Wednesday, October 17, 2007

Ray tree pruning

A time honored way to speed up your glass is to do "ray tree pruning" and you should do it if you aren't already. The way Whitted originally implemented his paper was like this:

color trace(ray r, depth d)
color c
if (r hits scene at point p)
c = direct_light(p)
if (depth < maxdepth)
c += ks*trace(reflected_ray) + kt*trace(refracted_ray)
c = background(r)
return c

If you hit glass that will be two branches per recursion level. The first efficiency improvement would be to only send the rays if ks and kt are non-zero. But that would not help you for glass. Instead, you need to kill whole paths down the recursion tree once they are sufficiently attenuated. For example, if kt=0.05, then after 3 transmissions the coefficient is (0.05)^3 and can be ignored. To implement this, you need to change the call to include an attenuation argument:

color trace(ray r, depth d, float atten)

and add

if (depth < maxdepth)
if (atten*ks.max_component() < maxatten)
c += ks*trace(refected_ray, atten*ks.max_component())
if (atten*kt.max_component() < maxatten)
c += kt*trace(refracted_ray, atten*kt.max_component())

You will be amazed at how much faster a scene like Whitted's glass ball is.


Adrian said...

If you're clipping based on attenuation, then why even bother with the depth limit on the recursion?

In my ray tracer, I've replaced the depth limit with an attenuation term rather than adding one. Works as long as you don't have a fluorescent material that "amplifies" the light.

Peter Shirley said...

Hi Adrian.

In real physics I agree, but in a program I think you should plan for a ray to sneak inside an object or to get some floating point error that allows it to internally reflect until your stack overflows. If the viewer chose a viewpoint inside a glass ball I think this would even happen for some viewpoints without a bug! Of course, if you always added some exponential attenuation that might accomplish the same thing as max depth.

Another reason for max_depth-- good for approximate fast renderings.

Thomas said...

the way i'd do glass is to first compute the fresnel coefficients for reflection and refraction, and probabilistically do one based on that.

Michael said...

Using just a single attenuation value is probably not the best way to do this. Imagine a fully red glass (color 1,0,0) in front of a blue glass (color 0,0,1). Always using the max component won´t kill the ray at all while using the max component of the product of the colors will kill the ray instantly.

Eric said...

Yes, this one's a big win. I believe it was first mentioned in:

A Testbed for Realistic Image Synthesis, R. A. Hall, D. P. Greenberg, IEEE Computer Graphics & Applications, 3(), November 1983, pp. 10-20.

(Or it was in Hall's thesis.)


sexy said...








Entertainment said...

home city trung kính

home city

chung cư trung kính

trung kính complex

5S Online - mùa 3 - Tập 66: Tình cậu duyên anh - Phần 1

5S Online - mùa 3 - Tập 65: Con nuôi sếp tổng - Phần 2

5S Online - mùa 3 - Tập 64: Con nuôi sếp tổng - Phần 1

5S Online - mùa 3 - Tập 63: Cắt đuôi sao cho xuôi - Phần 3

5S Online - mùa 3 - Tập 62: Cắt đuôi sao cho xuôi - Phần 2

5S Online - mùa 3 - Tập 61: Cắt đuôi sao cho xuôi - Phần 1

5S Online - Tập 54: Làm giàu quá khó cần có vận may - Phần 3

5S Online - Tập 53: Làm giàu quá khó cần có vận may - Phần 2

5S Online - Tập 52: Làm giàu quá khó cần có vận may - Phần 1

chung cư t&t riverside

chung cư 440 vĩnh hưng

chung cư t&t riverside 440 vĩnh hưng

bat dong san

bat dong san cho thue

bat dong san ban