RManNotes: Miscellaneous Stuff

Differences Between PRMan and BMRT

The main differences between PRMan and BMRT are outlined below (adapted from Gritz's BMRT manual):

PRMan and BMRT use incompatible Shading Language object files (".slo" and ".slc" extensions respectively).
PRMan uses a special format for texture files; BMRT uses TIFF files.
PRMan uses shadow (depth) maps for shadow generation; BMRT can use shadow maps or ray trace the shadows.
BMRT correctly supports area light sources; PRMan doesn't.
In BMRT, the trace() function is fully implemented (in PRMan, trace() just returns 0).

Shading Language Limitations

There are several limitations which apply to the RenderMan Shading Language in general. They are listed immediately below. Other caveats specific to the renderers are listed in their respective sections.

shader parameters should not be modified by a shader;
recursion is not allowed;
user function parameters are passed by reference;
global variables are not available inside user functions; some standard functions also cannot be called if they are specific to particular types of shaders;

With PRMan

PRMan's Shading Language compiler has several limitations which are itemized here. For additional information, refer to Section 5 of the PhotoRealistic RenderMan User's Manual.

Programmable imager, transformation, and volume shaders are not supported;
incident(), opposite(), bump(), and trace() all return 0.0;
There should only be one shader or function per Shading Language source file;
area functions should not be used inside conditionals (texture, environment, bump, shadow, Deriv, Du, Dv, area, calculatenormal);
shaders using a user function must be recompiled if the user function is modified;
user-defined functions should only have one return statement;
diffuse, specular, and solar functions vary from interface specification; and

With BMRT

BMRT's Shading Language compiler has a few limitations and they are listed below. Please consult the most recent Blue Moon Rendering Tools: User Guide as the definitive reference.

Transformation shaders are not supported

Where Are the Shaders At ACCAD?

When a RenderMan renderer is executed, it must be able to find a Shading Language object for every shader needed to render the scene. In the default ACCAD environment, both PRMan and BMRT look for shader object files in the following directories in the following order:

the current working directory;
/usr/local/accad/shaders -- an ACCAD directory for shaders which have been contributed from sources including The RenderMan Companion, students, and staff; and
the standard shader directory -- /usr/local/prman/lib/shaders for PRMan and /usr/local/bmrt/shaders for BMRT. These are the shaders distributed with the software packages and include the shaders required by the RenderMan Interface Specification.

The search path used for finding shaders can be modified by the user. In PRMan, configuration files can be customized to change the search path. In BMRT, the environment variable SHADER specifies the search path and can be overridden by the user using setenv. The RenderMan Interface also allows search paths to specified, so in both packages, RIB can be used for this purpose.

Where Are the Textures At ACCAD?

In the default ACCAD environment PRMan looks for texture files in the following directories in the following order:

the current working directory;
/usr/local/prman/lib/textures -- a few textures distributed with PRMan

Most of the time, it's more flexible and space-saving to store textures as TIFFs (or some other image format). Then TIFFs can then be converted to texture files for use with PRMan (BMRT reads TIFFs directly as texture files) using txmake or RIB. You can find many textures stored as TIFFs in:

/usr/local/accad/textures -- many assorted texture types and sizes, most untilable; and
/usr/local/accad/textures/Pixar128 -- 512x512 tilable textures which are from Pixar's CD-ROM.

Like shaders, the search path used for finding textures can be modified by the user. In PRMan, configuration files can be customized to change the texture search path. Also like shaders, RIB commands can be used to specify texture file search paths for both PRMan and BMRT.

Any comments or suggestions appreciated.

Steve May (smay@pixar.com)