Blue Moon Rendering Tools 2.4

Part 7: Miscellaneous Tools and Goodies

revised 27 August 1998

Larry I. Gritz

7.1 Parsing Shader Arguments with libslcargs.a

Pixar's PhotoRealistic RenderMan implementation provides a linkable library which allows a developer to read a compiled shader file (.slo) to determine what type of shader it is and what parameter names and defaults belong to that shader. Since Pixar's .slo format is different from BMRT's .slc format, a similar library is provided to parse the .slc files. The library is libslcargs.a, and the C language header file is slc.h. The slc.h file should be fairly self-documenting, and certainly anybody with experience using Pixar's libsloargs.a library ought to have an easy time of using libslcargs.a.

7.2 Using slctell to list shader arguments

Another binary, slctell, reports the type of shader and its parameter names and default values. Usage is simple: just give the shader name on the command line (NOT the file name). For example,
	slctell plastic
	surface "shaders/plastic"
	    "Ka" "uniform float"
	                Default value: 1
	    "Kd" "uniform float"
	                Default value: 0.5
	    "Ks" "uniform float"
	                Default value: 0.5
	    "roughness" "uniform float"
	                Default value: 0.1
	    "specularcolor" "uniform color"
	                Default value: "rgb" [1 1 1]

7.3 Simple Image Compositing with composite

Starting with release 2.3.6, BMRT includes a program to perform elementary image compositing operations. If you render your images with alpha channels (i.e. "rgba"), then coverage information will be stored with every pixel in the image. For the purposes of composite, RGB images without alpha channels will be assumed to have an alpha of 1.0 at every pixel.

composite may be run as follows:

composite file1 over file2 -o output
composite file1 in file2 -o output
composite file1 out file2 -o output
composite file1 atop file2 -o output
composite file1 xor file2 -o output
Composite images file1 and file2 using one of the standard image compositing operators described in (Porter & Duff, "Digital Image Compositing", Proceedings of SIGGRAPH '84, pp. 253-259), storing the composited image in file output.

composite file1 plus file2 -o output
composite file1 minus file2 -o output
Add or subtract two files, storing the results in file output. Pixels are clamped to [0,maxval], where maxval==255 for 8 bit images, maxval==65535 for 16 bit images.

composite file1 scale float -o output
composite file1 dissolve float -o output
composite file1 opaque float -o output
These three unary operators take a floating point number, rather than a filename, as their second operand. They all scale the channels of the image, but in slightly different ways. The scale operator multiples the RGB channels, but leaves the alpha alone -- i.e. it can brighten or darken an image without changing its transparency. The dissolve operator scales the alpha along with the RGB. Finally, the opaque operator will scale only the alpha channel.

Hint for beginners: you probably want over.

7.4 Making tiled TIFF files with mkmip

BMRT has always used TIFF files for stored image textures (as opposed to PRMan, which requires you to convert to a proprietary texture format). Beginning with BMRT 2.4, though BMRT will continue to accept regular scanline (or strip) oriented TIFF files, it is able to perform certain optimizations if the TIFF files you supply happen to be tile-oriented. In particular, BMRT is able to significantly reduce the memory needed for texture mapping with tiled TIFF files.

A new program supplied with BMRT, mkmip, can convert scanline TIFF files into multiresolution, tiled TIFF files. Command line usage is:

        mkmip [options] infilename outfilename

where options include:

-smode wrapmode
-tmode wrapmode
-mode wrapmode
where wrapmode is one of: periodic, black, or clamp. This specifies the behavior of the texture when outside the [0,1] lookup range. Note that -smode and -tmode specify wrapping behavior separately for the s and t directions, while -mode specifies both at the same time.

7.5 Setting default options and attributes

Remember that both of BMRT's renderers (rendrib and rgl) read from a file called .rendribrc both in the local directory where it is run, and also in your home directory. This file can be plain RIB, which means that if you want to set any defaults (default resolution, shader search path, texture cache size, etc.) you can just put the Option or Attribute lines in this file in your home directory.

7.6 farm: Poor Man's Render Farm

Many people ask how they can divide rendering of a single frame among several processors or machines. Starting with BMRT 2.4, the Perl script farm accomplishes this task, in a relatively rudimentary way.

How to use farm

  1. Set the environment variable BMRT_FARM to be a blank-separated list of the names of machines which can be used as render servers. Machines with multiple processors should be listed multiple times. For example, if you have a machine named "fred" with two processors, and one named "wilma" with one processor, then run:

    setenv BMRT_FARM "fred fred wilma"

    if you use csh. If you use sh, try:

    export BMRT_FARM="fred fred wilma"

  2. Make sure that rendrib is in the default path of each remote machine, and that mkmosaic is in the path on the local machine.

  3. Run farm:

    farm myfile.rib

What farm does

First, farm will look at your RIB file to figure out the resolution and the name of the TIFF file that it will render. It will choose an appropriate number of subwindows to render.

One by one, it will send the frame to machines on your BMRT_FARM list, using the -crop and -of flags to make rendrib render particular crop windows. Machines whose load averages are too high will automatically refuse the frames.

When farm sees that all the subsections are finished (each will leave a little file indicating that it's done), it will assemble all the pieces using the mkmosaic program, and clean up all the cruft files.

Important farm restrictions

  1. Because farm relies on rsh, you can only use it on UNIX (or UNIX-like) operating systems.
  2. You can't use farm to render to the display (the -d flag). It must be rendering to a TIFF file.
  3. Don't try using any other rendrib command line flags. Request all image options (like radiosity options) in the RIB file with Option and Attribute statements.
  4. Hitting Control-C to interrupt farm will kill only farm, but will leave the individual crop windows rendering on the remote machines. Beware.


Upstill, Steve. The RenderMan Companion: A Programmer's Guide to Realistic Computer Graphics. Addison-Wesley, 1989.
Pixar. "The RenderMan Interface, version 3.1 official specification." Published by Pixar, 1989.
Apodaca, A., ed. "The RenderMan Interface and Shading Language," Siggraph '90 course notes (course 18), 1990.
Apodaca, A., ed. "Writing RenderMan Shaders," Siggraph '92 course notes (course 21), 1992.
Apodaca, A., ed. "Using RenderMan for Animation Production," Siggraph '95 course notes (course 4), 1995.
Apodaca, A. and L. Gritz, eds. "Advanced RenderMan: Beyond the Companion" Siggraph '98 course notes (course 11), 1998.
Gritz, Larry. "Computing Specular-to-Diffuse Illumination for Two-Pass Rendering," Master's Thesis, The George Washington University, Dept. of EE & CS, May, 1993.
Gritz, Larry. "RenderMan for Poets," Technical Report GWU-IIST-94-05, Dept. of EE&CS, The George Washington University, 1994.
Gritz, Larry and J. Hahn. "BMRT: A Global Illumination Implementation of the RenderMan Standard," Journal of Graphics Tools, Vol. 1, No. 3, pp. 29-47 (1996).

This document last updated 27 Aug 98 by

All BMRT components are Copyright 1990-1998 by Larry I. Gritz. All rights reserved.

The RenderMan Interface Procedures and RIB Protocol are: Copyright 1988, 1989, Pixar. All rights reserved. RenderMan is a registered trademark of Pixar.