rfc2531.txt

<VC++网络游戏建摸与实现>源代码

         Print and display applications where different media choices
         will be made depending on the size of the recipient device.

   -  Examples of typical use:

         This example describes the maximum scanned image width and
         height for Group 3 fax: 215x297 mm (8.46x11.69 inches):

         (size-x<=2150/254)
         (size-y<=2970/254)







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RFC 2531        Content Feature Schema for Internet Fax       March 1999


   -  Related standards or documents:

         The memo "Media Features for Display, Print, and Fax" [3]
         describes features (pix-x, pix-y) for measuring document size
         in pixels.

         Fax applications should declare physical dimensions using the
         features defined here.

   -  Considerations particular to use in individual applications,
      protocols, services, or negotiation mechanisms:

         Where no physical size is known or available, but a pixel size
         is known, a notional size should be declared based upon known
         pixel dimensions and a notional resolution of (say) 100dpi

         For example, to describe a 640x480 pixel display:

            (& (size-x<=640/100) (size-y<=480/100) (dpi=100) )

         The notional 100dpi resolution is used as it represents a
         fairly typical resolution for a pixel-limited display.
         Reducing the rational numbers to canonical form gives the
         following equivalent expression:

            (& (size-x<=32/5) (size-y<=24/5) (dpi=100) )

   -  Interoperability considerations:

         For interoperability with other (non-fax) applications that use
         only pixel-based measurements, pixel dimensions (pix-x, pix-y)
         may be declared in addition to physical measurements.

   -  Related feature tags:

         pix-x                 [3]
         pix-y                 [3]
         dpi                   [3]
         dpi-xyratio           [this document]

   -  Intended usage:

         Common

   -  Author/Change controller:

         IETF




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RFC 2531        Content Feature Schema for Internet Fax       March 1999


A.2 Resolution aspect ratio

   -  Media Feature tag name(s):

         dpi-xyratio

   -  ASN.1 identifier associated with this feature tag:

         1.3.6.1.8.1.9

   -  Summary of the media features indicated:

         This feature is used to indicate differential horizontal and
         vertical resolution capability.  In the absence of this
         feature, horizontal and vertical resolutions are presumed to be
         the same.

         When this feature tag is specified, any declared resolution
         (dpi) is presumed to apply to the horizontal axis, and the
         vertical resolution is obtained by dividing that declared
         resolution by the resolution ratio.

         The value of this feature is a pure number, since it represents
         the ratio of two resolution values.

   -  Values appropriate for use with this feature tag:

         Rational (>0)

   -  The feature tag is intended primarily for use in the following
      applications, protocols, services, or negotiation mechanisms:

         Internet fax, and other print or display applications that must
         handle differential horizontal and vertical resolution values.

   -  Examples of typical use:

         The following example describes a fax resolution of 204 dpi
         horizontally by 391 dpi vertically:

            (& (dpi=204) (dpi-xyratio=204/391) )

   -  Related standards or documents:

         The memo "Media Features for Display, Print, and Fax" [3]
         describes a feature (dpi) for measuring document resolution.





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RFC 2531        Content Feature Schema for Internet Fax       March 1999


   -  Interoperability considerations:

         When interoperating with an application that does not recognize
         the differential resolution feature, resolution matching may be
         performed on the basis of the horizontal resolution only, so
         aspect ratio information may be lost.

   -  Related feature tags:

         dpi                   [3]
         size-x                [this document]
         size-y                [this document]

   -  Intended usage:

         Internet fax

   -  Author/Change controller:

         IETF

A.3 Color levels

   -  Media Feature tag name(s):

         color-levels

   -  ASN.1 identifier associated with this feature tag:

         1.3.6.1.8.1.10

   -  Summary of the media features indicated:

         This feature tag is used to indicate a number of different
         image data pixel color values.

         When mapped (palettized) color is used, this is generally
         different from the number of different colors that can be
         represented through the color mapping function.

         This feature tag is used in conjunction with a 'color' feature
         having a value other than 'Binary'.

   -  Values appropriate for use with this feature tag:

         Integer  (>=2)





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RFC 2531        Content Feature Schema for Internet Fax       March 1999


   -  The feature tag is intended primarily for use in the following
      applications, protocols, services, or negotiation mechanisms:

         Color image printing or display applications where the data
         resource used may depend upon color handling capabilities of
         the recipient.

   -  Examples of typical use:

         To describe recipient capabilities:
         (& (color=limited) (color-levels<=6) )
         (& (color=grey)    (color-levels<=64) )
         (& (color=mapped)  (color-levels<=240) )
         (& (color=full)    (color-levels<=16777216) )

         To describe capabilities used by a document:
         (& (color=limited) (color-levels=4) )
         (& (color=grey)    (color-levels=48) )
         (& (color=mapped)  (color-levels=100) )
         (& (color=full)    (color-levels=32768) )

   -  Related standards or documents:

         The memo "Media Features for Display, Print, and Fax" [3]
         describes a feature (color) for indicating basic color
         capabilities.

   -  Interoperability considerations:

         The actual number of color values used by a document does not,
         in general, exactly match the number that can be handled by a
         recipient.  To achieve a feature match, at least one must be
         declared as an inequality.

         It is recommended that a recipient declares the number of color
         values that it can handle as an inequality (<=), and a data
         resource declares the number of colors that it uses with an
         equality, as shown in the examples above.

   -  Security considerations:

      - Privacy concerns, related to exposure of personal information:
        Where feature matching is used to select content applicable to
        the physical abilities of a user, unusual values for this
        feature tag might give an indication of a user's restricted
        abilities.





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RFC 2531        Content Feature Schema for Internet Fax       March 1999


   -  Related feature tags:

         color                 [3]
         color-space           [this document]

   -  Intended usage:

         Internet fax
         Color image scanning/rendering applications

   -  Author/Change controller:

         IETF

A.4 Color space

   -  Media Feature tag name(s):

         color-space

   -  ASN.1 identifier associated with this feature tag:

         1.3.6.1.8.1.11

   -  Summary of the media features indicated:

         This feature indicates a color space.

         A color space value provides two types of information:
         o  the color model used to represent a color value, including
         the number of color components
         o  a mapping between color values and their physical
         realizations

         Device color space values are defined for applications where
         the general color representation used is significant, but exact
         color rendering is left to the device used.  Device color
         spaces defined here have values of the form 'Device- xxx'.

         Calibrated color space values are provided for use with a
         rendering system that is calibrated with respect to some
         indicated definition, and capable of processing device-
         independent color information accordingly.

   -  Values appropriate for use with this feature tag:

         Token




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RFC 2531        Content Feature Schema for Internet Fax       March 1999


         Device color      Device-RGB   (device dependent RGB)
         spaces:           Device-CMY   (device dependent CMY)
                           Device-CMYK  (device dependent CMYK)

         Calibrated color  CIELAB       (per T.42 [9])
         space:

                           (may be extended by further registrations)

         'Color-space=CIELAB' indicates the CIE L*a*b* colour space,
         using CIED50 illuminant and its perfectly diffuse reflecting
         white point (per T.42 [9]).

   -  The feature tag is intended primarily for use in the following
      applications, protocols, services, or negotiation mechanisms:

         Color image printing and display applications where the data
         resource used may depend upon color handling capabilities of
         the recipient.

         Scanning applications where the data transferred may depend
         upon the image generation capabilities of the originator.

   -  Examples of typical use:

         To describe rendering or scanning capabilities:

         (color-space=[Device-RGB,CIELAB])

         To describe capabilities assumed by a document for which
         approximate color reproduction is required:

         (color-space=Device-RGB)

         To describe capabilities assumed by a document for which exact
         color reproduction is required:

         (color-space=CIELAB)

   -  Related standards or documents:

         CIELAB color space is defined in [19]

         CIELAB use for fax is described in ITU T.42 [9]







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RFC 2531        Content Feature Schema for Internet Fax       March 1999


   -  Interoperability considerations:

         A color-handling receiver should indicate at any appropriate
         device color space capability, in addition to any calibrated
         color spaces that it may support.

         Calibrated color spaces are intended to be used when precise
         color matching is required; otherwise, if applicable, a device
         color space (color-space=Device-xxx) should be indicated.

         Documents for which exact color matching is not important
         should indicate a device color space capability, if applicable.

         These principles allow sender/receiver feature matching to be
         achieved when exact color matching is not required.

   -  Security considerations:

      - Privacy concerns, related to exposure of personal
        information:
            Where feature matching is used to select content applicable
            to the physical abilities of a user, unusual values for this
            feature tag might give an indication of a user's restricted
            abilities.

      - Denial of service concerns related to consequences of
        specifying incorrect values:
            Failure to indicate a generic color space capability for a
            device may lead to failure to match color space for an
            application or document that does not require an exact color
            match.

   -  Related feature tags:

         color                 [3]

   -  Related media types or data formats:

         TIFF-FX               [7]

   -  Intended usage:

         Internet fax
         Color image scanning/rendering applications

   -  Author/Change controller: