Hi!
I think it was already asked before several times, but I couldn’t find a definite answer for my question anywhere.
So the Q is:
Is it possible to produce reflective caustics on a metal surface from HDRI-only lighting? (Or refractive GI from a glass.)
I have the usual HDRI setup (following some tutorials), Reflective GI Caustics is ON, and the Object Settings are checked, but for no avail.
Thanks for your help.
syllogz
You’re doing it right, but you need a strong enough map to be able to see the reflection caustics. For a glass object, if you only use HDRI for lighting, and the shadows underneath the object are colored (read: lighter then with a non transparant object), then this is because of refraction caustics from the enviro map.
GI caustics are never really visible, they just lighten up an area very smootly, but you can see them if you compare the render to one with GI caustics unchecked. If you have a very detailed HDRI map with very strong small lightsources in it, then you will have more pronounced GI caustics from that map.
Thanks flip!
I had come to the same conclusion, but wasn’t quite sure about it.
I even tried to increase the multiplier on the VRayHDRI map, and Environment, but only got that subtle effect you said.
Now I’ll try to find a better HDRI (actually, I’m using the one you shared with us 
).
If I have something nice I’ll upload it to the image gallery.
bye,
syllogz
Hehe yeah, that’s probabely the problem! They’re not the best out there, it seems that it has not a very high dynamic range…
Do you have the kitchen.hdr? That’s a good one, you sould be able to get more visible caustics with that I think.
ufizzi produces some nice caustics, rotate it vertically for a bit more interesting angle of the light.
But remember that GI caustics can only get as sharp as your highest irradiance map setting, and sharpness also depends a lot on settings like the color threshold. Try rendering the same scene with irradiance map and direct comp and you’ll see what I mean.
Thanks egz for your reply!
I was experimenting with it this afternoon, and the best pics came with the uffizi and galileo maps.
And I also managed to realize the effect of Color Treshold on a trial&error basis :). But a 0.01 value still didn’t contribute too much.
Though I shall see what direct comp produces.
Why do you want caustics from GI? Isn’t there a way to make a ‘caustics only’ light or something in vray? So you could have caustics from a direct light, without the light contributing to the lighting or shadowing of the scene. maybe render with light on, save photon map, switch light off and render again with saved photon map?
That’s easy just uncheck diffuse and specular int the “affect surfaces” are of Max lights the light will still emit photons 
Ok now I feel stupid! 
Why not?
Actually, egz’s right!
Why not!
I was/am just interested in whether it can be done ot not. :roll:
Now, I’m not far from a good result, though the stunning wavy caustics patterns are nowhere in site yet.
You will never get them with hdri. if you would render caustics with a normal vray light with a huge area, you will also get very smooth patterns, because the light is coming from a large region. Same with hdri, you’ll need a very good and very high resolution map!
Now you’re not talking about caustics in general, you’re talking about sharp caustics. There’s more to caustics than sharpness..
Yeah I’m talking about sharp caustics because syllogz was talking about them
hey don’t get me wrong, I love caustics! (gi or direct, whatever!)