Lecture 13: Global Illumination & Path Tracing (26)
grafour
I was confused the difference between sampling BRDF vs light sources and I found these procedures kinda helpful (note they are talking about glossy surface):
BSDF sampling (diagram (a)) procedure:
Cast a ray from the camera for a given pixel.
Upon hitting the glossy surface, sample the surface's BSDF, then continue along the newly sampled ray.
Hopefully, you hit the light, in which case you contribute energy to the given pixel.
This will obviously yield the noise profile we see in diagram (a); the noise goes down as the surface gets glossier (sharpening the distribution of the BSDF) and as the light gets bigger (making the target easier to hit by chance).
Light source sampling (diagram (b)) procedure(???):
Cast a ray from the camera for a given pixel.
Upon hitting the glossy surface, uniformly sample the solid angle that subtends the light source (i.e., a "shadow ray"), then continue along the newly sampled ray.
If you hit the light (i.e., no obstructions) contribute energy to the given pixel, weighted inversely by the BSDF of the glossy surface evaluated at the shadow ray, (and probably also multiplied by some other weight representing the fraction of our total solid angle that the light takes up, as in regular importance sampling).
when you generate a sample on a light source, you are doing it in terms of the surface area measure, whereas if you’re constructing a path, you’re normally doing it in terms of solid angle.
I was confused the difference between sampling BRDF vs light sources and I found these procedures kinda helpful (note they are talking about glossy surface):
BSDF sampling (diagram (a)) procedure: Cast a ray from the camera for a given pixel. Upon hitting the glossy surface, sample the surface's BSDF, then continue along the newly sampled ray. Hopefully, you hit the light, in which case you contribute energy to the given pixel. This will obviously yield the noise profile we see in diagram (a); the noise goes down as the surface gets glossier (sharpening the distribution of the BSDF) and as the light gets bigger (making the target easier to hit by chance).
Light source sampling (diagram (b)) procedure(???): Cast a ray from the camera for a given pixel. Upon hitting the glossy surface, uniformly sample the solid angle that subtends the light source (i.e., a "shadow ray"), then continue along the newly sampled ray. If you hit the light (i.e., no obstructions) contribute energy to the given pixel, weighted inversely by the BSDF of the glossy surface evaluated at the shadow ray, (and probably also multiplied by some other weight representing the fraction of our total solid angle that the light takes up, as in regular importance sampling).
when you generate a sample on a light source, you are doing it in terms of the surface area measure, whereas if you’re constructing a path, you’re normally doing it in terms of solid angle.