Fix T91398 Overlay: Camera BG jitter offset (regression)

This was caused by camera background being rendered in world space, causing
floating point imprecision issues when camera was far from origin.

Adding a uniform to change vertex shader to process everything in viewspace
to fix the problem.

2 files changed, 11 insertions, 3 deletions

diff --git a/source/blender/draw/engines/overlay/overlay_image.c b/source/blender/draw/engines/overlay/overlay_image.c
index ada07499d1c..2e9a9bc5c64 100644
--- a/source/blender/draw/engines/overlay/overlay_image.c
+++ b/source/blender/draw/engines/overlay/overlay_image.c
@@ -344,7 +344,6 @@ void OVERLAY_image_camera_cache_populate(OVERLAY_Data *vedata, Object *ob)
     if (tex) {
       image_camera_background_matrix_get(cam, bgpic, draw_ctx, aspect, mat);
 
-      mul_m4_m4m4(mat, modelmat, mat);
       const bool is_foreground = (bgpic->flag & CAM_BGIMG_FLAG_FOREGROUND) != 0;
       /* Alpha is clamped just below 1.0 to fix background images to interfere with foreground
        * images. Without this a background image with 1.0 will be rendered on top of a transparent
@@ -361,6 +360,7 @@ void OVERLAY_image_camera_cache_populate(OVERLAY_Data *vedata, Object *ob)
       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);
+      DRW_shgroup_uniform_bool_copy(grp, "isCameraBackground", true);
       DRW_shgroup_uniform_bool_copy(grp, "depthSet", true);
       DRW_shgroup_uniform_vec4_copy(grp, "color", color_premult_alpha);
       DRW_shgroup_call_obmat(grp, DRW_cache_quad_get(), mat);
@@ -446,6 +446,7 @@ void OVERLAY_image_empty_cache_populate(OVERLAY_Data *vedata, Object *ob)
     DRW_shgroup_uniform_texture(grp, "imgTexture", tex);
     DRW_shgroup_uniform_bool_copy(grp, "imgPremultiplied", use_alpha_premult);
     DRW_shgroup_uniform_bool_copy(grp, "imgAlphaBlend", use_alpha_blend);
+    DRW_shgroup_uniform_bool_copy(grp, "isCameraBackground", false);
     DRW_shgroup_uniform_bool_copy(grp, "depthSet", depth_mode != OB_EMPTY_IMAGE_DEPTH_DEFAULT);
     DRW_shgroup_uniform_vec4_copy(grp, "color", ob->color);
     DRW_shgroup_call_obmat(grp, DRW_cache_quad_get(), mat);
diff --git a/source/blender/draw/engines/overlay/shaders/image_vert.glsl b/source/blender/draw/engines/overlay/shaders/image_vert.glsl
index 621e1d8068b..a44ea7081ba 100644
--- a/source/blender/draw/engines/overlay/shaders/image_vert.glsl
+++ b/source/blender/draw/engines/overlay/shaders/image_vert.glsl
@@ -1,5 +1,6 @@
 
 uniform bool depthSet;
+uniform bool isCameraBackground;
 
 in vec3 pos;
 
@@ -7,8 +8,14 @@ out vec2 uvs;
 
 void main()
 {
-  vec3 world_pos = point_object_to_world(pos);
-  gl_Position = point_world_to_ndc(world_pos);
+  if (isCameraBackground) {
+    vec3 vP = (ModelMatrix * vec4(pos, 1.0)).xyz;
+    gl_Position = point_view_to_ndc(vP);
+  }
+  else {
+    vec3 world_pos = point_object_to_world(pos);
+    gl_Position = point_world_to_ndc(world_pos);
+  }
 
   if (depthSet) {
     /* Result in a position at 1.0 (far plane). Small epsilon to avoid precision issue.