Fix #127774: Flat Object matrix leads to incorrect culling

Object with degenerate transform matrix can lead to flat
bounds on GPU. This in turn lead to NaN intersection planes
inside `IsectBox`.

Compute (pseudo) size of matrix and bypass culling is any
axis is too small.

The other part of the patch makes sure that there is a
distinction between disabled culling and invalid
bounding boxes.

Pull Request: https://projects.blender.org/blender/blender/pulls/127807

source/blender/draw/intern/draw_shader_shared.hh

@@ -204,7 +204,16 @@ struct ObjectBounds {
 };
 BLI_STATIC_ASSERT_ALIGN(ObjectBounds, 16)
 
+/* Return true if `bounding_corners` are valid. Should be checked before accessing them.
+ * Does not guarantee that `bounding_sphere` is valid. */
 inline bool drw_bounds_are_valid(ObjectBounds bounds)
+{
+  return bounds.bounding_sphere.w != -1.0f;
+}
+
+/* Return true if bounds are ready for culling.
+ * In this case, both `bounding_corners` and `bounding_sphere` are valid. */
+inline bool drw_bounds_culling_enabled(ObjectBounds bounds)
 {
   return bounds.bounding_sphere.w >= 0.0f;
 }

source/blender/draw/intern/shaders/draw_resource_finalize_comp.glsl

@@ -6,7 +6,8 @@
  * Finish computation of a few draw resource after sync.
  */
 
-#pragma BLENDER_REQUIRE(common_math_lib.glsl)
+#pragma BLENDER_REQUIRE(gpu_shader_math_vector_lib.glsl)
+#pragma BLENDER_REQUIRE(gpu_shader_math_matrix_lib.glsl)
 
 void main()
 {
@@ -31,7 +32,7 @@ void main()
     p03.z = max(p03.z, 1e-4);
     vec3 diagonal = p01 + p02 + p03;
     vec3 center = p0 + diagonal * 0.5;
-    float min_axis = min_v3(abs(diagonal));
+    float min_axis = reduce_min(abs(diagonal));
     bounds.bounding_sphere.xyz = transform_point(model_mat, center);
     /* We have to apply scaling to the diagonal. */
     bounds.bounding_sphere.w = length(transform_direction(model_mat, diagonal)) * 0.5;
@@ -51,6 +52,16 @@ void main()
       bounds.bounding_sphere.w = -2.0;
     }
 
+    /* Bypass culling test for objects that are flattenned on one or more axes (see #127774).
+     * Fixing them is too much computation but might be worth doing if a use case for it.
+     * Do not compute the real length to save some instructions. */
+    vec3 object_scale = vec3(reduce_add(abs(model_mat[0].xyz)),
+                             reduce_add(abs(model_mat[1].xyz)),
+                             reduce_add(abs(model_mat[2].xyz)));
+    if (any(lessThan(abs(object_scale), vec3(1e-10)))) {
+      bounds.bounding_sphere.w = -2.0;
+    }
+
     /* Update bounds. */
     bounds_buf[resource_id] = bounds;
   }

source/blender/draw/intern/shaders/draw_visibility_comp.glsl

@@ -32,7 +32,7 @@ void main()
 
   ObjectBounds bounds = bounds_buf[gl_GlobalInvocationID.x];
 
-  if (drw_bounds_are_valid(bounds)) {
+  if (drw_bounds_culling_enabled(bounds)) {
     IsectBox box = isect_box_setup(bounds.bounding_corners[0].xyz,
                                    bounds.bounding_corners[1].xyz,
                                    bounds.bounding_corners[2].xyz,