functionality
   would be added.









Klyne & McIntyre            Standards Track                    [Page 12]

RFC 2531        Content Feature Schema for Internet Fax       March 1999


      (& (color=Binary)
         (image-file-structure=[TIFF-S,TIFF-F,TIFF-J])
         (| (& (dpi=200) (dpi-xyratio=200/100) )    -- 200*100
            (& (dpi=200) (dpi-xyratio=1) )          -- 200*200
            (& (dpi=204) (dpi-xyratio=204/391) )    -- 204*391
            (& (dpi=300) (dpi-xyratio=1) ) )        -- 300*300
         (| (image-coding=[MH,MR,MMR])
            (& (image-coding=JBIG)
               (image-coding-constraint=JBIG-T85)
               (JBIG-stripe-size=128) ) )
         (MRC-mode=0)
         (paper-size=[A4,B4]) )

4.3 Grey-scale Internet fax system

   This is the previous example extended to handle grey scale multi-
   level images.  In keeping with Group 3 fax, this example requires
   equal x- and y- resolutions for a multi-level image.

      (& (| (& (color=Binary)
               (image-file-structure=[TIFF-S,TIFF-F,TIFF-J])
               (| (image-coding=[MH,MR,MMR])
                  (& (image-coding=JBIG)
                     (image-coding-constraint=JBIG-T85)
                     (JBIG-stripe-size=128) ) )
               (| (& (dpi=200) (dpi-xyratio=200/100) )
                  (& (dpi=200) (dpi-xyratio=1) )
                  (& (dpi=204) (dpi-xyratio=204/391) )
                  (& (dpi=300) (dpi-xyratio=1) ) ) )
            (& (color=Grey)
               (image-file-structure=[TIFF-C,TIFF-L])
               (color-levels<=256)
               (color-space-CIELAB)
               (| (& (image-coding=JPEG)
                     (image-coding-constraint=JPEG-T4E) )
                  (& (image-coding=JBIG)
                     (image-coding-constraint=JBIG-T43)
                     (JBIG-stripe-size=128)
                     (image-interleave=stripe) ) )
               (dpi=[100,200,300])
               (dpi-xyratio=1) ) )
         (MRC-mode=0)
         (paper-size=[A4,B4]) )

4.4 Full-color Internet fax system

   This adds 16-bit full-color to the previous example.




Klyne & McIntyre            Standards Track                    [Page 13]

RFC 2531        Content Feature Schema for Internet Fax       March 1999


      (& (| (& (color=Binary)
               (image-file-structure=[TIFF-S,TIFF-F,TIFF-J])
               (| (image-coding=[MH,MR,MMR])
                  (& (image-coding=JBIG)
                     (image-coding-constraint=JBIG-T85)
                     (JBIG-stripe-size=128) ) )
               (| (& (dpi=200) (dpi-xyratio=200/100) )
                  (& (dpi=200) (dpi-xyratio=1) )
                  (& (dpi=204) (dpi-xyratio=204/391) )
                  (& (dpi=300) (dpi-xyratio=1) ) ) )
            (& (| (& (color=Grey) (color-levels<=256) )
                  (& (color=Full) (color-levels<=65536)
                     (color-subsampling=["1:1:1","4:1:1"]) ) )
               (image-file-structure=[TIFF-C,TIFF-L])
               (color-space=CIELAB)
               (| (& (image-coding=JPEG)
                     (image-coding-constraint=JPEG-T4E) )
                  (& (image-coding=JBIG)
                     (image-coding-constraint=JBIG-T43)
                     (JBIG-stripe-size=128)
                     (image-interleave=stripe) ) )
               (dpi=[100,200,300])
               (dpi-xyratio=1) ) )
         (MRC-mode=0)
         (paper-size=[A4,B4]) )

4.5 Full-color Internet fax system (MRC)

      (& (| (& (color=Binary)
               (image-file-structure=[TIFF-S,TIFF-F,TIFF-J])
               (MRC-mode=0)
               (image-coding=[MH,MMR])
               (| (& (dpi=200) (dpi-xyratio=[200/100,1]) )
                  (& (dpi=204) (dpi-xyratio=204/391) )
                  (& (dpi=300) (dpi-xyratio=1) )
                  (& (dpi=400) (dpi-xyratio=1) ) ) )
            (& (image-file-structure=[TIFF-C,TIFF-L])
               (| (& (color=Grey) (color-levels<=256) )
                  (& (color=Full) (color-levels<=65536)
                     (color-subsampling=["1:1:1","4:1:1"]) ) )
               (color-space=CIELAB)
               (MRC-mode=0)
               (image-coding=JPEG)
               (image-coding-constraint=JPEG-T4E)
               (dpi=[100,200,300,400])
               (dpi-xyratio=1) )