Fix T72154: Background Camera Images

The Camera Background Images uses a alpha under blending. For alpha
under blending to work correctly the framebuffer containing the result
of the render engine needs to be active
(`DefaultFramebufferList.default_fb`) and the source blend color needs
to be premultiplied with alpha.

Due to recent refactoring this wasn't the case and it seemed that the
background image was drawn in front of the scene. This patch sets the
correct state so it seems to be drawn behind the scene.

Reviewed By: fclem

Differential Revision: https://developer.blender.org/D6365

1 files changed, 14 insertions, 3 deletions

diff --git a/source/blender/draw/engines/overlay/overlay_image.c b/source/blender/draw/engines/overlay/overlay_image.c
index 34e382b2e4a..cda55fcfb5e 100644
--- a/source/blender/draw/engines/overlay/overlay_image.c
+++ b/source/blender/draw/engines/overlay/overlay_image.c
@@ -326,6 +326,9 @@ void OVERLAY_image_camera_cache_populate(OVERLAY_Data *vedata, Object *ob)
       mul_m4_m4m4(mat, norm_obmat, mat);
       const bool is_foreground = (bgpic->flag & CAM_BGIMG_FLAG_FOREGROUND) != 0;
 
+      float color_alpha[4] = {1.0f, 1.0f, 1.0f, bgpic->alpha};
+      float color_premult_alpha[4] = {bgpic->alpha, bgpic->alpha, bgpic->alpha, bgpic->alpha};
+
       /* When drawing background we do 2 passes.
        * - One alpha over, which works where background is visible.
        * - One alpha under, works under partially visible objects. (only in cycles)
@@ -337,7 +340,7 @@ void OVERLAY_image_camera_cache_populate(OVERLAY_Data *vedata, Object *ob)
                                                     psl->image_background_over_ps);
         GPUShader *sh = OVERLAY_shader_image();
         DRWShadingGroup *grp = DRW_shgroup_create(sh, pass);
-        float color[4] = {1.0f, 1.0f, 1.0f, bgpic->alpha};
+        float *color = (is_foreground || i == 1) ? color_alpha : color_premult_alpha;
         DRW_shgroup_uniform_texture(grp, "imgTexture", tex);
         DRW_shgroup_uniform_bool_copy(grp, "imgPremultiplied", use_alpha_premult);
         DRW_shgroup_uniform_bool_copy(grp, "imgAlphaBlend", true);
@@ -441,11 +444,19 @@ void OVERLAY_image_draw(OVERLAY_Data *vedata)
 {
   OVERLAY_PassList *psl = vedata->psl;
   OVERLAY_PrivateData *pd = vedata->stl->pd;
+  OVERLAY_FramebufferList *fbl = vedata->fbl;
 
-  DRW_view_set_active(pd->view_reference_images);
+  const DefaultFramebufferList *dfbl = DRW_viewport_framebuffer_list_get();
 
+  DRW_view_set_active(pd->view_reference_images);
   DRW_draw_pass(psl->image_background_over_ps);
-  DRW_draw_pass(psl->image_background_under_ps);
+
+  if (DRW_state_is_fbo() && !DRW_pass_is_empty(psl->image_background_under_ps)) {
+    GPU_framebuffer_bind(dfbl->default_fb);
+    DRW_draw_pass(psl->image_background_under_ps);
+    GPU_framebuffer_bind(fbl->overlay_default_fb);
+  }
+
   DRW_draw_pass(psl->image_empties_back_ps);
 
   DRW_draw_pass(psl->image_empties_ps);