The RenderMan Interface - Paul Bourke

15.7 Texture Mapping Functions
Texture maps are images that are mapped onto the surface of a geometric primitive. The
RenderMan Interface supports three primitive types of texture access: basic texture maps
(via texture), environment maps (via environment), and shadow or z-buffer maps (via
shadow). Texture maps are accessed using two-dimensional coordinates and return floats
or colors. Environment maps are accessed using a direction and return floats or colors.
Shadow maps are accessed using points and return floats.
For two-dimensional access (texture), the texture coordinates default to the texture coordinates
attached to the surface, (s,t). These default texture coordinates are equal to the
surface parameters, the current texture coordinates, or the texture coordinates passed with
the geometric primitive. Texture coordinates can also be computed in the Shading Language.
This generality allows for many different types of coordinate mappings. Images
stored in various map projections can be accessed by computing the map projection given
a point on a sphere. This allows basic texture maps to be used as environment maps. Images
can also be mapped onto surfaces using a two step process. First the surface of the
geometric primitive is mapped to the surface of a parametric primitive, such as a plane or
cylinder, and then the parameters of this primitive are used as the texture coordinates. This
is sometimes called a decal projection.
For three-dimensional access (environment and shadow), the texture coordinates must always
be explicitly specified.
There is no restriction on how texture map values are used in the Shading Language. For
example, displacement mapping can be performed by moving a point on the surface in
the direction of the normal by the amount returned by a basic texture map. Transparency
mapping differs from color mapping only in which variable, either Os or Cs, the texture
is assigned to. There is also, in principle, no limit on the number of texture accesses per
shader or the number of texture maps per shader or per frame.
Texture maps are created in advance from image data via three types of MakeTexture procedures
that are defined as part of the RenderMan Interface. These are described in Part
I in the section on Texture Map Utilities. RiMakeTexture creates a texture map for access
via texture. RiMakeCubeFaceEnvironment and RiMakeLatLongEnvironment create an
environment map for access via environment. RiMakeShadow creates a shadow map for
access via shadow. A texture file may contain several channels of information and have any
horizontal or vertical resolution. This information is normally inherited from the image
from which the texture is made. The s coordinate is assigned to the horizontal direction
with increasing values moving right. The t coordinate is assigned to the vertical direction
with increasing values moving down. These coordinates are normalized to lie in the range
0 to 1 so that changing the resolution of the texture map has no effect on the shaders that
access the texture map. When a texture map is created, the wrap mode is also specified.
The wrap mode controls what values are returned if the texture coordinates fall outside the
unit square. Allowed wrap modes are: periodic, black and clamp. periodic causes the texture
data to tile the plane, black causes accesses outside the unit square to return the value 0, and
clamp causes the texture coordinates to be clamped to the closest point on the unit square
and the texture value associated with that point to be returned.
The texture access functions normally pass the texture map through a low-pass filter to
143