EEVEE: ScreenSpaceReflections: Add back multi ray-hitpoint reuse

We now reuse 9 hitpoints from the neighboorhood using a blue noise
sample distribution as mentionned in the reference presentation.

Reusing more rays does however make some area a bit more blury.

The resulting noise is quite lower compared to previous implementation
which was only reusing 4 hits.

2 files changed, 73 insertions, 7 deletions

diff --git a/source/blender/draw/engines/eevee/shaders/effect_reflection_lib.glsl b/source/blender/draw/engines/eevee/shaders/effect_reflection_lib.glsl
index 0f3f04596a8..cc73bbbfe29 100644
--- a/source/blender/draw/engines/eevee/shaders/effect_reflection_lib.glsl
+++ b/source/blender/draw/engines/eevee/shaders/effect_reflection_lib.glsl
@@ -46,3 +46,56 @@ HitData decode_hit_data(vec4 hit_data, float hit_depth)
   data.is_hit = (hit_data.w > 0.0);
   return data;
 }
+
+/* Blue noise categorised into 4 sets of samples.
+ * See "Stochastic all the things" presentation slide 32-37. */
+const int resolve_samples_count = 9;
+const vec2 resolve_sample_offsets[36] = vec2[36](
+    /* Set 1. */
+    /* First Ring (2x2). */
+    vec2(0, 0),
+    /* Second Ring (6x6). */
+    vec2(-1, 3),
+    vec2(1, 3),
+    vec2(-1, 1),
+    vec2(3, 1),
+    vec2(-2, 0),
+    vec2(3, 0),
+    vec2(2, -1),
+    vec2(1, -2),
+    /* Set 2. */
+    /* First Ring (2x2). */
+    vec2(1, 1),
+    /* Second Ring (6x6). */
+    vec2(-2, 3),
+    vec2(3, 3),
+    vec2(0, 2),
+    vec2(2, 2),
+    vec2(-2, -1),
+    vec2(1, -1),
+    vec2(0, -2),
+    vec2(3, -2),
+    /* Set 3. */
+    /* First Ring (2x2). */
+    vec2(0, 1),
+    /* Second Ring (6x6). */
+    vec2(0, 3),
+    vec2(3, 2),
+    vec2(-2, 1),
+    vec2(2, 1),
+    vec2(-1, 0),
+    vec2(-2, -2),
+    vec2(0, -1),
+    vec2(2, -2),
+    /* Set 4. */
+    /* First Ring (2x2). */
+    vec2(1, 0),
+    /* Second Ring (6x6). */
+    vec2(2, 3),
+    vec2(-2, 2),
+    vec2(-1, 2),
+    vec2(1, 2),
+    vec2(2, 0),
+    vec2(-1, -1),
+    vec2(3, -1),
+    vec2(-1, -2));
\ No newline at end of file
diff --git a/source/blender/draw/engines/eevee/shaders/effect_reflection_resolve_frag.glsl b/source/blender/draw/engines/eevee/shaders/effect_reflection_resolve_frag.glsl
index 45057109cdf..d2f4245c603 100644
--- a/source/blender/draw/engines/eevee/shaders/effect_reflection_resolve_frag.glsl
+++ b/source/blender/draw/engines/eevee/shaders/effect_reflection_resolve_frag.glsl
@@ -24,6 +24,8 @@ uniform sampler2D specroughBuffer;
 uniform sampler2D hitBuffer;
 uniform sampler2D hitDepth;
 
+uniform int samplePoolOffset;
+
 in vec4 uvcoordsvar;
 
 out vec4 fragColor;
@@ -75,8 +77,8 @@ void resolve_reflection_sample(int planar_index,
                                inout float weight_accum,
                                inout vec4 ssr_accum)
 {
-  vec4 hit_data = texture(hitBuffer, sample_uv * ssrUvScale);
-  float hit_depth = texture(hitDepth, sample_uv * ssrUvScale).r;
+  vec4 hit_data = texture(hitBuffer, sample_uv);
+  float hit_depth = texture(hitDepth, sample_uv).r;
   HitData data = decode_hit_data(hit_data, hit_depth);
 
   float hit_dist = length(data.hit_dir);
@@ -86,9 +88,12 @@ void resolve_reflection_sample(int planar_index,
 
   float weight = bsdf * data.ray_pdf_inv;
 
+  /* Do not reuse hitpoint from planar reflections for normal reflections and vice versa. */
+  if ((planar_index == -1 && data.is_planar) || (planar_index != -1 && !data.is_planar)) {
+    return;
+  }
   /* Do not add light if ray has failed but still weight it. */
-  if (!data.is_hit || (planar_index == -1 && data.is_planar) ||
-      (planar_index != -1 && !data.is_planar)) {
+  if (!data.is_hit) {
     weight_accum += weight;
     return;
   }
@@ -155,10 +160,18 @@ void raytrace_resolve(ClosureInputGlossy cl_in,
     float cone_tan = sqrt(1.0 - cone_cos * cone_cos) / cone_cos;
     cone_tan *= mix(saturate(dot(vN, -vV) * 2.0), 1.0, roughness); /* Elongation fit */
 
-    vec2 sample_uv = uvcoordsvar.xy;
+    int sample_pool = int((uint(gl_FragCoord.x) & 1u) + (uint(gl_FragCoord.y) & 1u) * 2u);
+    sample_pool = (sample_pool + (samplePoolOffset / 5)) % 4;
 
-    resolve_reflection_sample(
-        planar_index, sample_uv, vP, vN, vV, roughness_squared, cone_tan, weight_acc, ssr_accum);
+    for (int i = 0; i < resolve_samples_count; i++) {
+      int sample_id = sample_pool * resolve_samples_count + i;
+      vec2 texture_size = vec2(textureSize(hitBuffer, 0));
+      vec2 sample_texel = texture_size * uvcoordsvar.xy * ssrUvScale;
+      vec2 sample_uv = (sample_texel + resolve_sample_offsets[sample_id]) / texture_size;
+
+      resolve_reflection_sample(
+          planar_index, sample_uv, vP, vN, vV, roughness_squared, cone_tan, weight_acc, ssr_accum);
+    }
   }
 
   /* Compute SSR contribution */