Compositor: Port GLSL SMAA to CPU compositor #119414

Omar Emara · 2024-03-13T10:54:28+01:00

Omar Emara commented

2024-03-13 10:54:28 +01:00

This patch ports the GLSL SMAA library to the CPU compositor in order to
unify the anti-aliasing behavior between the CPU and GPU compositor.
Additionally, the SMAA texture generator was removed since it is now
unused.

Previously, we used an external C++ library for SMAA anti-aliasing,
which is itself a port of the GLSL SMAA library. However, the code
structure and results of the library were different, which made it quite
difficult to match results between CPU and GPU, hence the decision to
port the library ourselves.

The port was performed through a complete copy of the library to C++,
retaining the same function and variable names, even if they are
different from Blender's naming conversions. The necessary code changes
were done to make it work in C++, including manually doing swizzling
which changes the code structure a bit.

Even after porting the library, there were still major differences
between CPU and GPU, due to different arithmetic precision. To fix this
some of the bilinear samplers used in branches and selections were
carefully changed to use point samplers to avoid discontinuities around
branches, also resulting in a nice performance improvement. Some slight
differences still exist due to different bilinear interpolation, but
they shall be looked into later once we have a baseline implementation.

The new implementation is slower than the existing implementation, most
likely due to the liberal use of bilinear interpolation, since it is
quite cheap on GPUs and the code even does more work to use bilinear
interpolation to avoid multiple texture fetches, except this causes a
slow down on CPUs. Some of those were alleviated as mentioned in the
previous section, but we can probably look into optimizing it further.

This patch ports the GLSL SMAA library to the CPU compositor in order to unify the anti-aliasing behavior between the CPU and GPU compositor. Additionally, the SMAA texture generator was removed since it is now unused. Previously, we used an external C++ library for SMAA anti-aliasing, which is itself a port of the GLSL SMAA library. However, the code structure and results of the library were different, which made it quite difficult to match results between CPU and GPU, hence the decision to port the library ourselves. The port was performed through a complete copy of the library to C++, retaining the same function and variable names, even if they are different from Blender's naming conversions. The necessary code changes were done to make it work in C++, including manually doing swizzling which changes the code structure a bit. Even after porting the library, there were still major differences between CPU and GPU, due to different arithmetic precision. To fix this some of the bilinear samplers used in branches and selections were carefully changed to use point samplers to avoid discontinuities around branches, also resulting in a nice performance improvement. Some slight differences still exist due to different bilinear interpolation, but they shall be looked into later once we have a baseline implementation. The new implementation is slower than the existing implementation, most likely due to the liberal use of bilinear interpolation, since it is quite cheap on GPUs and the code even does more work to use bilinear interpolation to avoid multiple texture fetches, except this causes a slow down on CPUs. Some of those were alleviated as mentioned in the previous section, but we can probably look into optimizing it further.

Omar Emara added the

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Compositor: Port GLSL SMAA to CPU compositor #119414