Solari: More accurate ReSTIR DI resampling (#22033)

* Adds MIS during reservoir merge which increases emissive light
brightness, bringing it a little closer to the pathtraced reference
* Tests final reservoir visibility for shading, but _not_ for
resampling, which greatly improves shadow accuracy compared to the
pathtraced reference, at the unfortunate cost of higher noise

Author: JMS55

crates/bevy_solari/src/realtime/restir_di.wgsl

@@ -9,7 +9,7 @@
 #import bevy_solari::brdf::evaluate_brdf
 #import bevy_solari::gbuffer_utils::{gpixel_resolve, pixel_dissimilar, permute_pixel}
 #import bevy_solari::presample_light_tiles::{ResolvedLightSamplePacked, unpack_resolved_light_sample}
-#import bevy_solari::sampling::{LightSample, calculate_resolved_light_contribution, resolve_and_calculate_light_contribution, resolve_light_sample, trace_light_visibility}
+#import bevy_solari::sampling::{LightSample, calculate_resolved_light_contribution, resolve_and_calculate_light_contribution, resolve_light_sample, trace_light_visibility, balance_heuristic}
 #import bevy_solari::scene_bindings::{light_sources, previous_frame_light_id_translations, LIGHT_NOT_PRESENT_THIS_FRAME}
 
 @group(1) @binding(0) var view_output: texture_storage_2d<rgba16float, read_write>;
@@ -49,8 +49,9 @@ fn initial_and_temporal(@builtin(workgroup_id) workgroup_id: vec3<u32>, @builtin
 
     let diffuse_brdf = surface.material.base_color / PI;
     let initial_reservoir = generate_initial_reservoir(surface.world_position, surface.world_normal, diffuse_brdf, workgroup_id.xy, &rng);
-    let temporal_reservoir = load_temporal_reservoir(global_id.xy, depth, surface.world_position, surface.world_normal);
-    let merge_result = merge_reservoirs(initial_reservoir, temporal_reservoir, surface.world_position, surface.world_normal, diffuse_brdf, &rng);
+    let temporal = load_temporal_reservoir(global_id.xy, depth, surface.world_position, surface.world_normal);
+    let merge_result = merge_reservoirs(initial_reservoir, surface.world_position, surface.world_normal, diffuse_brdf,
+        temporal.reservoir, temporal.world_position, temporal.world_normal, temporal.diffuse_brdf, &rng);
 
     store_reservoir_b(global_id.xy, merge_result.merged_reservoir);
 }
@@ -71,16 +72,25 @@ fn spatial_and_shade(@builtin(global_invocation_id) global_id: vec3<u32>) {
 
     let diffuse_brdf = surface.material.base_color / PI;
     let input_reservoir = load_reservoir_b(global_id.xy);
-    let spatial_reservoir = load_spatial_reservoir(global_id.xy, depth, surface.world_position, surface.world_normal, &rng);
-    let merge_result = merge_reservoirs(input_reservoir, spatial_reservoir, surface.world_position, surface.world_normal, diffuse_brdf, &rng);
+    let spatial = load_spatial_reservoir(global_id.xy, depth, surface.world_position, surface.world_normal, &rng);
+    let merge_result = merge_reservoirs(input_reservoir, surface.world_position, surface.world_normal, diffuse_brdf,
+        spatial.reservoir, spatial.world_position, spatial.world_normal, spatial.diffuse_brdf, &rng);
     var combined_reservoir = merge_result.merged_reservoir;
 
+    // More accuracy, less stability
+#ifndef BIASED_RESAMPLING
+    store_reservoir_a(global_id.xy, combined_reservoir);
+#endif
+
     if reservoir_valid(combined_reservoir) {
         let resolved_light_sample = resolve_light_sample(combined_reservoir.sample, light_sources[combined_reservoir.sample.light_id >> 16u]);
         combined_reservoir.unbiased_contribution_weight *= trace_light_visibility(surface.world_position, resolved_light_sample.world_position);
     }
 
+    // More stability, less accuracy (shadows extend further out than they should)
+#ifdef BIASED_RESAMPLING
     store_reservoir_a(global_id.xy, combined_reservoir);
+#endif
 
     let wo = normalize(view.world_position - surface.world_position);
     let brdf = evaluate_brdf(surface.world_normal, wo, merge_result.wi, surface.material);
@@ -135,63 +145,67 @@ 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>) -> NeighborInfo {
     let motion_vector = textureLoad(motion_vectors, pixel_id, 0).xy;
     let temporal_pixel_id_float = round(vec2<f32>(pixel_id) - (motion_vector * view.main_pass_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.main_pass_viewport.zw) || bool(constants.reset) {
-        return empty_reservoir();
+        return NeighborInfo(empty_reservoir(), vec3(0.0), vec3(0.0), vec3(0.0));
     }
 
     let permuted_temporal_pixel_id = permute_pixel(vec2<u32>(temporal_pixel_id_float), constants.frame_index, view.viewport.zw);
-    var temporal_reservoir = load_temporal_reservoir_inner(permuted_temporal_pixel_id, depth, world_position, world_normal);
+    var temporal = load_temporal_reservoir_inner(permuted_temporal_pixel_id, depth, world_position, world_normal);
 
     // If permuted reprojection failed (tends to happen on object edges), try point reprojection
-    if !reservoir_valid(temporal_reservoir) {
-        temporal_reservoir = load_temporal_reservoir_inner(vec2<u32>(temporal_pixel_id_float), depth, world_position, world_normal);
+    if !reservoir_valid(temporal.reservoir) {
+        temporal = load_temporal_reservoir_inner(vec2<u32>(temporal_pixel_id_float), depth, world_position, world_normal);
     }
 
     // Check if the light selected in the previous frame no longer exists in the current frame (e.g. entity despawned)
-    let previous_light_id = temporal_reservoir.sample.light_id >> 16u;
-    let triangle_id = temporal_reservoir.sample.light_id & 0xFFFFu;
+    let previous_light_id = temporal.reservoir.sample.light_id >> 16u;
+    let triangle_id = temporal.reservoir.sample.light_id & 0xFFFFu;
     let light_id = previous_frame_light_id_translations[previous_light_id];
     if light_id == LIGHT_NOT_PRESENT_THIS_FRAME {
-        return empty_reservoir();
+        return NeighborInfo(empty_reservoir(), vec3(0.0), vec3(0.0), vec3(0.0));
     }
-    temporal_reservoir.sample.light_id = (light_id << 16u) | triangle_id;
+    temporal.reservoir.sample.light_id = (light_id << 16u) | triangle_id;
 
-    temporal_reservoir.confidence_weight = min(temporal_reservoir.confidence_weight, CONFIDENCE_WEIGHT_CAP);
+    temporal.reservoir.confidence_weight = min(temporal.reservoir.confidence_weight, CONFIDENCE_WEIGHT_CAP);
 
-    return temporal_reservoir;
+    return temporal;
 }
 
-fn load_temporal_reservoir_inner(temporal_pixel_id: vec2<u32>, depth: f32, world_position: vec3<f32>, world_normal: vec3<f32>) -> Reservoir {
+fn load_temporal_reservoir_inner(temporal_pixel_id: vec2<u32>, depth: f32, world_position: vec3<f32>, world_normal: vec3<f32>) -> NeighborInfo {
     // 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_surface = gpixel_resolve(textureLoad(previous_gbuffer, temporal_pixel_id, 0), temporal_depth, temporal_pixel_id, view.main_pass_viewport.zw, previous_view.world_from_clip);
+    let temporal_diffuse_brdf = temporal_surface.material.base_color / PI;
     if pixel_dissimilar(depth, world_position, temporal_surface.world_position, world_normal, temporal_surface.world_normal, view) {
-        return empty_reservoir();
+        return NeighborInfo(empty_reservoir(), vec3(0.0), vec3(0.0), vec3(0.0));
     }
 
-    return load_reservoir_a(temporal_pixel_id);
+    let temporal_reservoir = load_reservoir_a(temporal_pixel_id);
+    return NeighborInfo(temporal_reservoir, temporal_surface.world_position, temporal_surface.world_normal, temporal_diffuse_brdf);
 }
 
-fn load_spatial_reservoir(pixel_id: vec2<u32>, depth: f32, world_position: vec3<f32>, world_normal: vec3<f32>, rng: ptr<function, u32>) -> Reservoir {
+fn load_spatial_reservoir(pixel_id: vec2<u32>, depth: f32, world_position: vec3<f32>, world_normal: vec3<f32>, rng: ptr<function, u32>) -> NeighborInfo {
     for (var i = 0u; i < 5u; i++) {
         let spatial_pixel_id = get_neighbor_pixel_id(pixel_id, rng);
 
         let spatial_depth = textureLoad(depth_buffer, spatial_pixel_id, 0);
         let spatial_surface = gpixel_resolve(textureLoad(gbuffer, spatial_pixel_id, 0), spatial_depth, spatial_pixel_id, view.main_pass_viewport.zw, view.world_from_clip);
+        let spatial_diffuse_brdf = spatial_surface.material.base_color / PI;
         if pixel_dissimilar(depth, world_position, spatial_surface.world_position, world_normal, spatial_surface.world_normal, view) {
             continue;
         }
 
-        return load_reservoir_b(spatial_pixel_id);
+        let spatial_reservoir = load_reservoir_b(spatial_pixel_id);
+        return NeighborInfo(spatial_reservoir, spatial_surface.world_position, spatial_surface.world_normal, spatial_diffuse_brdf);
     }
 
-    return empty_reservoir();
+    return NeighborInfo(empty_reservoir(), world_position, world_normal, vec3(0.0));
 }
 
 fn get_neighbor_pixel_id(center_pixel_id: vec2<u32>, rng: ptr<function, u32>) -> vec2<u32> {
@@ -200,6 +214,13 @@ fn get_neighbor_pixel_id(center_pixel_id: vec2<u32>, rng: ptr<function, u32>) ->
     return vec2<u32>(spatial_id);
 }
 
+struct NeighborInfo {
+    reservoir: Reservoir,
+    world_position: vec3<f32>,
+    world_normal: vec3<f32>,
+    diffuse_brdf: vec3<f32>,
+}
+
 struct Reservoir {
     sample: LightSample,
     confidence_weight: f32,
@@ -252,42 +273,56 @@ struct ReservoirMergeResult {
 
 fn merge_reservoirs(
     canonical_reservoir: Reservoir,
+    canonical_world_position: vec3<f32>,
+    canonical_world_normal: vec3<f32>,
+    canonical_diffuse_brdf: vec3<f32>,
     other_reservoir: Reservoir,
-    world_position: vec3<f32>,
-    world_normal: vec3<f32>,
-    diffuse_brdf: vec3<f32>,
+    other_world_position: vec3<f32>,
+    other_world_normal: vec3<f32>,
+    other_diffuse_brdf: vec3<f32>,
     rng: ptr<function, u32>,
 ) -> ReservoirMergeResult {
-    let canonical_contribution = reservoir_contribution(canonical_reservoir, world_position, world_normal, diffuse_brdf);
-    let other_contribution = reservoir_contribution(other_reservoir, world_position, world_normal, diffuse_brdf);
-
-    let mis_weight_denominator = 1.0 / (canonical_reservoir.confidence_weight + other_reservoir.confidence_weight);
-
-    let canonical_mis_weight = canonical_reservoir.confidence_weight * mis_weight_denominator;
-    let canonical_resampling_weight = canonical_mis_weight * (canonical_contribution.target_function * canonical_reservoir.unbiased_contribution_weight);
-
-    let other_mis_weight = other_reservoir.confidence_weight * mis_weight_denominator;
-    let other_resampling_weight = other_mis_weight * (other_contribution.target_function * other_reservoir.unbiased_contribution_weight);
+    // Contributions for resampling
+    let canonical_contribution_canonical_sample = reservoir_contribution(canonical_reservoir, canonical_world_position, canonical_world_normal, canonical_diffuse_brdf);
+    let canonical_contribution_other_sample = reservoir_contribution(other_reservoir, canonical_world_position, canonical_world_normal, canonical_diffuse_brdf);
+
+    // Extra contributions for MIS
+    let other_contribution_canonical_sample = reservoir_contribution(canonical_reservoir, other_world_position, other_world_normal, other_diffuse_brdf);
+    let other_contribution_other_sample = reservoir_contribution(other_reservoir, other_world_position, other_world_normal, other_diffuse_brdf);
+
+    // Resampling weight for canonical sample
+    let canonical_sample_mis_weight = balance_heuristic(
+        canonical_reservoir.confidence_weight * canonical_contribution_canonical_sample.target_function,
+        other_reservoir.confidence_weight * other_contribution_canonical_sample.target_function,
+    );
+    let canonical_sample_resampling_weight = canonical_sample_mis_weight * canonical_contribution_canonical_sample.target_function * canonical_reservoir.unbiased_contribution_weight;
 
-    let weight_sum = canonical_resampling_weight + other_resampling_weight;
+    // Resampling weight for other sample
+    let other_sample_mis_weight = balance_heuristic(
+        other_reservoir.confidence_weight * other_contribution_other_sample.target_function,
+        canonical_reservoir.confidence_weight * canonical_contribution_other_sample.target_function,
+    );
+    let other_sample_resampling_weight = other_sample_mis_weight * canonical_contribution_other_sample.target_function * other_reservoir.unbiased_contribution_weight;
 
+    // Perform resampling
     var combined_reservoir = empty_reservoir();
     combined_reservoir.confidence_weight = canonical_reservoir.confidence_weight + other_reservoir.confidence_weight;
+    let weight_sum = canonical_sample_resampling_weight + other_sample_resampling_weight;
 
-    if rand_f(rng) < other_resampling_weight / weight_sum {
+    if rand_f(rng) < other_sample_resampling_weight / weight_sum {
         combined_reservoir.sample = other_reservoir.sample;
 
-        let inverse_target_function = select(0.0, 1.0 / other_contribution.target_function, other_contribution.target_function > 0.0);
+        let inverse_target_function = select(0.0, 1.0 / canonical_contribution_other_sample.target_function, canonical_contribution_other_sample.target_function > 0.0);
         combined_reservoir.unbiased_contribution_weight = weight_sum * inverse_target_function;
 
-        return ReservoirMergeResult(combined_reservoir, other_contribution.radiance, other_contribution.wi);
+        return ReservoirMergeResult(combined_reservoir, canonical_contribution_other_sample.radiance, canonical_contribution_other_sample.wi);
     } else {
         combined_reservoir.sample = canonical_reservoir.sample;
 
-        let inverse_target_function = select(0.0, 1.0 / canonical_contribution.target_function, canonical_contribution.target_function > 0.0);
+        let inverse_target_function = select(0.0, 1.0 / canonical_contribution_canonical_sample.target_function, canonical_contribution_canonical_sample.target_function > 0.0);
         combined_reservoir.unbiased_contribution_weight = weight_sum * inverse_target_function;
 
-        return ReservoirMergeResult(combined_reservoir, canonical_contribution.radiance, canonical_contribution.wi);
+        return ReservoirMergeResult(combined_reservoir, canonical_contribution_canonical_sample.radiance, canonical_contribution_canonical_sample.wi);
     }
 }
 

crates/bevy_solari/src/realtime/restir_gi.wgsl

@@ -7,7 +7,7 @@
 #import bevy_render::view::View
 #import bevy_solari::brdf::evaluate_diffuse_brdf
 #import bevy_solari::gbuffer_utils::{gpixel_resolve, pixel_dissimilar, permute_pixel}
-#import bevy_solari::sampling::{sample_random_light, trace_point_visibility}
+#import bevy_solari::sampling::{sample_random_light, trace_point_visibility, balance_heuristic}
 #import bevy_solari::scene_bindings::{trace_ray, resolve_ray_hit_full, RAY_T_MIN, RAY_T_MAX}
 #import bevy_solari::world_cache::{query_world_cache, WORLD_CACHE_CELL_LIFETIME}
 
@@ -321,11 +321,3 @@ fn merge_reservoirs(
         return ReservoirMergeResult(combined_reservoir, canonical_sample_radiance);
     }
 }
-
-fn balance_heuristic(x: f32, y: f32) -> f32 {
-    let sum = x + y;
-    if sum == 0.0 {
-        return 0.0;
-    }
-    return x / sum;
-}

crates/bevy_solari/src/realtime/specular_gi.wgsl

@@ -146,7 +146,7 @@ fn nee_mis_weight(inverse_p_light: f32, brdf_rays_can_hit: bool, wo_tangent: vec
 
     let p_light = 1.0 / inverse_p_light;
     let p_bounce = ggx_vndf_pdf(wo_tangent, wi_tangent, ray_hit.material.roughness);
-    return max(0.0, power_heuristic(p_light, p_bounce));
+    return power_heuristic(p_light, p_bounce);
 }
 
 // Don't adjust the size of this struct without also adjusting GI_RESERVOIR_STRUCT_SIZE.

crates/bevy_solari/src/realtime/world_cache_query.wgsl

@@ -46,16 +46,19 @@ struct WorldCacheGeometryData {
 @group(1) @binding(23) var<storage, read_write> world_cache_active_cells_count: u32;
 
 #ifndef WORLD_CACHE_NON_ATOMIC_LIFE_BUFFER
-fn query_world_cache(world_position: vec3<f32>, world_normal: vec3<f32>, view_position: vec3<f32>, cell_lifetime: u32, rng: ptr<function, u32>) -> vec3<f32> {
+fn query_world_cache(world_position_in: vec3<f32>, world_normal: vec3<f32>, view_position: vec3<f32>, cell_lifetime: u32, rng: ptr<function, u32>) -> vec3<f32> {
+    var world_position = world_position_in;
     var cell_size = get_cell_size(world_position, view_position);
 
     // https://tomclabault.github.io/blog/2025/regir, jitter_world_position_tangent_plane
+#ifndef NO_JITTER_WORLD_CACHE
     let TBN = orthonormalize(world_normal);
     let offset = (rand_vec2f(rng) * 2.0 - 1.0) * cell_size * 0.5;
-    let jittered_position = world_position + offset.x * TBN[0] + offset.y * TBN[1];
-    cell_size = get_cell_size(jittered_position, view_position);
+    world_position += offset.x * TBN[0] + offset.y * TBN[1];
+    cell_size = get_cell_size(world_position, view_position);
+#endif
 
-    let world_position_quantized = bitcast<vec3<u32>>(quantize_position(jittered_position, cell_size));
+    let world_position_quantized = bitcast<vec3<u32>>(quantize_position(world_position, cell_size));
     let world_normal_quantized = bitcast<vec3<u32>>(quantize_normal(world_normal));
     var key = compute_key(world_position_quantized, world_normal_quantized);
     let checksum = compute_checksum(world_position_quantized, world_normal_quantized);
@@ -77,7 +80,7 @@ fn query_world_cache(world_position: vec3<f32>, world_normal: vec3<f32>, view_po
             return world_cache_radiance[key].rgb;
         } else if existing_checksum == WORLD_CACHE_EMPTY_CELL {
             // Cell is empty - initialize it
-            world_cache_geometry_data[key].world_position = jittered_position;
+            world_cache_geometry_data[key].world_position = world_position;
             world_cache_geometry_data[key].world_normal = world_normal;
             return vec3(0.0);
         } else {

crates/bevy_solari/src/scene/sampling.wgsl

@@ -6,11 +6,15 @@
 #import bevy_solari::scene_bindings::{trace_ray, RAY_T_MIN, RAY_T_MAX, light_sources, directional_lights, LightSource, LIGHT_SOURCE_KIND_DIRECTIONAL, resolve_triangle_data_full, ResolvedRayHitFull}
 
 fn power_heuristic(f: f32, g: f32) -> f32 {
-    return f * f / (f * f + g * g);
+    return balance_heuristic(f * f, g * g);
 }
 
 fn balance_heuristic(f: f32, g: f32) -> f32 {
-    return f / (f + g);
+    let sum = f + g;
+    if sum == 0.0 {
+        return 0.0;
+    }
+    return max(0.0, f / sum);
 }
 
 // https://gpuopen.com/download/Bounded_VNDF_Sampling_for_Smith-GGX_Reflections.pdf (Listing 1)