Cycles Volume Render: scattering support.

This is done by adding a Volume Scatter node. In many cases you will want to
add together a Volume Absorption and Volume Scatter node with the same color
and density to get the expected results.

This should work with branched path tracing, mixing closures, overlapping
volumes, etc. However there's still various optimizations needed for sampling.
The main missing thing from the volume branch is the equiangular sampling for
homogeneous volumes.

The heterogeneous scattering code was arranged such that we can use a single
stratified random number for distance sampling, which gives less noise than
pseudo random numbers for each step. For volumes where the color is textured
there still seems to be something off, needs to be investigated.

1 files changed, 46 insertions, 28 deletions

diff --git a/intern/cycles/kernel/kernel_path_state.h b/intern/cycles/kernel/kernel_path_state.h
index fe4d38b10d6..5def19f2c42 100644
--- a/intern/cycles/kernel/kernel_path_state.h
+++ b/intern/cycles/kernel/kernel_path_state.h
@@ -42,6 +42,8 @@ ccl_device_inline void path_state_init(KernelGlobals *kg, PathState *state, RNG
 
 #ifdef __VOLUME__
 	if(kernel_data.integrator.use_volumes) {
+		state->volume_bounce = 0;
+
 		/* initialize volume stack with volume we are inside of */
 		kernel_volume_stack_init(kg, state->volume_stack);
 		/* seed RNG for cases where we can't use stratified samples */
@@ -61,6 +63,9 @@ ccl_device_inline void path_state_next(KernelGlobals *kg, PathState *state, int
 		state->flag |= PATH_RAY_TRANSPARENT;
 		state->transparent_bounce++;
 
+		/* random number generator next bounce */
+		state->rng_offset += PRNG_BOUNCE_NUM;
+
 		if(!kernel_data.integrator.transparent_shadows)
 			state->flag |= PATH_RAY_MIS_SKIP;
 
@@ -69,39 +74,51 @@ ccl_device_inline void path_state_next(KernelGlobals *kg, PathState *state, int
 
 	state->bounce++;
 
-	/* reflection/transmission */
-	if(label & LABEL_REFLECT) {
-		state->flag |= PATH_RAY_REFLECT;
-		state->flag &= ~(PATH_RAY_TRANSMIT|PATH_RAY_CAMERA|PATH_RAY_TRANSPARENT);
+#ifdef __VOLUME__
+	if(label & LABEL_VOLUME_SCATTER) {
+		/* volume scatter */
+		state->flag |= PATH_RAY_VOLUME_SCATTER;
+		state->flag &= ~(PATH_RAY_REFLECT|PATH_RAY_TRANSMIT|PATH_RAY_CAMERA|PATH_RAY_TRANSPARENT|PATH_RAY_DIFFUSE|PATH_RAY_GLOSSY|PATH_RAY_SINGULAR|PATH_RAY_MIS_SKIP);
 
-		if(label & LABEL_DIFFUSE)
-			state->diffuse_bounce++;
-		else
-			state->glossy_bounce++;
+		state->volume_bounce++;
 	}
-	else {
-		kernel_assert(label & LABEL_TRANSMIT);
+	else
+#endif
+	{
+		/* surface reflection/transmission */
+		if(label & LABEL_REFLECT) {
+			state->flag |= PATH_RAY_REFLECT;
+			state->flag &= ~(PATH_RAY_TRANSMIT|PATH_RAY_VOLUME_SCATTER|PATH_RAY_CAMERA|PATH_RAY_TRANSPARENT);
+
+			if(label & LABEL_DIFFUSE)
+				state->diffuse_bounce++;
+			else
+				state->glossy_bounce++;
+		}
+		else {
+			kernel_assert(label & LABEL_TRANSMIT);
 
-		state->flag |= PATH_RAY_TRANSMIT;
-		state->flag &= ~(PATH_RAY_REFLECT|PATH_RAY_CAMERA|PATH_RAY_TRANSPARENT);
+			state->flag |= PATH_RAY_TRANSMIT;
+			state->flag &= ~(PATH_RAY_REFLECT|PATH_RAY_VOLUME_SCATTER|PATH_RAY_CAMERA|PATH_RAY_TRANSPARENT);
 
-		state->transmission_bounce++;
-	}
+			state->transmission_bounce++;
+		}
 
-	/* diffuse/glossy/singular */
-	if(label & LABEL_DIFFUSE) {
-		state->flag |= PATH_RAY_DIFFUSE|PATH_RAY_DIFFUSE_ANCESTOR;
-		state->flag &= ~(PATH_RAY_GLOSSY|PATH_RAY_SINGULAR|PATH_RAY_MIS_SKIP);
-	}
-	else if(label & LABEL_GLOSSY) {
-		state->flag |= PATH_RAY_GLOSSY|PATH_RAY_GLOSSY_ANCESTOR;
-		state->flag &= ~(PATH_RAY_DIFFUSE|PATH_RAY_SINGULAR|PATH_RAY_MIS_SKIP);
-	}
-	else {
-		kernel_assert(label & LABEL_SINGULAR);
+		/* diffuse/glossy/singular */
+		if(label & LABEL_DIFFUSE) {
+			state->flag |= PATH_RAY_DIFFUSE|PATH_RAY_DIFFUSE_ANCESTOR;
+			state->flag &= ~(PATH_RAY_GLOSSY|PATH_RAY_SINGULAR|PATH_RAY_MIS_SKIP);
+		}
+		else if(label & LABEL_GLOSSY) {
+			state->flag |= PATH_RAY_GLOSSY|PATH_RAY_GLOSSY_ANCESTOR;
+			state->flag &= ~(PATH_RAY_DIFFUSE|PATH_RAY_SINGULAR|PATH_RAY_MIS_SKIP);
+		}
+		else {
+			kernel_assert(label & LABEL_SINGULAR);
 
-		state->flag |= PATH_RAY_GLOSSY|PATH_RAY_SINGULAR|PATH_RAY_MIS_SKIP;
-		state->flag &= ~PATH_RAY_DIFFUSE;
+			state->flag |= PATH_RAY_GLOSSY|PATH_RAY_SINGULAR|PATH_RAY_MIS_SKIP;
+			state->flag &= ~PATH_RAY_DIFFUSE;
+		}
 	}
 
 	/* random number generator next bounce */
@@ -136,7 +153,8 @@ ccl_device_inline float path_state_terminate_probability(KernelGlobals *kg, Path
 		if((state->bounce >= kernel_data.integrator.max_bounce) ||
 		   (state->diffuse_bounce >= kernel_data.integrator.max_diffuse_bounce) ||
 		   (state->glossy_bounce >= kernel_data.integrator.max_glossy_bounce) ||
-		   (state->transmission_bounce >= kernel_data.integrator.max_transmission_bounce))
+		   (state->transmission_bounce >= kernel_data.integrator.max_transmission_bounce) ||
+		   (state->volume_bounce >= kernel_data.integrator.max_volume_bounce))
 		{
 			return 0.0f;
 		}