The RenderMan Interface - Paul Bourke

Section 7
EXTERNAL RESOURCES
7.1 Texture Map Utilities
The format of the various texture map files is implementation dependent. However, there
are standard utilities that convert image files into texture map files.
During two-dimensional texture access, texture coordinates (s, t) are mapped onto the texture
such that s=0 maps to xmin, s=1 maps to xmax+1, t=0 maps to ymin, and t=1 maps to
ymax+1. To be precise, all accesses to the half-open interval [0,1) in s and t will lie within
the picture data.
A wrapmode describes how the texture is accessed if the texture coordinates are outside the
unit square (less than zero, or greater than or equal to one). The swrap and twrap strings
specify the wrapping behavior of the s and t coordinates. The standard wrapping behavior
for s and t, ”black”, is to return the value zero for all accesses outside the unit square.
(Thus an RGBa texture will be transparent black, zero on all four channels.) The keyword
”periodic” indicates that values of s (or t) outside [0,1) will be mapped into [0,1) by subtracting
the largest integer less than or equal to the coordinate (the “floor” of the coordinate).
This will wrap the value 1 back to 0, the value 1.25 to 0.25, and the value -0.1 to 0.9. The
result will be to repeat the texture as a tile that fills texture space in the s (or t) direction.
The keyword ”clamp” indicates that values of s (or t) outside [0,1) will be mapped into [0,1)
by clamping them at their minimum and maximum values. All values below zero will
be clamped to zero and all values greater than or equal to one will be clamped to a value
slightly less than one (at the last texture pixel).
Textures are often prefiltered so that subsequent antialiasing calculations can be done more
quickly at run-time. This is controlled by giving a filterfunc, which is the same as the filterfunc
used in RiPixelFilter, and an swidth and twidth.
7.1.1 Making texture maps
Surface textures are used to modify the properties of a surface, such as color and opacity.
A surface texture is accessed using the surface texture coordinates (see the section on Texture
coordinates) or any other two-dimensional coordinates computed by a user-defined
shader. A surface texture consists of one or more channels. A single channel or a group of n
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