3D graphics questions that do not deserve their own thread (OpenGL / Dx10)

The Something Awful Forums > Discussion > Serious Hardware/Software Crap > The Cavern of COBOL > 3D graphics questions that do not deserve their own thread (OpenGL / Dx10)

piratepilates: Mar 28, 2004; So I will learn to live with it. Because I can live with it. I can live with it.

I don't know how much you guys know about path tracing and teh rendering equation but I've got a question about it:

In all of the simple algorithms for path tracing using lots of monte carlo samples that I see in lecture notes the tracing function of the algorithm randomly chooses between returning with the emitted value for the current surface and continuing by tracing another ray from that surface's hemisphere (for example in the slides here). Like so:

code:

TracePath(p, d) returns (r,g,b) [and calls itself recursively]:
    Trace ray (p, d) to find nearest intersection p�
    Select with probability (say) 50%:
        Emitted:
            return 2 * (Le_red, Le_green, Le_blue) // 2 = 1/(50%)
        Reflected: 
             generate ray in random direction d�
             return 2 * fr(d ->d�) * (n dot d�) * TracePath(p�, d�)

Why does it randomly choose between returning with the emissive element and continuing the path? Is this just a way of using russian roulette to terminate paths without bias? Surely it would make more sense to count the emissive and reflective properties for all ray paths together and use russian roulette just to decide whether to continue tracing or not.

Also why do the algorithms that don't use this choosing between emission and reflective components instead only count the first emissive element? The rendering equation for a point is the emissive element plus the integral of the incoming light, which is itself the emissive element for a point and the integral of incoming light of other surfaces, so why not count it for each bounce of each path?

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The Something Awful Forums > Discussion > Serious Hardware/Software Crap > The Cavern of COBOL > 3D graphics questions that do not deserve their own thread (OpenGL / Dx10)