Fix T46760: Branched Path Tracing converges to different result than plain Path Tracing

Multiple importance sampling for branched path tracing light samples needs to be
calculated separately per BSDF, not with Veach's one sample model.

2 files changed, 53 insertions, 13 deletions

diff --git a/intern/cycles/kernel/kernel_emission.h b/intern/cycles/kernel/kernel_emission.h
index de9e8d77ec8..3863cf221c5 100644
--- a/intern/cycles/kernel/kernel_emission.h
+++ b/intern/cycles/kernel/kernel_emission.h
@@ -99,23 +99,23 @@ ccl_device_noinline bool direct_emission(KernelGlobals *kg, ShaderData *sd,
 		return false;
 
 	/* evaluate BSDF at shading point */
-	float bsdf_pdf;
 
 #ifdef __VOLUME__
 	if(ccl_fetch(sd, prim) != PRIM_NONE)
-		shader_bsdf_eval(kg, sd, ls->D, eval, &bsdf_pdf);
-	else
+		shader_bsdf_eval(kg, sd, ls->D, eval, ls->pdf, ls->shader & SHADER_USE_MIS);
+	else {
+		float bsdf_pdf;
 		shader_volume_phase_eval(kg, sd, ls->D, eval, &bsdf_pdf);
+		if(ls->shader & SHADER_USE_MIS) {
+			/* Multiple importance sampling. */
+			float mis_weight = power_heuristic(ls->pdf, bsdf_pdf);
+			light_eval *= mis_weight;
+		}
+	}
 #else
-	shader_bsdf_eval(kg, sd, ls->D, eval, &bsdf_pdf);
+	shader_bsdf_eval(kg, sd, ls->D, eval, ls->pdf, ls->shader & SHADER_USE_MIS);
 #endif
 
-	if(ls->shader & SHADER_USE_MIS) {
-		/* multiple importance sampling */
-		float mis_weight = power_heuristic(ls->pdf, bsdf_pdf);
-		light_eval *= mis_weight;
-	}
-	
 	bsdf_eval_mul(eval, light_eval/ls->pdf);
 
 #ifdef __PASSES__
diff --git a/intern/cycles/kernel/kernel_shader.h b/intern/cycles/kernel/kernel_shader.h
index 6b560f5fdb2..25e5822a21b 100644
--- a/intern/cycles/kernel/kernel_shader.h
+++ b/intern/cycles/kernel/kernel_shader.h
@@ -516,12 +516,52 @@ ccl_device_inline void _shader_bsdf_multi_eval(KernelGlobals *kg, const ShaderDa
 	*pdf = (sum_sample_weight > 0.0f)? sum_pdf/sum_sample_weight: 0.0f;
 }
 
-ccl_device void shader_bsdf_eval(KernelGlobals *kg, ShaderData *sd,
-	const float3 omega_in, BsdfEval *eval, float *pdf)
+#ifdef __BRANCHED_PATH__
+ccl_device_inline void _shader_bsdf_multi_eval_branched(KernelGlobals *kg,
+                                                        const ShaderData *sd,
+                                                        const float3 omega_in,
+                                                        BsdfEval *result_eval,
+                                                        float light_pdf,
+                                                        bool use_mis)
+{
+	for(int i = 0; i < ccl_fetch(sd, num_closure); i++) {
+		const ShaderClosure *sc = ccl_fetch_array(sd, closure, i);
+		if(CLOSURE_IS_BSDF(sc->type)) {
+			float bsdf_pdf = 0.0f;
+			float3 eval = bsdf_eval(kg, sd, sc, omega_in, &bsdf_pdf);
+			if(bsdf_pdf != 0.0f) {
+				float mis_weight = use_mis? power_heuristic(light_pdf, bsdf_pdf): 1.0f;
+				bsdf_eval_accum(result_eval,
+				                sc->type,
+				                eval * sc->weight * mis_weight);
+			}
+		}
+	}
+}
+#endif
+
+ccl_device void shader_bsdf_eval(KernelGlobals *kg,
+                                 ShaderData *sd,
+                                 const float3 omega_in,
+                                 BsdfEval *eval,
+                                 float light_pdf,
+                                 bool use_mis)
 {
 	bsdf_eval_init(eval, NBUILTIN_CLOSURES, make_float3(0.0f, 0.0f, 0.0f), kernel_data.film.use_light_pass);
 
-	_shader_bsdf_multi_eval(kg, sd, omega_in, pdf, -1, eval, 0.0f, 0.0f);
+#ifdef __BRANCHED_PATH__
+	if(kernel_data.integrator.branched)
+		_shader_bsdf_multi_eval_branched(kg, sd, omega_in, eval, light_pdf, use_mis);
+	else
+#endif
+	{
+		float pdf;
+		_shader_bsdf_multi_eval(kg, sd, omega_in, &pdf, -1, eval, 0.0f, 0.0f);
+		if(use_mis) {
+			float weight = power_heuristic(light_pdf, pdf);
+			bsdf_eval_mul(eval, make_float3(weight, weight, weight));
+		}
+	}
 }
 
 ccl_device int shader_bsdf_sample(KernelGlobals *kg, const ShaderData *sd,