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to Particles and Procedural Effects
Updated: Sept 3 2013 .
PBR, VEX and bsdf
A very popular technique right now is physically
based rendering. Houdini uses a physically based renderer inside
of mantra called PBR. This involved using an algorithm that is
more physically based to take advantage of global illumination
models. These kinds of algorithms have existed for several decades
but have become very production friendly now that hardware needs
have caught up.
Pixar, using renderman, has now started the process of using
physically based lighting in their pipeline (Siggraph 2013 - included Katana -
Pixar's
Fast Lighting Preview with NVIDIA Technology).
Lighting, shading and rendering are all heavily inter-related so
this is simply a brief overview of how to get up to speed with
bsdf in your vex/vops shaders for use with pbr in Houdini.
What is BSDF?
Bidirectional
scattering distribution function - was probably introduced
in 1991 by Paul Heckbert. It is often used to name the
mathematical function which describes how light is scattered by a
surface. In practice it is split into the reflected (BRDF) and
transmitted (BTDF) components.
The general concept is of an incoming/incident
ray and an outgoing/reflected or transmitted ray and a result of
some kind to describe what happens. Subsurface scattering is
a specialized case of BTDF.
There is an excellent explanatory article at
scratchapixel on this subject here.
Also below are excerpts from H12.5 document on understanding
mantra rendering.
PBR (physically based rendering) is a shading process that supports
accurate physical simulations of lighting, shadows, and shading. It
can work with either sampling methods (raytracing or micropolygon).
Whereas in traditional shading the shader programs control the
number and distribution of secondary rays, in PBR secondary rays are
controlled by the surface’s BSDF (Bidirectional Scattering
Distribution Function).
In PBR, the surface shader is responsible for computing the BSDF of
the surface. Whereas traditional surface shaders set Cf
(surface color) and Of (surface opacity) color values,
surface shaders for PBR set F to a bsdf
value (bsdf is a VEX datatype introduced in Houdini 9).
VEX
Below is a simple shader from sidefx
documentation that works for traditional and PBR algorithms.
001 surface simple()
002 {
003 vector nn = nomalize(frontface(N, I));
004 Cf = diffuse(nn);
005 F = diffuse();
006 }
Lines 3 and 4 are needed for the traditional
pipeline, however in PBR, only line 5 is needed. Thus if you
encounter an older shader without F set, it will render black in
PBR.
If you wanted to modify this to include a parameter for color it
might look like this:
surface simple( vector basecolor = {1,0,0} )
{
vector nn = normalize(frontface(N, I));
Cf = diffuse(nn) * basecolor;
F = diffuse() * basecolor;
}
Traditional surface shaders can cause light and
shadow shaders to run using an illuminance loop. Inside this loop
are Cl (light color), shadow(Cl) - uses the shadow shader, and
L(direction from the surface to the light).