Double precision pixel coordinate interpolation was added in 3a65d2f591 to
fix an issue of "wobbly" resulting image at very high scale factors. But
the root cause of that was the fact that the scanline loop was repeatedly
adding the floating point step for each pixel, instead of doing multiplication
by pixel index (repeated floating point additions can "drift" due to
imprecision, whereas multiplications are much more accurate).
Change all that math back to use single precision floats. I checked the
original issue the commit was fixing, it is still fine. Also added a gtest
to cover this situation: `imbuf_transform, nearest_very_large_scale`
This makes `IMB_transform` a tiny bit faster, on Windows/VS2022/Ryzen5950X
scaling an image at 4K resolution:
- Bilinear: 5.92 -> 5.83 ms
- Subsampled3x3: 53.6 -> 52.7 ms
Pull Request: https://projects.blender.org/blender/blender/pulls/117160
f16dbd0360