Trying to improve adaptive shadow bias

libraries/render-utils/src/LightStage.cpp

@@ -23,8 +23,6 @@ const glm::mat4 LightStage::Shadow::_biasMatrix{
     0.5, 0.5, 0.5, 1.0 };
 const int LightStage::Shadow::MAP_SIZE = 1024;
 
-static const auto MAX_BIAS = 0.006f;
-
 const LightStage::Index LightStage::INVALID_INDEX { render::indexed_container::INVALID_INDEX };
 
 LightStage::LightStage() {
@@ -63,7 +61,7 @@ LightStage::LightStage() {
 
 LightStage::Shadow::Schema::Schema() {
     ShadowTransform defaultTransform;
-    defaultTransform.bias = MAX_BIAS;
+    defaultTransform.fixedBias = 0.005f;
     std::fill(cascades, cascades + SHADOW_CASCADE_MAX_COUNT, defaultTransform);
     invMapSize = 1.0f / MAP_SIZE;
     cascadeCount = 1;
@@ -72,6 +70,18 @@ LightStage::Shadow::Schema::Schema() {
     maxDistance = 20.0f;
 }
 
+void LightStage::Shadow::Schema::updateCascade(int cascadeIndex, const ViewFrustum& shadowFrustum) {
+    auto& cascade = cascades[cascadeIndex];
+    cascade.frustumPosition = shadowFrustum.getPosition();
+    // The adaptative bias is computing how much depth offset we have to add to
+    // push back a coarsely sampled surface perpendicularly to the shadow direction,
+    // to not have shadow acnee. The final computation is done in the shader, based on the
+    // surface normal.
+    auto maxWorldFrustumSize = glm::max(shadowFrustum.getWidth(), shadowFrustum.getHeight());
+    cascade.adaptiveBiasUnitScale = maxWorldFrustumSize * 0.5f * invMapSize;
+    cascade.adaptiveBiasTransformScale = shadowFrustum.getNearClip() * shadowFrustum.getFarClip() / (shadowFrustum.getFarClip() - shadowFrustum.getNearClip());
+}
+
 LightStage::Shadow::Cascade::Cascade() : 
     _frustum{ std::make_shared<ViewFrustum>() },
     _minDistance{ 0.0f },
@@ -210,13 +220,15 @@ void LightStage::Shadow::setKeylightFrustum(const ViewFrustum& viewFrustum,
 
     // Position the keylight frustum
     auto position = viewFrustum.getPosition() - (nearDepth + farDepth)*lightDirection;
+    // Update the buffer
+    auto& schema = _schemaBuffer.edit<Schema>();
+    auto cascadeIndex = 0;
     for (auto& cascade : _cascades) {
         cascade._frustum->setOrientation(orientation);
         cascade._frustum->setPosition(position);
+        schema.cascades[cascadeIndex].frustumPosition = position;
+        cascadeIndex++;
     }
-    // Update the buffer
-    auto& schema = _schemaBuffer.edit<Schema>();
-    schema.lightDirInViewSpace = glm::inverse(viewFrustum.getView()) * glm::vec4(lightDirection, 0.f);
 }
 
 void LightStage::Shadow::setKeylightCascadeFrustum(unsigned int cascadeIndex, const ViewFrustum& viewFrustum,
@@ -269,8 +281,10 @@ void LightStage::Shadow::setKeylightCascadeFrustum(unsigned int cascadeIndex, co
 
     // Update the buffer
     auto& schema = _schemaBuffer.edit<Schema>();
-    schema.cascades[cascadeIndex].reprojection = _biasMatrix * ortho * shadowViewInverse.getMatrix();
-    schema.cascades[cascadeIndex].bias = baseBias;
+    auto& schemaCascade = schema.cascades[cascadeIndex];
+    schemaCascade.reprojection = _biasMatrix * ortho * shadowViewInverse.getMatrix();
+    schemaCascade.fixedBias = baseBias;
+    schema.updateCascade(cascadeIndex, *cascade._frustum);
 }
 
 void LightStage::Shadow::setCascadeFrustum(unsigned int cascadeIndex, const ViewFrustum& shadowFrustum) {
@@ -281,7 +295,10 @@ void LightStage::Shadow::setCascadeFrustum(unsigned int cascadeIndex, const View
 
     *cascade._frustum = shadowFrustum;
     // Update the buffer
-    _schemaBuffer.edit<Schema>().cascades[cascadeIndex].reprojection = _biasMatrix * shadowFrustum.getProjection() * viewInverse.getMatrix();
+    auto& schema = _schemaBuffer.edit<Schema>();
+    auto& schemaCascade = schema.cascades[cascadeIndex];
+    schemaCascade.reprojection = _biasMatrix * shadowFrustum.getProjection() * viewInverse.getMatrix();
+    schema.updateCascade(cascadeIndex, shadowFrustum);
 }
 
 LightStage::Index LightStage::findLight(const LightPointer& light) const {

libraries/render-utils/src/LightStage.h

@@ -110,6 +110,8 @@ public:
 
             Schema();
 
+            void updateCascade(int cascadeIndex, const ViewFrustum& shadowFrustum);
+
         };
         UniformBufferView _schemaBuffer = nullptr;
     };
@@ -213,6 +215,7 @@ protected:
     Index _sunOffLightId;
 
     Index _defaultLightId;
+
 };
 using LightStagePointer = std::shared_ptr<LightStage>;
 

libraries/render-utils/src/RenderShadowTask.cpp

@@ -256,7 +256,9 @@ void RenderShadowSetup::run(const render::RenderContextPointer& renderContext) {
 }
 
 void RenderShadowCascadeSetup::configure(const Config& configuration) {
-    _baseBias = configuration.bias * configuration.bias * 0.01f;
+    // I'm not very proud of this empirical adjustment
+    auto cascadeBias = configuration.bias * powf(1.1f, _cascadeIndex);
+    _baseBias = cascadeBias * cascadeBias * 0.01f;
 }
 
 void RenderShadowCascadeSetup::run(const render::RenderContextPointer& renderContext, Outputs& output) {

libraries/render-utils/src/RenderShadowTask.h

@@ -67,7 +67,7 @@ class RenderShadowCascadeSetupConfig : public render::Job::Config {
         Q_PROPERTY(float bias MEMBER bias NOTIFY dirty)
 public:
 
-    float bias{ 0.5f };
+    float bias{ 0.25f };
 
 signals:
     void dirty();

libraries/render-utils/src/Shadow.slh

@@ -83,11 +83,17 @@ float evalShadowAttenuationPCF(int cascadeIndex, ShadowSampleOffsets offsets, ve
     return shadowAttenuation;
 }
 
-float evalShadowCascadeAttenuation(int cascadeIndex, ShadowSampleOffsets offsets, vec4 shadowTexcoord, float bias) {
+float evalShadowCascadeAttenuation(int cascadeIndex, ShadowSampleOffsets offsets, vec4 shadowTexcoord, float bias,
+                                   vec3 worldPosition, vec3 worldLightDir) {
     if (!isShadowCascadeProjectedOnPixel(shadowTexcoord)) {
         // If a point is not in the map, do not attenuate
         return 1.0;
     }
+    // After this, bias is in view units
+    bias *= getShadowAdaptiveBiasUnitScale(cascadeIndex);
+    // Transform to texcoord space (between 0 and 1)
+    float shadowDepth = abs(dot(worldPosition-getShadowFrustumPosition(cascadeIndex), worldLightDir));
+    bias = transformShadowAdaptiveBias(cascadeIndex, shadowDepth, bias);
     // Add fixed bias
     bias += getShadowBias(cascadeIndex);
     return evalShadowAttenuationPCF(cascadeIndex, offsets, shadowTexcoord, bias);
@@ -101,12 +107,11 @@ float evalShadowAttenuation(vec3 worldLightDir, vec4 worldPosition, float viewDe
 	
     // Adjust bias if we are at a grazing angle with light
     float ndotl = dot(worldLightDir, worldNormal);
-    float bias = 0.5*(1.0/(ndotl*ndotl)-1.0);
-    bias *= getShadowScale();
+    float bias = 1.0/(ndotl*ndotl)-1.0;
     vec2 cascadeAttenuations = vec2(1.0, 1.0);
-    cascadeAttenuations.x = evalShadowCascadeAttenuation(cascadeIndices.x, offsets, cascadeShadowCoords[0], bias);
+    cascadeAttenuations.x = evalShadowCascadeAttenuation(cascadeIndices.x, offsets, cascadeShadowCoords[0], bias, worldPosition.xyz, worldLightDir);
     if (cascadeMix > 0.0 && cascadeIndices.y < getShadowCascadeCount()) {
-        cascadeAttenuations.y = evalShadowCascadeAttenuation(cascadeIndices.y, offsets, cascadeShadowCoords[1], bias);
+        cascadeAttenuations.y = evalShadowCascadeAttenuation(cascadeIndices.y, offsets, cascadeShadowCoords[1], bias, worldPosition.xyz, worldLightDir);
     }
     float attenuation = mix(cascadeAttenuations.x, cascadeAttenuations.y, cascadeMix);
     // Falloff to max distance

libraries/render-utils/src/ShadowCore.slh

@@ -38,11 +38,19 @@ float getShadowScale() {
 }
 
 float getShadowBias(int cascadeIndex) {
-	return shadow.cascades[cascadeIndex].bias;
+	return shadow.cascades[cascadeIndex].fixedBias;
 }
 
-vec3 getShadowDirInViewSpace() {
-    return shadow.lightDirInViewSpace;
+vec3 getShadowFrustumPosition(int cascadeIndex) {
+	return shadow.cascades[cascadeIndex].frustumPosition;
+}
+
+float getShadowAdaptiveBiasUnitScale(int cascadeIndex) {
+	return shadow.cascades[cascadeIndex].adaptiveBiasUnitScale;
+}
+
+float transformShadowAdaptiveBias(int cascadeIndex, float shadowDepth, float depthBias) {
+    return shadow.cascades[cascadeIndex].adaptiveBiasTransformScale * depthBias / (shadowDepth*shadowDepth);
 }
 
 // Compute the texture coordinates from world coordinates

libraries/render-utils/src/Shadows_shared.slh

@@ -11,16 +11,16 @@
 
 struct ShadowTransform {
 	MAT4 reprojection;
-
-	float bias;
+    VEC3 frustumPosition;
+	float fixedBias;
+    float adaptiveBiasUnitScale;
+    float adaptiveBiasTransformScale;
     float _padding1;
     float _padding2;
-    float _padding3;
 };
 
 struct ShadowParameters {
     ShadowTransform cascades[SHADOW_CASCADE_MAX_COUNT];
-    VEC3 lightDirInViewSpace;
     int cascadeCount;
     float invMapSize;
     float invCascadeBlendWidth;