Fix #34233: bad alpha blending for 3D texture painting. Same changes as the

previous fix for 2D image painting were done, and also:

* For brushes that do masking (keeping the max contribution to a pixel limited),
  the alpha from textures is now part of the mask. In many cases the logic
  worked out the same and where it didn't it used to cause artifacts.

* Color interpolation for smear and soften tools now happens in premultipied
  space, to avoid bleeding of RGB colors from zero alpha areas.

* Fix a few cases where byte <=> float conversion was not doing the proper
  straight <=> premul conversion.

* Replace some float division by constants with multiplications, is faster.

Note: float texture painting seems to have some issues updating the OpenGL
texture, but issue was already there before this commit.

1 files changed, 2 insertions, 2 deletions

diff --git a/source/blender/blenlib/intern/math_color_inline.c b/source/blender/blenlib/intern/math_color_inline.c
index e9a1c0abc38..bef4e5adaaa 100644
--- a/source/blender/blenlib/intern/math_color_inline.c
+++ b/source/blender/blenlib/intern/math_color_inline.c
@@ -309,8 +309,8 @@ MINLINE void straight_to_premul_v4(float color[4])
 
 MINLINE void straight_uchar_to_premul_float(float result[4], const unsigned char color[4])
 {
-	float alpha = color[3] / 255.0f;
-	float fac = alpha / 255.0f;
+	float alpha = color[3] * (1.0f / 255.0f);
+	float fac = alpha * (1.0f / 255.0f);
 
 	result[0] = color[0] * fac;
 	result[1] = color[1] * fac;