Cycles: random walk subsurface scattering.

It is basically brute force volume scattering within the mesh, but part
of the SSS code for faster performance. The main difference with actual
volume scattering is that we assume the boundaries are diffuse and that
all lighting is coming through this boundary from outside the volume.

This gives much more accurate results for thin features and low density.
Some challenges remain however:

* Significantly more noisy than BSSRDF. Adding Dwivedi sampling may help
  here, but it's unclear still how much it helps in real world cases.
* Due to this being a volumetric method, geometry like eyes or mouth can
  darken the skin on the outside. We may be able to reduce this effect,
  or users can compensate for it by reducing the scattering radius in
  such areas.
* Sharp corners are quite bright. This matches actual volume rendering
  and results in some other renderers, but maybe not so much real world
  objects.

Differential Revision: https://developer.blender.org/D3054

1 files changed, 12 insertions, 0 deletions

diff --git a/intern/cycles/kernel/kernel_volume.h b/intern/cycles/kernel/kernel_volume.h
index 2af4c9a5e7a..7b67a37adc5 100644
--- a/intern/cycles/kernel/kernel_volume.h
+++ b/intern/cycles/kernel/kernel_volume.h
@@ -35,6 +35,8 @@ typedef struct VolumeShaderCoefficients {
 	float3 emission;
 } VolumeShaderCoefficients;
 
+#ifdef __VOLUME__
+
 /* evaluate shader to get extinction coefficient at P */
 ccl_device_inline bool volume_shader_extinction_sample(KernelGlobals *kg,
                                                        ShaderData *sd,
@@ -92,6 +94,8 @@ ccl_device_inline bool volume_shader_sample(KernelGlobals *kg,
 	return true;
 }
 
+#endif /* __VOLUME__ */
+
 ccl_device float3 volume_color_transmittance(float3 sigma, float t)
 {
 	return make_float3(expf(-sigma.x * t), expf(-sigma.y * t), expf(-sigma.z * t));
@@ -102,6 +106,8 @@ ccl_device float kernel_volume_channel_get(float3 value, int channel)
 	return (channel == 0)? value.x: ((channel == 1)? value.y: value.z);
 }
 
+#ifdef __VOLUME__
+
 ccl_device bool volume_stack_is_heterogeneous(KernelGlobals *kg, ccl_addr_space VolumeStack *stack)
 {
 	for(int i = 0; stack[i].shader != SHADER_NONE; i++) {
@@ -239,6 +245,8 @@ ccl_device_noinline void kernel_volume_shadow(KernelGlobals *kg,
 		kernel_volume_shadow_homogeneous(kg, state, ray, shadow_sd, throughput);
 }
 
+#endif /* __VOLUME__ */
+
 /* Equi-angular sampling as in:
  * "Importance Sampling Techniques for Path Tracing in Participating Media" */
 
@@ -369,6 +377,8 @@ ccl_device int kernel_volume_sample_channel(float3 albedo, float3 throughput, fl
 	}
 }
 
+#ifdef __VOLUME__
+
 /* homogeneous volume: assume shader evaluation at the start gives
  * the volume shading coefficient for the entire line segment */
 ccl_device VolumeIntegrateResult kernel_volume_integrate_homogeneous(
@@ -1346,4 +1356,6 @@ ccl_device_inline void kernel_volume_clean_stack(KernelGlobals *kg,
 	}
 }
 
+#endif /* __VOLUME__ */
+
 CCL_NAMESPACE_END