Under the hood

Listening To : 'Sensitive Artist' by King Missile.

To paraphrase an email from a work colleague..

Pixel samples have no effect on when and where the shader gets executed. The shading rate and other dicing settings determine how the geometry is tessellated into micropolygons. Then the shader is executed at each vertex on the micropolygon. The pixel samples then just determine how this pre-shaded micropolygonal geometry is sampled for each pixel.

Not only do pixel samples have nothing to do with shading, they also have nothing to do with raytracing. Shooting rays into a cone an angle of 0 will always hit the same spot regardless of samples - the pixar docs appear incorrect about this, based on experimentation. Also, it's not necessary to specify the cone angle at all if using the simpler environment() and trace() routines. I believe renderman will compute the optimal cone angle automatically based on shading rate and surface derivatives. Although in tests, geometric aliasing still occurred with trace, and only making a coneangle based on the change in incident angle across a micropolygon with gather seemed to remove it.

Watch those param orderings!

Listening To : Shooby Taylor, the human horn.

Refract and Faceforward have I and N paramaters specified in different orders! How many times has that caught me out ?!?!?

     vector refract( vector I, N; float eta )
     vector faceforward( vector N, I )

Also clamp and mix/smoothstep have different orderings.

     type clamp( type a, min, max )
vs
     type mix( type x, y; float alpha )
     type smoothstep( type min, max, value )