The Disney BRDF Explorer is an application that allows the development and analysis of bidirectional reflectance distribution functions (BRDFs). It can load and plot analytic BRDF functions (coded as snippets of OpenGL GLSL programs), measured material data from the MERL database, and anisotropic measured material data from MIT CSAIL.
Most of the application should be (hopefully) self explanatory, so the goal here is to document the less obvious bits of usage, as well as providing some useful info.
WHICH BRDF(S) YOU'RE SEEING
In the BRDF Parameters window, each BRDF has a colored background. The plotted values (for the 3D, polar, and cartesian plots) are drawn in the corresponding colors.
In the Image Slice, Lit Object, and Lit Sphere windows, you're seeing the first (topmost in the parameters window) enabled BRDF.
NAVIGATING POLAR/CARTESIAN PLOTS
Use the left mouse button to pan around in the plot. Right dragging will zoom in/out around the centered location. Double-clicking anywhere resets the view to the default zoom and location. For cartesian plots, Control+left drag stretches ONLY the x-axis, while Control+right drag stretches only the y-axis.
NAVIGATING 3D PLOTS
Left-drag spins around the origin (there's no current way to change this). Right drag zooms in and out. Double clicking resets to the default view.
PARAMETER SLIDERS
Parameters sliders (a slider plus a text entry box) are all over the application. To reset one to its default value, Control+Click in the text box.
SOLOING BRDFS
Pressing the "solo" (circle) button for a given BRDF makes ONLY that BRDF visible, hiding all the others. The "Solo this BRDF's color channels" button also hides all the other channels, but shows a separate plot for the red, green, and blue channels (in their respective colors). Click the solo button again to exit solo mode.
LIT SPHERE VIEW
You can drag the left mouse button on the surface of the sphere to change the incident angle. If "Double Theta" is enabled, the specular highlight will track the position of the mouse.
LIT OBJECT VIEW
The lit object view allows an arbitrary object to be viewed under with a directional light from the incident direction (in "No IBL" mode) or under illumination from an arbitrary environment map. Left dragging rotates the object; right dragging zooms the object; Control+left dragging rotates the environment probe.
The combo box lets you choose "No IBL", "IBL: No IS" (quasirandom sampling from the environment map with no importance sampling) and "IBL: IBL IS" (importance sampling from the IBL). Multiple importance sampling is planned but not implemented. The "Keep Sampling" mode, when enabled, progressively refines the image until 4096 samples have been applied.
The buttons allow changing out the object (any OBJ should work) and the environment probe (which must be in ptex format).
ALBEDO VIEW
Albedo computation by brute force sampling proved too expensive to do interactively, so the application can use several different sampling strategies to compute the albedo. Use the combo box on the right to choose between these sampling strategies. The Resample x10 button adds more samples to that view of the graph.
IMAGE SLICE VIEW
An "image slice" is an alternative way of looking at BRDF data that we have found helpful at Disney. Along the x-axis, the half angle is varied from zero to 90 degrees; along the y-axis, the difference angle is varied from 0 to 90 degrees.
BRDF FILES
.brdf files consist of a set of parameters and a BRDF function written in GLSL. At runtime, the application creates UI elements for each parameter, and creates shaders for different views that incorporate the GLSL function.
The BRDF function looks like this (this example is for a lambertian BRDF):
::begin shader vec3 BRDF( vec3 L, vec3 V, vec3 N, vec3 X, vec3 Y ) { return vec3(reflectance / 3.14159265); } ::end shader
Anything valid in GLSL can go between "::begin shader" and "::end shader" as long as it's valid GLSL.
The application allows float, color, and boolean parameters. The float parameters have the form float [name] [min val] [max val] [default val] for example: float reflectance 0.0 1.0 1.0
Boolean parameters have the form bool [name] [default], where default is 0 or 1 (keywords such as true and false aren't recognized) for example: bool hasDiffuse 0
Color parameters have the form color [name] [defaultR] [defaultG] [defaultB], where defaultR/G/B are in [0..1] for example: color diffuseColor 0.5 0.1 1.0
Parameters are passed into the resulting GLSL shaders as uniforms of the same name (so parameter names must be valid GLSL variable names, although the application doesn't enforce this). Float parameters come in as uniform floats, color parameters as vec3s, and boolean parameters as bools. The application declares them when constructing shaders, so your GLSL BRDF functions can refer to them knowing that they'll exist and have the proper values at runtime.
LIGHT PROBE ATTRIBUTION
HDR Light Probe Images Copyright 1998 courtesy of Paul Debevec, www.debevec.org, used with permission.
Please see: Paul Debevec. Rendering Synthetic Objects Into Real Scenes: Bridging Traditional and Image-Based Graphics With Global Illumination and High Dynamic Range Photography. Proceedings of SIGGRAPH 98, Computer Graphics Proceedings, Annual Conference Series, July 1998, pp. 189-198.
WHERE TO GET MEASURED BRDF DATA
MERL data can be requested here: http://www.merl.com/brdf/
MIT CSAIL data is here: http://people.csail.mit.edu/addy/research/brdf/
BUILD INSTRUCTIONs
To install brdf run:
make install [prefix=<some-directory>]
If you do not specify "prefix=..." then brdf will be installed into the current directory.