Compositor: CPU vs GPU Differences #118548
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Reference: blender/blender#118548
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Each of the following sections describe one difference between the CPU and GPU compositors, its
problem, its potential solution, the nodes it affects, and the failed regression tests where it
manifests.
Anisotropic Filtering
Problem
GPU uses the hardware anisotropic filtering capabilities of the GPU. Since each vendor, GPU, and
driver might have a different implementation, we can't really unify the implementations.
Solution
Reimplement our own anisotropic filter.
Affected Nodes
This affects the following nodes:
Failed Tests
Jitter Anti-Aliasing (blender/blender!118853)
Problem
CPU uses an 8-sample jitter multi-sample anti-aliasing algorithm to anti-alias some masks. GPU
compositor uses bilinear interpolation with zero boundary to achieve anti-aliasing.
Solution
Use SMAA in all compositor operations that require anti-aliasing.
Affected Nodes
Failed Tests
SMAA (blender/blender!119414)
Problem
SMAA operation produce different results between CPU and GPU. GPU uses the original SMAA library,
CPU uses a C++ port of the library.
Solution
No solution known yet. Needs investigation to figure out where the difference originates.
Affected Nodes
Failed Tests
Fast Gaussian (
382131fef2
)Problem
GPU does not implement the Fast Gaussian blur mode and falls back to the slow convolution algorithm.
But, CPU implementation seems broken and could better match standard Gaussian for low dynamic range
images. Further, CPU sometimes uses Fast Gaussian as a utility in other operations, like the Glare
node, while a normal blur should be used instead since the blur radius is very small.
Solution
Implement Fast Gaussian for GPU, port the same implementation to CPU.
Affected Nodes
Failed Tests
Curve Maps (Indirectly fixed by blender/blender!118624)
Problem
CPU and GPU evaluates curve maps slightly differently, which can produce a tiny difference in the
0.001 magnitude. Further, curve maps are stored as half floats in the GPU, so we lose some
precision, making the difference worse.
Solution
Don't use the hardware sampler and interpolate the curve maps manually. Use full float for curve
maps.
Affected Nodes
Failed Tests
Clipped Transformations (blender/blender!119278)
Problem
The result of some transformations is clipped in CPU, but only in background rendering, so this is a
bug in the CPU implementation rather than a difference.
Solution
Unknown, still needs investigation.
Affected Nodes
Failed Tests
Linear Space Color Conversion (blender/blender!118624)
Problem
CPU produces slightly different values after sRGB to Linear color transformations, for instance,
pixels of value 1.0 becomes 1.00002408. According to Sergey:
Solution
Do the color space conversion on the CPU using the same OIIO method. Or move to a more accurate method on the CPU.
Affected Nodes
Failed Tests
Legacy Cryptomatte (blender/blender!118570)
Problem
GPU doesn't have an implementation.
Solution
Unclear what this node represents or if it will be removed. Just do an implementation regardless.
Affected Nodes
Failed Tests
Fog Glare (
f0c379e1d3
)Problem
CPU convolves the image with an arbitrary point spread function. GPU uses a Bloom algorithm.
Solution
Move Bloom as to separate option, port implementation to CPU. Add an FFT implementation for GPU and
use that to do the convolution, getting rid of the arbitrary PSF for both CPU and GPU.
Affected Nodes
Failed Tests
Denoise (blender/blender!118553)
Problem
CPU always adds the normal and albedo passes even if they are not connected, and inflates their
values to full buffers.
Solution
Only add the normal and albedo passes if they are actually connected and are full buffers.
Affected Nodes
Failed tests
Interpolation
Problem
CPU and GPU interpolate images differently, but it is likely that the GPU is more correct.
Solution
Unknown, needs more investigation.
Affected Nodes
Failed Tests
Vector Blur (blender/blender#120135)
Problem
GPU uses an implementation similar to EEVEE, while CPU uses a more compute intensive operation.
Solution
Formalize GPU implementation and port it to CPU.
Affected Nodes
Failed Tests
Variable Transformations (blender/blender#120314)
Problem
CPU allow the inputs of transform nodes to be variable, GPU does not.
Solution
Allow variable transformations for GPU.
Affected Nodes
Failed Tests
Only sampling and interpolation differences remain now. But those are hard to match given the different designs of the CPU and GPU compositor. So they will be handled after unifying CPU and GPU compositors.