Caustics

[^Lele^]

Hi Lele, yes yes I know all of these.
Thank you for the explanation.
Advise: If I have more than one pool (even in different position and not close to each other) or I would like caustics in some "sea/beach" area...can I just increase the emit radius as I want without any problems?

A bigger radius simply means you'll be casting the same number of photons over a broader area.
This is a minor problem for the progressive sampler (it'll take longer to converge.), but it could be a deal breaker for the traditional photon mapping.

Corona suffers from the same kind of issue, however: put a caustic-enabled object far away from your subject, even not seen by the camera, and Corona will have to take it into account, meaning your caustics will immediately look different. (see attached).
The two images show the same number of passes.
The first scene contains only what you see (some cosmos asset with refractive properties), the second also has a glass, caustic-enabled sphere somewhat far from the subject.
The number of photons reaching the subjects is dramatically lower, and so you can tell most of the left-side caustics are absent, and the right-side diffraction isn't nearly as resolved.
It's likely it will converge eventually, but there's no magic at play with either V-Ray or Corona: it's all down to proper set-up (and, in some case, the amount of memory, or time, available.).

[nicola_barbano]

Cool!
So my "wishlist" could be just improve the speed of Caustics in Progressive mode.
I never used the caustics too much in daily production, so this is why I wanted to understand it better and to find a more stable/direct workflow with them. I found really different results depends using Displacement or Noise modify for example, and which scale ect
...but from my latest tests using "noise" and Progressive mode I have good and fast results.

Thank you for the explanation!
Nico

[^Lele^]

We are aware of the performance issues with the progressive caustic sampler (particularly when the core count grows, and ghz shrink), we're researching ways to make it better.
You are right in mentioning displacement and bump as major defining aspects for caustics: those are by nature *exceptionally* sensitive to normals, and minor changes in the aspect of a mesh will produce major changes in the caustic patterns.
Particularly for reflective caustics (as they have a higher tendency to spread out.).
There is no "fix" for that (it's the way it should work.), so one has to take great care in preparing the caustic casters just right.

[nicola_barbano]

"core count grows, and ghz shrink"...it sounds like ET language to me! ...No I'm joking...I understand the concept but not the practice.

Well, so...I hope you can the speed! It would be great!

[^Lele^]

"core count grows, and ghz shrink"...it sounds like ET language to me! ...No I'm joking...I understand the concept but not the practice.

I meant it gets slower with a 64core CPU which operates at, f.e., 2.4ghz on all cores, compared to an 8core that works at 4ghz, f.e.

[nicola_barbano]

Oh ok, understood...I have a Ryzen9 5950X...it works fine/speed for my daily work

[markus_s_cg]

Blender 3.2 has implemented something called shadow caustics. It's not full-blown reflective caustics, but it looks really useful and fast. Take a look at the water caustics at the end of this video.
https://www.youtube.com/watch?v=nF3xvlAC8Lg
I wonder if implementing something like this in Vray could be useful as a cheap alternative to progressive caustics, and photon-mapped caustics in many cases?

[seandunderdale]

Yeh thats where I guess there is difficulty in making a renderer. Do you create something as accurate as possible, but maybe slow to get a result? Or something that is fast, almost real time, and looks "good enough". I guess they're not mutually exclusive?..perhaps there is room for fake fast caustics, and slow accurate caustics.

[nicola_barbano]

The "shadow caustic effect" in Blender is awesome.
I have to say that also the progressive mode is not very slow or at least it's fast enough for my purpose.

[^Lele^]

Wow, very very pretty, and very very quick.
There are a couple of important limitations, to be sure (no reflective caustics, caustics appear only inside the shadows' footprint, and no more than 4 caustic bounces.), but definitely quite nice.