Caustic settings

[SonyBoy]

Just trying out caustics for the first time. I'm noticing that the default settings seem to be MUCH lower than typical settings used to be for photon emission in Mental Ray, e.g. only 60 max photons in the globals, and only 500 subdivs in each light (although is this latter number the square root of the actual number, following Vray's paradigm on other settings?).

Mental Ray often required tens of thousands of photons in a scene to generate good GI and caustics. So my question is, what are some average settings per-light and globally to get decent results? At first testing it seems to me that the defaults are too low, but I don't want to bump them up to something impractical? Obviously it depends on the situation but just looking for some pointers as to what sorts of ranges I'm looking at.

[yolov]

Caustics defaults are a bit delicate. I agree that they are low-ish, but just enabling them when the defaults are higher may make one wait... for quite a bit as they are generated and the free RAM slightly runs out. So the lower defaults are mostly there for a starting point where something becomes visible and then get to a better result by increasing the settings where necessary.

Yes, 500 subdivs means 500 is the square root of the total number of photons. Btw, the photon subdivs is not what you need for caustics - this parameter controls photon emission when Photon Map is used as a GI engine. For caustics (although it uses the same photon mapping method) you need the caustic subdivs (default 1000 = 1000^2 = 1,000,000 photons).

Like you've guessed, good caustics require a *lot* of photons, so anywhere between 3000 and 7000 subdivs per light usually gets you there. Now, the more caustic subdivs, the more RAM required, which is where the max density parameter comes into play (increase it slightly above zero to save on RAM).
Then max photons and search area will determine how smooth/sharp the caustics will be.

It's mostly a trial-and-error process, as this is very, very scene dependent, hence no good recipe for specific settings and value can be given.
I would usually start with the default, add my Caustics render element too and look at the RGB and Caustics channels to see where I'm getting. Then I'd increase the caustics subdivs per light to at least 2500-3000, so it still generates somewhat quick without needing too much RAM. Then play with the search distance, then play with max photons. I'd then have to decide if I need more photons,which means more caustic subdivs per light (usually needed). So my next step would be seeing if 4500 does it. Anything above that would require a lot of RAM (say on a 32G machine, I'd start increasing the max density slightly when I get to a point where I'd need, say, 6000 or more subdivs per light).

Hope this helps at least a bit.

[SonyBoy]

Helps a lot! Thank you.

[SonyBoy]

One further question -- I notice that turning caustics on causes some noticeable darkening in my glass object. A quick search online found posts that said simply turning off "Affects Shadows" on the material would fix this, as caustics take over transparent shadow calculations when they are enabled (?). However I am not finding this makes any difference. What's the fix for this, beyond shader adjustment?

[yolov]

Hmm, it should. For proper caustics, you'll need to disable 'affect shadows'. I'll check on this first thing tomorrow, but if you can post an example or a simple scene in the meantime, it would make things a lot easier.

[SonyBoy]

Sure, here's a stripped down scene and two renders -- one without caustics, and one with caustics and affect shadows turned off on the glass shader. I can't send the HDRI used to render the scene, but otherwise this is the render scene.

[yolov]

This turned out to be more complicated than expected. I suspect there's a bug somewhere in V-Ray, but so far I've only been able to pin it down to the dome light.
I'll try and see if I can find something else tomorrow, just wanted to let you know that I'm (slowly) making progress and that it's most probably a bug.

[SonyBoy]

Ah, OK. Thanks for looking into it! Glad it wasn't something stupid I was overlooking, at least

[yolov]

OK, done some more digging. We haven't broken anything, so it's been rendering like this for at least a few years. Which means that the domelight somehow behaves unexpectedly.
I've logged an issue so that our developers can look into this.

Unfortunately I can't come up with a workaround for this other than not using the dome light in this case, until we see what can be done about this.

[SonyBoy]

OK then, thanks for the support. Much appreciated.

[yolov]

Just some news: this is fixed in the internal builds and will be included in the next major update.

[SonyBoy]

Good to know, thanks!