turn on multiple importance sampling for the sun lamp
It was on. Everything is on. I even removed left over custom properties everywhere that had left from previous renderers in this scene.
Edit: Hey, I got shadows on by changing sun size from default 1.000 to 0.010 (should remember that by now), but still no caustics.
Caustics only render off of mesh lamps.
Actually it works with sun lamp, but it’s kind of ridiculous. With 100 samples you can see couple of white spots. I guess you need something like 100000 samples, but it’s silly compared to everything else. You can get a noise free rendering with that 100 samples, but caustics don’t even work closely. I don’t know, but for me it’s a problem in Cycles (or its pathtracing model for caustics).
You know, Lightwave could be a solution for this and other Blender related problems (old school smiley).
It’s a path tracing thing. You need bi-directional path tracing with MLT to get accurate caustics. Not at all a Blender thing, it’s just how light paths are calculated. Super bright paths have a much lower chance of connecting with a small light source than their neighbors.
On a side note: If that really is a default glass shader, then there seems to be an issue with the normals on your meshes… Inverted? I would agree that Cycles might (right now) not be the best choice for caustics renderings, but of course using physically implausible geometry doesn’t really help with that…
For razor sharp caustics you would need e. g. a tiny mesh light, which unfortunately takes forever to converge in standard path tracing. The mesh light in this example is not even that small, but with 10000 samples it is still super grainy:
Yes, they were inverted. When you don’t even see the light reflected unless using mesh emitter you can get confused and flip them the wrong direction.
I don’t care about the technical details. I only know that Lightwave in 1998 had better caustics than Cycles now. And it’s not even the only problem I have with Blender.
Then… use Lightwave? A lack of understanding of the “why” on your part doesn’t make one tool better than the other. Lightwave’s renderer uses faked GI and faked caustics. They are, by definition, not “better”, merely faster. For that speed, you trade a good deal of accuracy, and in return get a large bundle of technical headaches if you want to animate. If you want photon mapped caustics in Blender, there are a number of other compatible renderers (Vray, Lux, Thea, and Yafaray, off the top of my head) that allow for it. Again, this isn’t a Blender problem, it’s a problem with your lack of understanding on the subject at hand. But this is not a discussion for this thread, and it’s getting off topic.
EDIT
Never mind: Getting off topic.