Fixed a banding problem when using sphere light sources with very sma…

…ll radius (issue LuxCoreRender/BlendLuxCore#477)
LuxCoreRender · Jun 4, 2020 · e15e45c · e15e45c
1 parent 68e1ae4
commit e15e45c
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Showing 3 changed files with 56 additions and 43 deletions.
diff --git a/include/slg/lights/light_funcs.cl b/include/slg/lights/light_funcs.cl
@@ -626,34 +626,40 @@ OPENCL_FORCE_INLINE float3 SphereLight_Illuminate(__global const LightSource *sp
 
 	// The point isn't inside the sphere
 
-	// Build a local coordinate system
-	const float3 localZ = toLight * (1.f / centerDistance);
-	Frame localFrame;
-	Frame_SetFromZ_Private(&localFrame, localZ);
-
-	// Sample sphere uniformly inside subtended cone
 	const float cosThetaMax = sqrt(max(0.f, 1.f - radiusSquared / centerDistanceSquared));
+	if (cosThetaMax > 1.f - DEFAULT_EPSILON_STATIC) {
+		// If the subtended angle is too small, I sample the light source like
+		// if it was a point light source in order to avoiding banding due to
+		// (lack of) numerical precision.
+		return PointLight_Illuminate(sphereLight, bsdf, time, shadowRay, directPdfW);
+	} else {
+		// Build a local coordinate system
+		const float3 localZ = toLight * (1.f / centerDistance);
+		Frame localFrame;
+		Frame_SetFromZ_Private(&localFrame, localZ);
+
+		// Sample sphere uniformly inside subtended cone
+		const float3 localShadowRayDir = UniformSampleConeLocal(u0, u1, cosThetaMax);
+		if (CosTheta(localShadowRayDir) < DEFAULT_COS_EPSILON_STATIC)
+			return BLACK;
 
-	const float3 localShadowRayDir = UniformSampleConeLocal(u0, u1, cosThetaMax);
-	if (CosTheta(localShadowRayDir) < DEFAULT_COS_EPSILON_STATIC)
-		return BLACK;
-
-	const float3 shadowRayDir = Frame_ToWorld_Private(&localFrame, localShadowRayDir);
+		const float3 shadowRayDir = Frame_ToWorld_Private(&localFrame, localShadowRayDir);
 
-	// Check the intersection with the sphere
-	const float3 shadowRayOrig = BSDF_GetRayOrigin(bsdf, shadowRayDir);
-	float shadowRayDistance;
-	if (!SphereLight_SphereIntersect(absolutePos, radiusSquared, shadowRayOrig, shadowRayDir, &shadowRayDistance))
-		shadowRayDistance = dot(toLight, shadowRayDir);
+		// Check the intersection with the sphere
+		const float3 shadowRayOrig = BSDF_GetRayOrigin(bsdf, shadowRayDir);
+		float shadowRayDistance;
+		if (!SphereLight_SphereIntersect(absolutePos, radiusSquared, shadowRayOrig, shadowRayDir, &shadowRayDistance))
+			shadowRayDistance = dot(toLight, shadowRayDir);
 
-	*directPdfW = UniformConePdf(cosThetaMax);
+		*directPdfW = UniformConePdf(cosThetaMax);
 
-	// Setup the shadow ray
-	Ray_Init4(shadowRay, shadowRayOrig, shadowRayDir, 0.f, shadowRayDistance, time);
+		// Setup the shadow ray
+		Ray_Init4(shadowRay, shadowRayOrig, shadowRayDir, 0.f, shadowRayDistance, time);
 
-	const float invArea = 1.f / (4.f * M_PI_F * radiusSquared);
+		const float invArea = 1.f / (4.f * M_PI_F * radiusSquared);
 
-	return VLOAD3F(sphereLight->notIntersectable.sphere.emittedFactor.c) * invArea * M_1_PI_F;
+		return VLOAD3F(sphereLight->notIntersectable.sphere.emittedFactor.c) * invArea * M_1_PI_F;
+	}
 }
 
 //------------------------------------------------------------------------------

diff --git a/release-notes.txt b/release-notes.txt
@@ -48,6 +48,7 @@
 * Fixed a problem with BlendLuxCore when parsing more than 9 image pipelines (#336)
 * Fixed a bug causing fireflies when using light tracing for caustics in some case (#329)
 * Fixed a crash when using RemoveUnusedTextures() with Volumes (issue #377)
+* Fixed a banding problem when using sphere light sources with very small radius (issue LuxCoreRender/BlendLuxCore#477)
 
 Note: due to Glossycoating updated support for bump mapping on GPUs, some old scene using this kind of material/bump map combination may require some fix.
 

diff --git a/src/slg/lights/spherelight.cpp b/src/slg/lights/spherelight.cpp
@@ -116,38 +116,44 @@ Spectrum SphereLight::Illuminate(const Scene &scene, const BSDF &bsdf,
 
 	// The point isn't inside the sphere
 
-	// Build a local coordinate system
-	const Vector localZ = toLight / centerDistance;
-	Frame localFrame(localZ);
-
-	// Sample sphere uniformly inside subtended cone
 	const float cosThetaMax = sqrtf(Max(0.f, 1.f - radiusSquared / centerDistanceSquared));
+	if (cosThetaMax > 1.f - DEFAULT_EPSILON_STATIC) {
+		// If the subtended angle is too small, I sample the light source like
+		// if it was a point light source in order to avoiding banding due to
+		// (lack of) numerical precision.
+		return PointLight::Illuminate(scene, bsdf, time, u0, u1, passThroughEvent, shadowRay, directPdfW, emissionPdfW, cosThetaAtLight);
+	} else {
+		// Build a local coordinate system
+		const Vector localZ = toLight / centerDistance;
+		Frame localFrame(localZ);
 
-	const Vector localRayDir = UniformSampleCone(u0, u1, cosThetaMax);
+		// Sample sphere uniformly inside subtended cone
+		const Vector localRayDir = UniformSampleCone(u0, u1, cosThetaMax);
 
-	if (CosTheta(localRayDir) < DEFAULT_COS_EPSILON_STATIC)
-		return Spectrum();
+		if (CosTheta(localRayDir) < DEFAULT_COS_EPSILON_STATIC)
+			return Spectrum();
 
-	const Vector shadowRayDir = localFrame.ToWorld(localRayDir);
-	const Point shadowRayOrig = bsdf.GetRayOrigin(shadowRayDir);
-	const Ray ray(shadowRayOrig, shadowRayDir);
+		const Vector shadowRayDir = localFrame.ToWorld(localRayDir);
+		const Point shadowRayOrig = bsdf.GetRayOrigin(shadowRayDir);
+		const Ray ray(shadowRayOrig, shadowRayDir);
 
-	// Check the intersection with the sphere
-	float shadowRayDistance;
-	if (!SphereIntersect(ray, shadowRayDistance))
-		shadowRayDistance = Dot(toLight, shadowRayDir);
+		// Check the intersection with the sphere
+		float shadowRayDistance;
+		if (!SphereIntersect(ray, shadowRayDistance))
+			shadowRayDistance = Dot(toLight, shadowRayDir);
 
-	if (cosThetaAtLight)
-		*cosThetaAtLight = CosTheta(localRayDir);
+		if (cosThetaAtLight)
+			*cosThetaAtLight = CosTheta(localRayDir);
 
-	directPdfW = UniformConePdf(cosThetaMax);
+		directPdfW = UniformConePdf(cosThetaMax);
 
-	if (emissionPdfW)
-		*emissionPdfW = invArea * CosTheta(localRayDir) * INV_PI;
+		if (emissionPdfW)
+			*emissionPdfW = invArea * CosTheta(localRayDir) * INV_PI;
 
-	shadowRay = Ray(shadowRayOrig, shadowRayDir, 0.f, shadowRayDistance, time);
+		shadowRay = Ray(shadowRayOrig, shadowRayDir, 0.f, shadowRayDistance, time);
 
-	return emittedFactor * invArea * INV_PI;
+		return emittedFactor * invArea * INV_PI;
+	}
 }
 
 Properties SphereLight::ToProperties(const ImageMapCache &imgMapCache, const bool useRealFileName) const {