Volume voxels visible in compositing (Cycles) #65793
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Reference: blender/blender#65793
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System Information
Operating system: Windows 10
Graphics card: GeForce GTX 950 OC
Blender Version
Blender 2.80
cc73d59ad5
Short description of error
First of all :
Not sure if this is a bug or if Im just not getting it.
Everything is done in Cycles
Im subtacting the DiffInd passes in compositing and I get the volxel shape of my smoke sim.
Here a screenshot of my Nodesetup (note the backdrop)
I do this to extract the Indirect lighting of for example fire or something else while excluding the sun and stuff.
This is the image (the white cuby thing to the right of the smoke)
Exact steps for others to reproduce the error
Add a basic smoke simm with high resolution and a ground and wall.
Add a sun light
Add a new view layer
Exclude the smoke domain from the 2nd layer
Enable the DiffInd pass for both layeres
In compositing :
Subtract the DiffInd passes from each other.
This is an example scene:
debug.blend
It seems like there is a problem how "null" surfaces get handeled.
If I set a layer to indirect only the indirect light only appear outside the voxels (where they are intersecting the geometry).
Added subscriber: @FATTOMCAT
Added subscriber: @brecht
Added subscriber: @mano-wii
I can confirm the effect of the smoke voxels:
But I am almost certain that this is a limitation in the system.
The attached file doesn't work in 4.x anymore, but I'm pretty sure that this is just a sampling artifact. In voxels that contain some volume density, Cycles has to perform volume sampling, and that affects the noise pattern.
On average, with enough samples, the result should average out to the same value. However, note that in the screenshot, the subtraction is clamped - therefore, negative differences are set to zero, while positive differences remain. This explains why the resulting compositing artifacts don't average out anymore.
All in all - this is just a limitation of how path tracing works. If you denoise the passes before subtracting, the effect should be imperceptible.