Fix T49187: inconsistent Normal Map node output for backfacing polygons.

There basically are two issues here: in smooth mode (and all non-tangent
normal map types) it doesn't invert the normal for backfacing polys;
on the other hand for flat shaded tangent type it is inverted too soon.

This fix does a brute force correction by checking the backfacing flag.

Reviewers: #cycles, brecht

Reviewed By: #cycles, brecht

Differential Revision: https://developer.blender.org/D2181

1 files changed, 12 insertions, 0 deletions

diff --git a/intern/cycles/kernel/svm/svm_tex_coord.h b/intern/cycles/kernel/svm/svm_tex_coord.h
index 01dede3fff5..6ea2539c543 100644
--- a/intern/cycles/kernel/svm/svm_tex_coord.h
+++ b/intern/cycles/kernel/svm/svm_tex_coord.h
@@ -277,6 +277,7 @@ ccl_device void svm_node_normal_map(KernelGlobals *kg, ShaderData *sd, float *st
 	float3 color = stack_load_float3(stack, color_offset);
 	color = 2.0f*make_float3(color.x - 0.5f, color.y - 0.5f, color.z - 0.5f);
 
+	bool is_backfacing = (ccl_fetch(sd, flag) & SD_BACKFACING) != 0;
 	float3 N;
 
 	if(space == NODE_NORMAL_MAP_TANGENT) {
@@ -306,6 +307,12 @@ ccl_device void svm_node_normal_map(KernelGlobals *kg, ShaderData *sd, float *st
 		}
 		else {
 			normal = ccl_fetch(sd, Ng);
+
+			/* the normal is already inverted, which is too soon for the math here */
+			if(is_backfacing) {
+				normal = -normal;
+			}
+
 			object_inverse_normal_transform(kg, sd, &normal);
 		}
 
@@ -332,6 +339,11 @@ ccl_device void svm_node_normal_map(KernelGlobals *kg, ShaderData *sd, float *st
 			N = safe_normalize(N);
 	}
 
+	/* invert normal for backfacing polygons */
+	if(is_backfacing) {
+		N = -N;
+	}
+
 	float strength = stack_load_float(stack, strength_offset);
 
 	if(strength != 1.0f) {