EEVEE: Screen Space Reflection: Improve self intersection cases.

This patch fix most self intersection comming from reflection rays.
We regenerate the ray if it goes below the shading normal (should be the
geometric normal but we have no access to it here).

Also add the same precision based bias we use for contact shadows.

This fix T81105 Eevee SSR quality regression in 2.91 alpha

1 files changed, 32 insertions, 4 deletions

diff --git a/source/blender/draw/engines/eevee/shaders/effect_ssr_frag.glsl b/source/blender/draw/engines/eevee/shaders/effect_ssr_frag.glsl
index f8dccb7511a..d50a4eaea3c 100644
--- a/source/blender/draw/engines/eevee/shaders/effect_ssr_frag.glsl
+++ b/source/blender/draw/engines/eevee/shaders/effect_ssr_frag.glsl
@@ -81,11 +81,34 @@ void do_planar_ssr(int index,
 void do_ssr(vec3 V, vec3 N, vec3 T, vec3 B, vec3 viewPosition, float a2, vec4 rand)
 {
   float NH;
-  vec3 H = sample_ggx(rand.xzw, a2, N, T, B, NH); /* Microfacet normal */
-  float pdf = pdf_ggx_reflect(NH, a2);
-
+  /* Microfacet normal */
+  vec3 H = sample_ggx(rand.xzw, a2, N, T, B, NH);
   vec3 R = reflect(-V, H);
-  pdfData = min(1024e32, pdf); /* Theoretical limit of 16bit float */
+
+  /* If ray is bad (i.e. going below the surface) regenerate. */
+  /* This threshold is a bit higher than 0 to improve self intersection cases. */
+  const float bad_ray_threshold = 0.085;
+  if (dot(R, N) <= bad_ray_threshold) {
+    H = sample_ggx(rand.xzw * vec3(1.0, -1.0, -1.0), a2, N, T, B, NH);
+    R = reflect(-V, H);
+  }
+
+  if (dot(R, N) <= bad_ray_threshold) {
+    H = sample_ggx(rand.xzw * vec3(1.0, 1.0, -1.0), a2, N, T, B, NH);
+    R = reflect(-V, H);
+  }
+
+  if (dot(R, N) <= bad_ray_threshold) {
+    H = sample_ggx(rand.xzw * vec3(1.0, -1.0, 1.0), a2, N, T, B, NH);
+    R = reflect(-V, H);
+  }
+
+  if (dot(R, N) <= bad_ray_threshold) {
+    /* Not worth tracing. */
+    return;
+  }
+
+  pdfData = min(1024e32, pdf_ggx_reflect(NH, a2)); /* Theoretical limit of 16bit float */
 
   vec3 hit_pos = raycast(-1, viewPosition, R * 1e16, ssrThickness, rand.y, ssrQuality, a2, true);
 
@@ -171,6 +194,11 @@ void main()
     }
   }
 
+  /* Constant bias (due to depth buffer precision). Helps with self intersection. */
+  /* Magic numbers for 24bits of precision.
+   * From http://terathon.com/gdc07_lengyel.pdf (slide 26) */
+  viewPosition.z = get_view_z_from_depth(depth - mix(2.4e-7, 4.8e-7, depth));
+
   do_ssr(V, N, T, B, viewPosition, a2, rand);
 }