EEVEE: ScreenSpaceReflections: Jitter starting texel

This make sure the rays are generated randomly from a fullres
texel center.

This creates more noise but increase the convergence when doing
half res tracing.

2 files changed, 18 insertions, 2 deletions

diff --git a/source/blender/draw/engines/eevee/eevee_screen_raytrace.c b/source/blender/draw/engines/eevee/eevee_screen_raytrace.c
index c49f9ccc48c..75219b88e78 100644
--- a/source/blender/draw/engines/eevee/eevee_screen_raytrace.c
+++ b/source/blender/draw/engines/eevee/eevee_screen_raytrace.c
@@ -130,6 +130,9 @@ void EEVEE_screen_raytrace_cache_init(EEVEE_ViewLayerData *sldata, EEVEE_Data *v
     struct GPUShader *trace_shader = EEVEE_shaders_effect_reflection_trace_sh_get();
     struct GPUShader *resolve_shader = EEVEE_shaders_effect_reflection_resolve_sh_get();
 
+    int hitbuf_size[3];
+    GPU_texture_get_mipmap_size(effects->ssr_hit_output, 0, hitbuf_size);
+
     /** Screen space raytracing overview
      *
      * Following Frostbite stochastic SSR.
@@ -155,6 +158,9 @@ void EEVEE_screen_raytrace_cache_init(EEVEE_ViewLayerData *sldata, EEVEE_Data *v
     DRW_shgroup_uniform_block(grp, "planar_block", sldata->planar_ubo);
     DRW_shgroup_uniform_block(grp, "common_block", sldata->common_ubo);
     DRW_shgroup_uniform_block(grp, "renderpass_block", sldata->renderpass_ubo.combined);
+    DRW_shgroup_uniform_vec2_copy(grp, "targetSize", (float[2]){hitbuf_size[0], hitbuf_size[1]});
+    DRW_shgroup_uniform_float_copy(
+        grp, "randomScale", effects->reflection_trace_full ? 0.0f : 0.5f);
     DRW_shgroup_call_procedural_triangles(grp, NULL, 1);
 
     eGPUSamplerState no_filter = GPU_SAMPLER_DEFAULT;
diff --git a/source/blender/draw/engines/eevee/shaders/effect_reflection_trace_frag.glsl b/source/blender/draw/engines/eevee/shaders/effect_reflection_trace_frag.glsl
index f30b7789043..b88a106e897 100644
--- a/source/blender/draw/engines/eevee/shaders/effect_reflection_trace_frag.glsl
+++ b/source/blender/draw/engines/eevee/shaders/effect_reflection_trace_frag.glsl
@@ -19,6 +19,8 @@
 
 uniform sampler2D normalBuffer;
 uniform sampler2D specroughBuffer;
+uniform vec2 targetSize;
+uniform float randomScale;
 
 in vec4 uvcoordsvar;
 
@@ -27,7 +29,16 @@ layout(location = 1) out float hitDepth;
 
 void main()
 {
-  vec2 uvs = uvcoordsvar.xy;
+  vec4 rand = texelfetch_noise_tex(gl_FragCoord.xy);
+  /* Decorrelate from AA. */
+  /* TODO(fclem) we should use a more general approach for more random number dimensions. */
+  vec2 random_px = floor(fract(rand.xy * 2.2074408460575947536) * 1.99999) - 0.5;
+  rand.xy = fract(rand.xy * 3.2471795724474602596);
+
+  /* Randomly choose the pixel to start the ray from when tracing at lower resolution.
+   * This method also make sure we always start from the center of a fullres texel. */
+  vec2 uvs = (gl_FragCoord.xy + random_px * randomScale) / (targetSize * ssrUvScale);
+
   float depth = textureLod(maxzBuffer, uvs * hizUvScale.xy, 0.0).r;
 
   HitData data;
@@ -88,7 +99,6 @@ void main()
     }
   }
 
-  vec4 rand = texelfetch_noise_tex(vec2(gl_FragCoord.xy));
   /* Gives *perfect* reflection for very small roughness */
   if (roughness < 0.04) {
     rand.xzw *= 0.0;