Fix T53600: Cycles shader mixing issue with principled BSDF and zero weights.

SVM nodes need to read all data to get the right offset for the following node.
This is quite weak, a more generic solution would be good in the future.

2 files changed, 21 insertions, 9 deletions

diff --git a/intern/cycles/kernel/svm/svm.h b/intern/cycles/kernel/svm/svm.h
index 9ff02c1586b..d3dac5706d0 100644
--- a/intern/cycles/kernel/svm/svm.h
+++ b/intern/cycles/kernel/svm/svm.h
@@ -211,9 +211,7 @@ ccl_device_noinline void svm_eval_nodes(KernelGlobals *kg, ShaderData *sd, ccl_a
 				break;
 			}
 			case NODE_CLOSURE_BSDF:
-				if(type == SHADER_TYPE_SURFACE) {
-					svm_node_closure_bsdf(kg, sd, stack, node, path_flag, &offset);
-				}
+				svm_node_closure_bsdf(kg, sd, stack, node, type, path_flag, &offset);
 				break;
 			case NODE_CLOSURE_EMISSION:
 				svm_node_closure_emission(sd, stack, node);
@@ -331,9 +329,7 @@ ccl_device_noinline void svm_eval_nodes(KernelGlobals *kg, ShaderData *sd, ccl_a
 				break;
 #  if NODES_FEATURE(NODE_FEATURE_VOLUME)
 			case NODE_CLOSURE_VOLUME:
-				if(type == SHADER_TYPE_VOLUME) {
-					svm_node_closure_volume(kg, sd, stack, node, path_flag);
-				}
+				svm_node_closure_volume(kg, sd, stack, node, type, path_flag);
 				break;
 #  endif  /* NODES_FEATURE(NODE_FEATURE_VOLUME) */
 #  ifdef __EXTRA_NODES__
diff --git a/intern/cycles/kernel/svm/svm_closure.h b/intern/cycles/kernel/svm/svm_closure.h
index f04c46ef7f9..4547894665e 100644
--- a/intern/cycles/kernel/svm/svm_closure.h
+++ b/intern/cycles/kernel/svm/svm_closure.h
@@ -56,7 +56,7 @@ ccl_device void svm_node_glass_setup(ShaderData *sd, MicrofacetBsdf *bsdf, int t
 	}
 }
 
-ccl_device void svm_node_closure_bsdf(KernelGlobals *kg, ShaderData *sd, float *stack, uint4 node, int path_flag, int *offset)
+ccl_device void svm_node_closure_bsdf(KernelGlobals *kg, ShaderData *sd, float *stack, uint4 node, ShaderType shader_type, int path_flag, int *offset)
 {
 	uint type, param1_offset, param2_offset;
 
@@ -67,8 +67,19 @@ ccl_device void svm_node_closure_bsdf(KernelGlobals *kg, ShaderData *sd, float *
 	/* note we read this extra node before weight check, so offset is added */
 	uint4 data_node = read_node(kg, offset);
 
-	if(mix_weight == 0.0f)
+	/* Only compute BSDF for surfaces, transparent variable is shared with volume extinction. */
+	if(mix_weight == 0.0f || shader_type != SHADER_TYPE_SURFACE) {
+		if(type == CLOSURE_BSDF_PRINCIPLED_ID) {
+			/* Read all principled BSDF extra data to get the right offset. */
+			read_node(kg, offset);
+			read_node(kg, offset);
+			read_node(kg, offset);
+			read_node(kg, offset);
+			read_node(kg, offset);
+		}
+
 		return;
+	}
 
 	float3 N = stack_valid(data_node.x)? stack_load_float3(stack, data_node.x): sd->N;
 
@@ -835,9 +846,14 @@ ccl_device void svm_node_closure_bsdf(KernelGlobals *kg, ShaderData *sd, float *
 	}
 }
 
-ccl_device void svm_node_closure_volume(KernelGlobals *kg, ShaderData *sd, float *stack, uint4 node, int path_flag)
+ccl_device void svm_node_closure_volume(KernelGlobals *kg, ShaderData *sd, float *stack, uint4 node, ShaderType shader_type, int path_flag)
 {
 #ifdef __VOLUME__
+	/* Only sum extinction for volumes, variable is shared with surface transparency. */
+	if(shader_type != SHADER_TYPE_VOLUME) {
+		return;
+	}
+
 	uint type, param1_offset, param2_offset;
 
 	uint mix_weight_offset;