Yep - the way caustics work in the experimental 3.1 build are as follows:
- Your light must have “caustic light” enabled (I believe this also works scenes lit with HDRI - but there is a separate toggle on the world panel to enable that - under sampling)
- Any glass objects you want to enable caustics for must be set to “caustic caster”
- Your ground plane or any objects that the caustics may fall on must be set to “caustic receiver”
Some additional important points:
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In the render panel, under light paths - your “filter glossy” value must not be zero. I have found values of 0.01 and above work just fine (I believe the default is 1, so you shouldn’t need to touch this - the actual value appears to make no difference for these types of caustics).
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You must also have refractive caustics turned on in this panel. This should be on by default if you have selected the “full global illumination” preset.
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Setting an object to be both a caustic caster and receiver at the same time can cause some weird behaviour. I have found that glass objects can become much darker, although it doesn’t seem consistent (we have to remember this function is at an early stage of development). I’d avoid doing this for now.
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This technique doesn’t currently work for reflective caustics.
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Be careful if you scale your objects. If you inadvertently invert the scale , it can screw up the caustics even though the object face normal appear to be facing the correct way. If you are unsure - apply all transforms to your object - then check the external face normals are blue.
There is also a thread running here for more examples of this in use - including some testing around the scaling issue in point 5 above.