Lenovo Y580 Intel Core i7-3630QM CPU, 15,6 GiB RAM, Nvidia GeForce GTX 660M (GK107M), Linux Mint 17 64-bit
Broken: Blender 2.71 from buildbot (Hash 4299b00) (and several earlier versions)
Worked: Blender 2.71 stable (Hash: 9337574)
Short description of error
Cycles is not rendering direct light with volumetrics, only indirect.
Exact steps for others to reproduce the error
Just starting with the default cube, add to it Object/quick effects/quick smoke, Add a volume scatter shader to created domain, with attribute node with density parameter as density of the shader, Simulate several frames, F12 for render.
You might want to simplify this a little.
Instead of using a smoke object, just add a volumetric material to the cube so verify as to whether or not the issue might be specific to smoke rendering.
Also, if the lamp size is small, you might have trouble getting any sort of lighting on it even if using MIS, though regressions in terms of direct lighting abilities are always possible.
Thank you ace_dragon.
Yes, simplified scene reproduces same bug. Just connect a volume scatter shader to default cube (default scene), removing diffuse one. No direct light. Lamp size makes no difference.
Blender 2.71 render is all fine, later versions are not.
Ok. I changed light to point and increased its strenhgt. Its working. Still I dont understand why sun does not work with volumes like such domain. I understand why it doesnt work with world volume (pretty reasonable -light at infinite distance and volumetric world) but why doesnt it work here? Volume domain has its thickness, and it should be considered in all calculations. Why is the sunlight in this case???
BTW @Thomas Dinges (dingto), You are doing such a great job on Cycles!
You can see for yourself if you 'build' the sun into the environmental light and make it a really small source.
Cycles is a unidirectional pathtracer and that combined with a volume means it will be very hard to illuminate the surface with a source that small.
It's kind of unfortunate really because it means we still have to use stacked cards, the motion blur workaround, or the old internal engine if we want real volumetric cloud and smoke effects in outdoor scenes without tricky compositing tricks, and I'm not sure if there's any papers covering the topic of successfully lighting a cloud from a distant source without a sort of shading cache.