After seeing this thread i decided to try it myself cause i never did an "underwater" before :pinch:
Used the Popcorn FX God Rays to try to simulate sun from above...
Lots of Popcorn FX (Bubbles, Clouds, Fog, ...) and in post a pinch of "Hitfilm Fluid Distortion"...






Toystorylab on Vimeo :     Crassitudes (my "Alter Ego") on Youtube: 

@TSL - That was very cool.  I thought the underwater effects were very fine.


Here is a different implementation of caustics, used on a "spirit lady" proof-of-concept...

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iClone 7... Character Creator... Substance Designer/Painter... Blender... Audacity...
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IllusionLAB - I've tried several different ways to use spotlight shining through opacity object but I am not getting the impressive results I see in your post.  Would you be willing to give us a general overview of the steps you took to achieve this effect (the caustic shadows and the light rays)?  I just noticed the Shadow Catcher as one of your props -- maybe that is the key?  Thanks,  

Reallusion Certified Director / Reallusion Best Visual Award / Reallusion Certified Content Developer
See all the G-Tools Plug-Ins available for iClone here: G-Tools Plug-Ins

It's not the shadow catcher -- you can get rid of that.

I won't speak for IllusionLAB but the sample he sent me seems to indicate that you need a VERY fine "mesh" to get the reflections to show properly from the spotlight.  What he did was create a plane and put this very fine opacity map on it that the spotlight shone through (and used an emmissive GI surface to get the "blue light" but, again, this isn't the important part -- the important part is that very detailed mesh).

I've actually tried this myself and, even using his project, I can't get the same underwater results I would need using more sophisticated caustics (he uses a very simple noise pattern and "animates" it just by rotating.  I want actual caustics and while I have a pattern for them I can't get it to look right.  Yet.  I'm still hopeful).

In case anyone else wants to try, what I'm using is this:  https://www.dualheights.se/caustics/

There's a freeware version there that is more than adequate for testing -- and if I could ever get this to work I'd pay for the more advanced version, but I can't yet.  Yet.

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Alienware Aurora R16, Win 11, i9-149000KF, 3.20GHz CPU, 64GB RAM, RTX 4090 (24GB), Samsung 870 Pro 8TB, Gen3 MVNe M-2 SSD, 4TBx2, 39" Alienware Widescreen Monitor
Mike "ex-genius" Kelley

Here is fairly quick and dirty. Bottom caustics only. Quality of the caustics video is lousy (ripped first available from YouTube). Hi-res videos from Mike's link would probably do better.
Basically I have a plain up high with the video in opacity channel (had to play with that video in HitFilm, inverting it, desaturating,.. etc). And a spot light above it:



I like the idea of illusionLAB. Could be fun to play with opacity channel. Here is static one, but rotated:

we have some very talented animators here that can think right outside the box to overcome some of iClone's limitations
nice work on those underwater scenes

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