Cubo Prism

#4
Thanks Pod & Zoo!

To me caustics are a way to (unexpectedly) produce most interesting effects with the simplest of means, the Luxrender Cubo file being the prime example.

Indeed this is done with the Cheetah renderer, one area light for illumination and a pure photon spotlight with maximum sample count for caustics (settings 1.4/1.0/100).
That's still not clean but I cheated with a procedural rough ground texture :smile:

The caustic prepass takes ~40 seconds but the render above is still a slow one because of 128 DOF samples.
 

podperson

Active member
#5
This was a relatively quick render (I modeled the cubo in Cheetah 3d — it's a cube with a bulge, with bevels — took a couple of minutes).

What I was going for is the spectral caustics I saw here.

I did this by rendering a 3 frame "animation" where the area light is only present in frame 1, the photon light is red in frame one, green in frame two, and blue in frame three, and the index of refraction goes down slightly (which is the wrong way around; I've fixed this and am rendering a new images as I type this). Finally, I composited the three frames in Acorn using the "screen" composition for frames 2 and 3.

I've used maximum photons here but it's still pretty noisy. I believe I can get a better result by replacing the glass with a "material" material (which will render a lot slower than dielectric glass) and animating the material's transparency and reflection (so six frames total, 3 with reflection, 3 with refraction) — this is a trick I suggest in my book by the way.

One of the benefits of adding a depth of field pass is that you're effectively increasing the number of photons beyond the limit (the downside is the hugely increased rendering time, of course).
 

Attachments

podperson

Active member
#6
This is downsampled from 4K resolution with slightly different settings, refraction the correct way around, and composited using "exclusion" instead of "screen" (what I really want is "add" which I could do in Photoshop).
 

Attachments

podperson

Active member
#7
Final render for tonight — I carefully narrowed the photon light beam which probably got me my single biggest improvement in noise. (Should have thought of that sooner…) I also tried rendering six spectral colors instead of 3, but they don't balance properly so I need to work on that (this composite only uses red, green, and blue).

Rendered at "4K" and reduced to 1080p.
 

Attachments

#8
(I modeled the cubo in Cheetah 3d — it's a cube with a bulge, with bevels — took a couple of minutes).
That I tried also but somehow failed - the caustics looked worse than the original - maybe I should have tried different bevel radii...

What I was going for is the spectral caustics I saw here.

I did this by rendering a 3 frame "animation" where the area light is only present in frame 1, the photon light is red in frame one, green in frame two, and blue in frame three, and the index of refraction goes down slightly (which is the wrong way around; I've fixed this and am rendering a new images as I type this). Finally, I composited the three frames in Acorn using the "screen" composition for frames 2 and 3.
Great idea!

I remember trying this years ago, but I simply did three different IOR renders and copied those into the three channels of a photoshop document - that grants correct blending but my caustics were too sharp and showed gaps between the color stripes.

Your idea with more than three samples and layer blending is very helpful,
I made a sequence from IOR 1.52 to 1.53 in 11 steps and combined them in PS:

cubospectr3-.jpg

Do you think you could make a script or an app (renderbuddy add-on) that would automatically grab a Cheetah IOR sequence and do a correct spectral merging?
 

podperson

Active member
#10
Do you think you could make a script or an app (renderbuddy add-on) that would automatically grab a Cheetah IOR sequence and do a correct spectral merging?
Very easy in Photoshop (you can just record a sequence of actions and turn it into a button).

P.S. OMG I just opened Photoshop for the first time in months. It's gotten so… awful.
 
Last edited:
#11
Thanks Chris and Pod :smile:

(Now I was secretly hoping for a Not-Photoshoppers-Only-Solution but then again the demand for Cheetah spectral caustic rendering may be not that overwhelming anyways...)

cubo8.jpg
 
Last edited:

podperson

Active member
#12
Yeah I’m thinking if I wrote a spectral caustic rendering script I would spend more time on it than everyone in the history of the world would spend doing it manually :)

That said, I wrote a photoshop-like JavaScript library a few years back which has support for pixel-level operations so it’s something that could be done pretty easily should the need arise.
 
#15
Final render for tonight — I carefully narrowed the photon light beam which probably got me my single biggest improvement in noise. (Should have thought of that sooner…) I also tried rendering six spectral colors instead of 3, but they don't balance properly so I need to work on that (this composite only uses red, green, and blue).

Rendered at "4K" and reduced to 1080p.
Very nice POD!
 
Top