Fix T38710: volume render issue with transparent surfaces.

2 files changed, 10 insertions, 2 deletions

diff --git a/intern/cycles/kernel/kernel_random.h b/intern/cycles/kernel/kernel_random.h
index ef397269ec2..31cb6ff6abd 100644
--- a/intern/cycles/kernel/kernel_random.h
+++ b/intern/cycles/kernel/kernel_random.h
@@ -120,6 +120,9 @@ ccl_device_inline float path_rng_1D(KernelGlobals *kg, RNG *rng, int sample, int
 	/* Cranly-Patterson rotation using rng seed */
 	float shift;
 
+	/* using the same *rng value to offset seems to give correlation issues,
+	 * we could hash it with the dimension but this has a performance impact,
+	 * we need to find a solution for this */
 	if(dimension & 1)
 		shift = (*rng >> 16) * (1.0f/(float)0xFFFF);
 	else
diff --git a/intern/cycles/kernel/kernel_volume.h b/intern/cycles/kernel/kernel_volume.h
index 4d058763f22..ec0c3114213 100644
--- a/intern/cycles/kernel/kernel_volume.h
+++ b/intern/cycles/kernel/kernel_volume.h
@@ -536,12 +536,17 @@ ccl_device VolumeIntegrateResult kernel_volume_integrate_heterogeneous(KernelGlo
 ccl_device_noinline VolumeIntegrateResult kernel_volume_integrate(KernelGlobals *kg,
 	PathState *state, ShaderData *sd, Ray *ray, PathRadiance *L, float3 *throughput, RNG *rng)
 {
+	/* workaround to fix correlation bug in T38710, can find better solution
+	 * in random number generator later, for now this is done here to not impact
+	 * performance of rendering without volumes */
+	RNG tmp_rng = cmj_hash(*rng, state->rng_offset);
+
 	shader_setup_from_volume(kg, sd, ray, state->bounce);
 
 	if(volume_stack_is_heterogeneous(kg, state->volume_stack))
-		return kernel_volume_integrate_heterogeneous(kg, state, ray, sd, L, throughput, rng);
+		return kernel_volume_integrate_heterogeneous(kg, state, ray, sd, L, throughput, &tmp_rng);
 	else
-		return kernel_volume_integrate_homogeneous(kg, state, ray, sd, L, throughput, rng);
+		return kernel_volume_integrate_homogeneous(kg, state, ray, sd, L, throughput, &tmp_rng);
 }
 
 /* Volume Stack