Dirty RGB (VrayDiffuseFilter) Render Element

[tysonibele]

I have multiple smoke sources I'm trying to mix together. I've set red/blue/green values for their RGB input into the simulation. The idea was to then use the VrayDiffuseFilter render element in order to separate them in comp and do color correction on each individual smoke source....that way I can tweak the colors of each smoke source in comp without having to commit to a specific color prior to simming.

However, my render element is coming back "dirty"...it has all kinds of black shading and the primary colors are not fully luminous (therefore not usable as a matte). It seems this is because as the voxels are rendered, they do a trilinear sample of surrounding voxels to determine their final output values...and part of that calculation is causing ambient voxels to bleed their default 'black' RGB value into my voxels containing my custom RGB values.

While I can see the importance of this blending in theory (if a red voxel is next to a green voxel, you'd want the center pixels to come out cyan)...it would be nice if there was a way to "ignore" black voxels....so that if a red voxel is next to a black voxel, all pixels sampled in the red voxel will return 100% red). If my assumptions are correct, this would allow for proper RGB matte passes. I could see this just being a checkbox or something in the render settings, so that default behavior is unchanged from what it is now.

Attached are examples of a raw render, and the resulting VrayDiffuseFilter render element for a couple of smoke sources, to show the issue.

[georgi.zhekov]

Hello,

Does it get better if you switch your Sampler Type in the Rendering rollout to Linear? If it's still happening - can you send us the scene so we can take a look?

Thank you,

[tysonibele]

So, in setting up a scene file for you to test I've discovered a couple things, and also realized I forgot to mention a couple of things.

Firstly, my render settings have opacity set to temperature. Not sure if that makes a difference, since it seems like temperature is advected around the grid the same as smoke, but worth mentioning.

Secondly, I'm getting inconsistent results if I discard the smoke channel in the output settings. If I'm rendering from temperature, intuitively I'd imagine I wouldn't need the smoke channel at all once the sim is finished...but you can see I'm getting a lot more black shading in the render element pass when the smoke channel is discarded (ie, different results when nothing in the sim changes, other than the 'smoke' channel checkbox being on/off).

So while my hunch was this error was related to sampling described in my first post...maybe it's a different issue altogether?

Attached is the max file (2017), and 4 images showing the differences. Left is rgb element, right is main render. You can see that both rgb elements are dirty, and one moreso than the other (the one without smoke channel saved is really dirty).

[georgi.zhekov]

Ah, that explains it. Indeed currently your setup won't work. This is still in the to do list.

Until we implement some changes - only the Smoke channel will return the correct result.

Thank you,

[Svetlin.Nikolov]

Yup, at the moment you need both the Smoke channel and the RGB so the edges will be cleaned up, but I hope sometime soon we'll be able to throw that in the garbage bin and implement it right.

Cheers!