CODEWITHSUNDEEP
c++ctraceraytracing
crapper
crapper

Reputation: 179

Ray Tracing: Only use single ray instead of both reflection & refraction rays

I am currently trying to understand the ray tracer developed by Kevin Beason (smallpt: http://www.kevinbeason.com/smallpt/) and if I understand the code correctly he randomly chooses to either reflect or refract the ray (if the surface is both reflective and refractive).

Line 71-73:

return obj.e + f.mult(depth>2 ? (erand48(Xi)<P ?   // Russian roulette
radiance(reflRay,depth,Xi)*RP:radiance(Ray(x,tdir),depth,Xi)*TP) :
radiance(reflRay,depth,Xi)*Re+radiance(Ray(x,tdir),depth,Xi)*Tr);

Can anybody please explain the disadvantages of only casting a single ray instead of both of them? I had never heard of this technique and I am curious what the trade-off is, given that it results in a huge complexity reduction.

Upvotes: 5

Views: 925

Answers (2)

Michael Anderson
Michael Anderson

Reputation: 73440

This is a monte-carlo ray tracer. Its advantages are that you don't spawn an exponentially increasing number of rays - which can occur in some simple geometries.. The down side is that you need to average over a large number of samples. Typically you sample until the expected deviation from the true value is "low enough". Working out how many samples is required requires some stats - or you just take a lot of samples.

Upvotes: 3

George Duckett
George Duckett

Reputation: 32418

Presumably he's relying on super-sampling pixels and trusting that the average colour will work out roughly correct, although not as accurate.

i.e. fire 4 rays through one pixel and on average 2 are reflected, 2 are refracted.
Combine them to get an approximation of one ray reflected and refracted.

Upvotes: 2

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