- the feature set of the current request,

     - a collection of quality values assigned to media types,
       languages, and charsets for the current request, following the
       model of the corresponding HTTP/1.1 [1] Accept- headers,

     - a table which lists `forbidden' combinations of media types and
       charsets, i.e. combinations which cannot be displayed because of
       some internal user agent limitation.

   The output of the algorithm is either the best variant, or the
   conclusion that none of the variants are acceptable.

19.1 Computing overall quality values

   As a first step in the local variant selection algorithm, the overall
   qualities associated with all variant descriptions in the list are
   computed.

   The overall quality Q of a variant description is the value

      Q = round5( qs * qt * qc * ql * qf * qa )

   where rounds5 is a function which rounds a floating point value to 5
   decimal places after the point.  It is assumed that the user agent
   can run on multiple platforms: the rounding function makes the
   algorithm independent of the exact characteristics of the underlying
   floating point hardware.

   The factors qs, qt, qc, ql, qf, and qa are determined as follows.

      qs Is the source quality factor in the variant description.

      qt The media type quality factor is 1 if there is no type
         attribute in the variant description.  Otherwise, it is the
         quality value assigned to this type by the configuration
         database.  If the database does not assign a value, then the
         factor is 0.





 
RFC 2295            Transparent Content Negotiation           March 1998


      qc The charset quality factor is 1 if there is no charset
         attribute in the variant description.  Otherwise, it is the
         quality value assigned to this charset by the configuration
         database.  If the database does not assign a value, then the
         factor is 0.

      ql The language quality factor is 1 if there is no language
         attribute in the variant description.  Otherwise, it is the
         highest quality value the configuration database assigns to any
         of the languages listed in the language attribute.  If the
         database does not assign a value to any of the languages
         listed, then the factor is 0.

      qf The features quality factor is 1 if there is no features
         attribute in the variant description.  Otherwise, it is the
         quality degradation factor computed for the features attribute
         using the feature set of the current request.

      qa The quality adjustment factor is 0 if the variant description
         lists a media type - charset combination which is `forbidden'
         by the table, and 1 otherwise.

   As an example, if a variant list contains the variant description

     {"paper.2" 0.7 {type text/html} {language fr}}

   and if the configuration database contains the quality value
   assignments

     types:     text/html;q=1.0, type application/postscript;q=0.8
     languages: en;q=1.0, fr;q=0.5

   then the local variant selection algorithm will compute the overall
   quality for the variant description as follows:

     {"paper.2" 0.7 {type text/html} {language fr}}
                 |           |                 |
                 |           |                 |
                 V           V                 V
       round5 ( 0.7   *     1.0        *      0.5 ) = 0.35000

   With same configuration database, the variant list

     {"paper.1" 0.9 {type text/html} {language en}},
     {"paper.2" 0.7 {type text/html} {language fr}},
     {"paper.3" 1.0 {type application/postscript} {language en}}

   would yield the following computations: