A layer with Blending Options › Blend If made the planner refuse the
whole document, so every refresh of it ran the CPU compositor: on the
#209 layered document an opacity edit took 140 ms instead of 6 ms.
Blend If is compose::apply_blend_if: the composite with the layer mixed
back towards the backdrop by a per-pixel weight from the layer's own
content with its masks ("This Layer"; an adjustment's result) and the
backdrop ("Underlying Layer"). The planner now keeps the backdrop,
composites the layer as before (channel restrictions included, as on
the CPU), renders its own content and emits one Lerp pass with the new
F_BLEND_IF flag and the ranges packed exactly in its parameters. Base
and clipped layers both go through it; Advanced Blending still falls
back to the CPU.
Hard points: compositing can leave an 8-bit level a few millionths
below itself (a level-20 backdrop at partial alpha reads 19.999998),
which put it on the wrong side of a hard point at that level on the
CPU, and the GPU's arithmetic differs there. Both now treat a value
within 1e-4 of a level as that level; 8/16-bit values off a level are
at least 1/257 away. On 16/32-bit documents the GPU reads layers as
RGBA16F, so a value within about a tenth of a level of a hard point can
still land on the other side, as with the GPU Threshold adjustment.
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
Co-authored-by: Brandon Thomas <echelon@gmail.com>
