Blender codebase had two ways to convert half (FP16) to float (FP32):
- BLI_math_bits.h half_to_float. Out of 64k possible half values, it converts
4096 of them incorrectly. Mostly denormals and NaNs, which is perhaps not too
relevant. But more importantly, it converts half zero to float 0.000030517578
which does not sound ideal.
- Functions in Vulkan vk_data_conversion.hh. This one converts 2046 possible
half values incorrectly.
Function to convert float (FP32) to half (FP16) was in Vulkan
vk_data_conversion.hh, and it got a bunch of possible inputs wrong. I guess it
did not do proper "round to nearest even" that CPU/GPU hardware does.
This PR:
- Adds BLI_math_half.hh with float_to_half and half_to_float functions.
- Documentation and test coverage.
- When compiling on ARM NEON, use hardware VCVT instructions.
- Removes the incorrect half_to_float from BLI_math_bits.h and replaces single
usage of it in View3D color picking to use the new function.
- Changes Vulkan FP32<->FP16 conversion code to use the new functions, to fix
correctness issues (makes eevee_next_bsdf_vulkan test pass). This makes it
faster too.
Pull Request: https://projects.blender.org/blender/blender/pulls/127708
92544d6d76