Fix T86429 EEVEE: Ambient occlusion broken on some platform

This was cause by a division by 0 if the ray direction had no
depth difference. Adding a small epsilon fix the issue.

1 files changed, 5 insertions, 0 deletions

diff --git a/source/blender/draw/engines/eevee/shaders/raytrace_lib.glsl b/source/blender/draw/engines/eevee/shaders/raytrace_lib.glsl
index e2aad867901..c47a8bdc4e9 100644
--- a/source/blender/draw/engines/eevee/shaders/raytrace_lib.glsl
+++ b/source/blender/draw/engines/eevee/shaders/raytrace_lib.glsl
@@ -48,6 +48,11 @@ void raytrace_screenspace_ray_finalize(inout ScreenSpaceRay ray)
   if (len_squared(ray.direction.xy) < 0.00001) {
     ray.direction.xy = vec2(0.0, 0.01);
   }
+  /* Avoid divide by 0 error in line_unit_box_intersect_dist, leading to undefined behavior
+   * (see T86429). */
+  if (ray.direction.z == 0.0) {
+    ray.direction.z = 0.0001;
+  }
   float ray_len_sqr = len_squared(ray.direction.xyz);
   /* Make ray.direction cover one pixel. */
   bool is_more_vertical = abs(ray.direction.x) < abs(ray.direction.y);