That is an example (only a spot light and a mask for the godrays) with 10.000 Samples and 5,5 hours rendertime (with 8 CPUs because i cant`t render with cuda). It looks horrible. A lot of users have the same problem with rendering in cycles how you can read in this threads. The using of clamps in the case of godrays is not really good because the clamps have a negative effect to the godrays or … they don´t look good.
My idea is know that everyone download the Blend-File and test his own ideas to render faster and better this file an upload his (or her ;)) result here. I will test it naturaly too. It is better that a lot of people test and render but only one. Or perhaps someone knows how godrays are to render in cycles (like Greg Zaal : http://www.blendernation.com/2014/02/21/blender-2-70-test-build-is-now-available/). I tried to ask him to the parameters for his picture. But i just havent`t an answer since yet.
-The world is a homogeneous volume, so check the “homogeneous” box in the world panel so Cycles knows not to ray-march it (expensive!!!). And change the homogeneous sampling type for “distance” to “equiangular” in the volume sampling panel (under render settings).
-The stuff you want to clamp is all indirect light, so set indirect clamp at 2.5 or so, and leave direct clamp off.
-Turn off ray visibility settings for the mask and give it a black shader so it’s not reflecting light and adding more fireflies
If you want to compare with BI, it’s not sampling indirect scattering. You can turn that off in Cycles if you want, just set volume depth under the light paths tab to 0.
