The RenderMan Interface - Paul Bourke

prevent aliasing. If one set of texture coordinates is given to the access function, the texture
will be filtered over the area of the surface element being shaded (see the Shading
Rate section in Part I). Four sets of texture coordinates can also be given to the access procedure,
in which case the texture is filtered over the quadrilateral determined by those
four points. The quality of texture antialiasing is controlled in the same way as spatial
antialiasing. Parameters control how true the answer is, the effective number of samples
used before filtering, and the type and width of the filter used. For this to be done properly
(since texture maps are normally prefiltered), these filtering parameters are best given to
the appropriate RiMake... procedure. For flexibility, however, they can also be changed at
access time. Table 15.1, Texture Access Parameters gives the standard parameters to all the
texture access functions; particular implementations may have additional parameters. If a
parameter is encountered by an implementation that does not support its functionality, it
should be ignored.
Name Type Description
”blur” varying float Specifies an additional area to be added to the texture area
filtered in both the s and t directions, expressed in units of
texture coordinates. A value of 1.0 would request that the
entire texture file be blurred into the result. A value of
0.001 would request that one extra texture pixel be added
in the case of a one-thousand by one-thousand texture file.
”sblur” varying float Specifies ”blur” individually in the s direction.
”tblur” varying float Specifies ”blur” individually in the t direction.
”width” uniform float This value multiplies the width of the area being filtered
over in both the s and t directions. A value of 0 effectively
turns off texture antialiasing. The default value is 1.0.
”swidth” uniform float Specifies ”width” individually in the s direction.
”twidth” uniform float Specifies ”width” individually in the t direction.
”filter” uniform string Specifies the name of the filter to use for filtering over an
area of the texture. The default is ”box”. Individual implementations
may allow additional filters.
”fill” uniform float Specifies the value to be provided for channels requested
that are not present in the texture file. This is most often
useful for a shader that knows how to use a texture containing
an alpha channel. If no alpha channel is present in
the texture file, the texture system quietly provides a default
value of 0.0 to requests for the alpha value resulting
in a completely tranparent texture. Such a shader could
provide its own ”fill” value of 1.0 so that textures without
an alpha channel would be opaque by default.
Table 15.1: Texture and Environment Map Access Parameters
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