Fix T78124 Overlay: Image: Camera background image transparency not working

This changes to premultiplied blending for all cases and put the premult
in the shader.

2 files changed, 6 insertions, 3 deletions

diff --git a/source/blender/draw/engines/overlay/overlay_image.c b/source/blender/draw/engines/overlay/overlay_image.c
index be3510967b6..a754f7cbd49 100644
--- a/source/blender/draw/engines/overlay/overlay_image.c
+++ b/source/blender/draw/engines/overlay/overlay_image.c
@@ -61,11 +61,11 @@ void OVERLAY_image_cache_init(OVERLAY_Data *vedata)
   state = DRW_STATE_WRITE_COLOR | DRW_STATE_WRITE_DEPTH | DRW_STATE_DEPTH_LESS;
   DRW_PASS_CREATE(psl->image_empties_ps, state | pd->clipping_state);
 
-  state = DRW_STATE_WRITE_COLOR | DRW_STATE_DEPTH_LESS_EQUAL | DRW_STATE_BLEND_ALPHA;
+  state = DRW_STATE_WRITE_COLOR | DRW_STATE_DEPTH_LESS_EQUAL | DRW_STATE_BLEND_ALPHA_PREMUL;
   DRW_PASS_CREATE(psl->image_empties_back_ps, state | pd->clipping_state);
   DRW_PASS_CREATE(psl->image_empties_blend_ps, state | pd->clipping_state);
 
-  state = DRW_STATE_WRITE_COLOR | DRW_STATE_BLEND_ALPHA;
+  state = DRW_STATE_WRITE_COLOR | DRW_STATE_BLEND_ALPHA_PREMUL;
   DRW_PASS_CREATE(psl->image_empties_front_ps, state);
   DRW_PASS_CREATE(psl->image_foreground_ps, state);
 }
@@ -342,7 +342,7 @@ void OVERLAY_image_camera_cache_populate(OVERLAY_Data *vedata, Object *ob)
       mul_m4_m4m4(mat, modelmat, mat);
       const bool is_foreground = (bgpic->flag & CAM_BGIMG_FLAG_FOREGROUND) != 0;
 
-      float color_premult_alpha[4] = {bgpic->alpha, bgpic->alpha, bgpic->alpha, bgpic->alpha};
+      float color_premult_alpha[4] = {1.0f, 1.0f, 1.0f, bgpic->alpha};
 
       DRWPass *pass = is_foreground ? psl->image_foreground_ps : psl->image_background_ps;
 
diff --git a/source/blender/draw/engines/overlay/shaders/image_frag.glsl b/source/blender/draw/engines/overlay/shaders/image_frag.glsl
index 298ba1e27ed..4d37f1599a3 100644
--- a/source/blender/draw/engines/overlay/shaders/image_frag.glsl
+++ b/source/blender/draw/engines/overlay/shaders/image_frag.glsl
@@ -26,4 +26,7 @@ void main()
       fragColor.a = 1.0;
     }
   }
+
+  /* Pre-multiplied blending. */
+  fragColor.rgb *= fragColor.a;
 }