Cycles: volume light sampling

* Volume multiple importace sampling support to combine equiangular and distance
  sampling, for both homogeneous and heterogeneous volumes.

* Branched path "Sample All Direct Lights" and "Sample All Indirect Lights" now
  apply to volumes as well as surfaces.

Implementation note:

For simplicity this is all done with decoupled ray marching, the only case we do
not use decoupled is for distance only sampling with one light sample. The
homogeneous case should still compile on the GPU because it only requires fixed
size storage, but the heterogeneous case will be trickier to get working.

1 files changed, 20 insertions, 6 deletions

diff --git a/intern/cycles/kernel/kernel_light.h b/intern/cycles/kernel/kernel_light.h
index ac432d3fe04..0adf9ed4666 100644
--- a/intern/cycles/kernel/kernel_light.h
+++ b/intern/cycles/kernel/kernel_light.h
@@ -208,8 +208,8 @@ ccl_device float lamp_light_pdf(KernelGlobals *kg, const float3 Ng, const float3
 	return t*t/cos_pi;
 }
 
-ccl_device void lamp_light_sample(KernelGlobals *kg, int lamp,
-	float randu, float randv, float3 P, LightSample *ls)
+ccl_device bool lamp_light_sample(KernelGlobals *kg, int lamp,
+	float randu, float randv, float3 P, LightSample *ls, bool for_volume)
 {
 	float4 data0 = kernel_tex_fetch(__light_data, lamp*LIGHT_SIZE + 0);
 	float4 data1 = kernel_tex_fetch(__light_data, lamp*LIGHT_SIZE + 1);
@@ -224,6 +224,11 @@ ccl_device void lamp_light_sample(KernelGlobals *kg, int lamp,
 	ls->v = randv;
 
 	if(type == LIGHT_DISTANT) {
+#ifdef __VOLUME__
+		if(for_volume)
+			return false;
+#endif
+
 		/* distant light */
 		float3 lightD = make_float3(data0.y, data0.z, data0.w);
 		float3 D = lightD;
@@ -244,6 +249,11 @@ ccl_device void lamp_light_sample(KernelGlobals *kg, int lamp,
 	}
 #ifdef __BACKGROUND_MIS__
 	else if(type == LIGHT_BACKGROUND) {
+#ifdef __VOLUME__
+		if(for_volume)
+			return false;
+#endif
+
 		/* infinite area light (e.g. light dome or env light) */
 		float3 D = background_light_sample(kg, randu, randv, &ls->pdf);
 
@@ -299,6 +309,8 @@ ccl_device void lamp_light_sample(KernelGlobals *kg, int lamp,
 		ls->eval_fac *= kernel_data.integrator.inv_pdf_lights;
 		ls->pdf *= lamp_light_pdf(kg, ls->Ng, -ls->D, ls->t);
 	}
+
+	return true;
 }
 
 ccl_device bool lamp_light_eval(KernelGlobals *kg, int lamp, float3 P, float3 D, float t, LightSample *ls)
@@ -514,7 +526,7 @@ ccl_device int light_distribution_sample(KernelGlobals *kg, float randt)
 
 /* Generic Light */
 
-ccl_device void light_sample(KernelGlobals *kg, float randt, float randu, float randv, float time, float3 P, LightSample *ls)
+ccl_device bool light_sample(KernelGlobals *kg, float randt, float randu, float randv, float time, float3 P, LightSample *ls, bool for_volume)
 {
 	/* sample index */
 	int index = light_distribution_sample(kg, randt);
@@ -533,10 +545,12 @@ ccl_device void light_sample(KernelGlobals *kg, float randt, float randu, float
 		ls->D = normalize_len(ls->P - P, &ls->t);
 		ls->pdf = triangle_light_pdf(kg, ls->Ng, -ls->D, ls->t);
 		ls->shader |= shader_flag;
+
+		return true;
 	}
 	else {
 		int lamp = -prim-1;
-		lamp_light_sample(kg, lamp, randu, randv, P, ls);
+		return lamp_light_sample(kg, lamp, randu, randv, P, ls, for_volume);
 	}
 }
 
@@ -546,9 +560,9 @@ ccl_device int light_select_num_samples(KernelGlobals *kg, int index)
 	return __float_as_int(data3.x);
 }
 
-ccl_device void light_select(KernelGlobals *kg, int index, float randu, float randv, float3 P, LightSample *ls)
+ccl_device bool light_select(KernelGlobals *kg, int index, float randu, float randv, float3 P, LightSample *ls, bool for_volume)
 {
-	lamp_light_sample(kg, index, randu, randv, P, ls);
+	return lamp_light_sample(kg, index, randu, randv, P, ls, for_volume);
 }
 
 ccl_device int lamp_light_eval_sample(KernelGlobals *kg, float randt)