The vzTMTangentSpaceShader is a built-in shader to be used with the
vzTMRenderAction. This shader implements a gradientless-shading
algorithm to provide volumetric lighting.
The given volume texture is rendered with an infinite directional light
source. The shader also uses texture lookup tables to modulate the output
color values.
The shader uses a two-pass algorithm to approximate the dot product of the
gradient vector with the light vector, at each point within the volume. This
dot product is then used to modulate the texture values during rendering.
For details, refer to Peercy et.al, 'Efficient Bump Mapping Hardware',
Computer Graphics, Proc. SIGGRAPH '97.