Experimental bilinear reprojection

crates/bevy_solari/src/realtime/restir_di.wgsl

@@ -48,7 +48,7 @@ fn initial_and_temporal(@builtin(global_invocation_id) global_id: vec3<u32>) {
     let diffuse_brdf = base_color / PI;
 
     let initial_reservoir = generate_initial_reservoir(world_position, world_normal, diffuse_brdf, &rng);
-    let temporal_reservoir = load_temporal_reservoir(global_id.xy, depth, world_position, world_normal);
+    let temporal_reservoir = load_temporal_reservoir(global_id.xy, depth, world_position, world_normal, &rng);
     let combined_reservoir = merge_reservoirs(initial_reservoir, temporal_reservoir, world_position, world_normal, diffuse_brdf, &rng);
 
     di_reservoirs_b[pixel_index] = combined_reservoir.merged_reservoir;
@@ -119,14 +119,47 @@ fn generate_initial_reservoir(world_position: vec3<f32>, world_normal: vec3<f32>
     return reservoir;
 }
 
-fn load_temporal_reservoir(pixel_id: vec2<u32>, depth: f32, world_position: vec3<f32>, world_normal: vec3<f32>) -> Reservoir {
+fn load_temporal_reservoir(pixel_id: vec2<u32>, depth: f32, world_position: vec3<f32>, world_normal: vec3<f32>, rng: ptr<function, u32>) -> Reservoir {
     let motion_vector = textureLoad(motion_vectors, pixel_id, 0).xy;
-    let temporal_pixel_id_float = round(vec2<f32>(pixel_id) - (motion_vector * view.viewport.zw));
-    let temporal_pixel_id = vec2<u32>(temporal_pixel_id_float);
+    let temporal_pixel_id_float = vec2<f32>(pixel_id) - (motion_vector * view.viewport.zw);
+
+    // Check if the current pixel was off screen during the previous frame (current pixel is newly visible),
+    // or if all temporal history should assumed to be invalid
     if any(temporal_pixel_id_float < vec2(0.0)) || any(temporal_pixel_id_float >= view.viewport.zw) || bool(constants.reset) {
         return empty_reservoir();
     }
 
+    // https://en.wikipedia.org/wiki/Bilinear_interpolation#On_the_unit_square
+    let tl = vec2<u32>(temporal_pixel_id_float);
+    let tr = tl + vec2(1u, 0u);
+    let bl = tl + vec2(0u, 1u);
+    let br = tl + vec2(1u, 1u);
+    let f = fract(temporal_pixel_id_float);
+    let tl_w = (1.0 - f.x) * (1.0 - f.y);
+    let tr_w = f.x * (1.0 - f.y);
+    let bl_w = (1.0 - f.x) * f.y;
+
+    // Choose a random pixel from the 2x2 quad, weighted by the bilinear weights
+    // This gives better results than always using the nearest pixel
+    var temporal_pixel_id = tl;
+    var weight_sum = tl_w;
+    let r = rand_f(rng);
+    if (r > weight_sum) {
+        temporal_pixel_id = tr;
+        weight_sum += tr_w;
+    }
+    if (r > weight_sum) {
+        temporal_pixel_id = bl;
+        weight_sum += bl_w;
+    }
+    if (r > weight_sum) {
+        temporal_pixel_id = br;
+    }
+
+    // Clamp to view size, since 2x2 quad may go off screen
+    temporal_pixel_id = min(temporal_pixel_id, vec2<u32>(view.viewport.zw - 1.0));
+
+    // Check if the pixel features have changed heavily between the current and previous frame
     let temporal_depth = textureLoad(previous_depth_buffer, temporal_pixel_id, 0);
     let temporal_gpixel = textureLoad(previous_gbuffer, temporal_pixel_id, 0);
     let temporal_world_position = reconstruct_previous_world_position(temporal_pixel_id, temporal_depth);
@@ -138,6 +171,7 @@ fn load_temporal_reservoir(pixel_id: vec2<u32>, depth: f32, world_position: vec3
     let temporal_pixel_index = temporal_pixel_id.x + temporal_pixel_id.y * u32(view.viewport.z);
     var temporal_reservoir = di_reservoirs_a[temporal_pixel_index];
 
+    // Check if the light selected in the previous frame no longer exists in the current frame (e.g. entity despawned)
     temporal_reservoir.sample.light_id.x = previous_frame_light_id_translations[temporal_reservoir.sample.light_id.x];
     if temporal_reservoir.sample.light_id.x == LIGHT_NOT_PRESENT_THIS_FRAME {
         return empty_reservoir();

crates/bevy_solari/src/realtime/restir_gi.wgsl

@@ -43,7 +43,7 @@ fn initial_and_temporal(@builtin(global_invocation_id) global_id: vec3<u32>) {
     let base_color = pow(unpack4x8unorm(gpixel.r).rgb, vec3(2.2));
     let diffuse_brdf = base_color / PI;
 
-    let temporal_reservoir = load_temporal_reservoir(global_id.xy, depth, world_position, world_normal);
+    let temporal_reservoir = load_temporal_reservoir(global_id.xy, depth, world_position, world_normal, &rng);
 
     let ray_direction = sample_uniform_hemisphere(world_normal, &rng);
     let ray_hit = trace_ray(world_position, ray_direction, RAY_T_MIN, RAY_T_MAX, RAY_FLAG_NONE);
@@ -127,14 +127,47 @@ fn spatial_and_shade(@builtin(global_invocation_id) global_id: vec3<u32>) {
     textureStore(view_output, global_id.xy, pixel_color);
 }
 
-fn load_temporal_reservoir(pixel_id: vec2<u32>, depth: f32, world_position: vec3<f32>, world_normal: vec3<f32>) -> Reservoir {
+fn load_temporal_reservoir(pixel_id: vec2<u32>, depth: f32, world_position: vec3<f32>, world_normal: vec3<f32>, rng: ptr<function, u32>) -> Reservoir {
     let motion_vector = textureLoad(motion_vectors, pixel_id, 0).xy;
-    let temporal_pixel_id_float = round(vec2<f32>(pixel_id) - (motion_vector * view.viewport.zw));
-    let temporal_pixel_id = vec2<u32>(temporal_pixel_id_float);
+    let temporal_pixel_id_float = vec2<f32>(pixel_id) - (motion_vector * view.viewport.zw);
+
+    // Check if the current pixel was off screen during the previous frame (current pixel is newly visible),
+    // or if all temporal history should assumed to be invalid
     if any(temporal_pixel_id_float < vec2(0.0)) || any(temporal_pixel_id_float >= view.viewport.zw) || bool(constants.reset) {
         return empty_reservoir();
     }
 
+    // https://en.wikipedia.org/wiki/Bilinear_interpolation#On_the_unit_square
+    let tl = vec2<u32>(temporal_pixel_id_float);
+    let tr = tl + vec2(1u, 0u);
+    let bl = tl + vec2(0u, 1u);
+    let br = tl + vec2(1u, 1u);
+    let f = fract(temporal_pixel_id_float);
+    let tl_w = (1.0 - f.x) * (1.0 - f.y);
+    let tr_w = f.x * (1.0 - f.y);
+    let bl_w = (1.0 - f.x) * f.y;
+
+    // Choose a random pixel from the 2x2 quad, weighted by the bilinear weights
+    // This gives better results than always using the nearest pixel
+    var temporal_pixel_id = tl;
+    var weight_sum = tl_w;
+    let r = rand_f(rng);
+    if (r > weight_sum) {
+        temporal_pixel_id = tr;
+        weight_sum += tr_w;
+    }
+    if (r > weight_sum) {
+        temporal_pixel_id = bl;
+        weight_sum += bl_w;
+    }
+    if (r > weight_sum) {
+        temporal_pixel_id = br;
+    }
+
+    // Clamp to view size, since 2x2 quad may go off screen
+    temporal_pixel_id = min(temporal_pixel_id, vec2<u32>(view.viewport.zw - 1.0));
+
+    // Check if the pixel features have changed heavily between the current and previous frame
     let temporal_depth = textureLoad(previous_depth_buffer, temporal_pixel_id, 0);
     let temporal_gpixel = textureLoad(previous_gbuffer, temporal_pixel_id, 0);
     let temporal_world_position = reconstruct_previous_world_position(temporal_pixel_id, temporal_depth);