Cycles: multiple importance sampling for lamps, which helps reduce noise for

big lamps and sharp glossy reflections. This was already supported for mesh
lights and the background, so lamps should do it too.

This is not for free and it's a bit slower than I hoped even though there is
no extra BVH ray intersection. I'll try to optimize it more later.

* Area lights look a bit different now, they had the wrong shape before.
* Also fixes a sampling issue in the non-progressive integrator.
* Only enabled for the CPU, will test on the GPU later.
* An option to disable this will be added for situations where it does not help.

Same time comparison before/after:
http://www.pasteall.org/pic/show.php?id=43313
http://www.pasteall.org/pic/show.php?id=43314

1 files changed, 45 insertions, 16 deletions

diff --git a/intern/cycles/kernel/kernel_emission.h b/intern/cycles/kernel/kernel_emission.h
index d5506ad1dd0..54bc0717b60 100644
--- a/intern/cycles/kernel/kernel_emission.h
+++ b/intern/cycles/kernel/kernel_emission.h
@@ -66,6 +66,8 @@ __device float3 direct_emissive_eval(KernelGlobals *kg, float rando,
 		else
 			eval = make_float3(0.0f, 0.0f, 0.0f);
 	}
+	
+	eval *= ls->eval_fac;
 
 	shader_release(kg, &sd);
 
@@ -74,29 +76,29 @@ __device float3 direct_emissive_eval(KernelGlobals *kg, float rando,
 
 __device bool direct_emission(KernelGlobals *kg, ShaderData *sd, int lindex,
 	float randt, float rando, float randu, float randv, Ray *ray, BsdfEval *eval,
-	bool *is_lamp)
+	int *lamp)
 {
 	LightSample ls;
 
-	float pdf = -1.0f;
-
 #ifdef __NON_PROGRESSIVE__
 	if(lindex != -1) {
 		/* sample position on a specified light */
-		light_select(kg, lindex, randu, randv, sd->P, &ls, &pdf);
+		light_select(kg, lindex, randu, randv, sd->P, &ls);
 	}
 	else
 #endif
 	{
 		/* sample a light and position on int */
-		light_sample(kg, randt, randu, randv, sd->time, sd->P, &ls, &pdf);
+		light_sample(kg, randt, randu, randv, sd->time, sd->P, &ls);
 	}
 
-	/* compute pdf */
-	if(pdf < 0.0f)
-		pdf = light_sample_pdf(kg, &ls, -ls.D, ls.t);
+	/* return lamp index for MIS */
+	if(ls.use_mis)
+		*lamp = ls.lamp;
+	else
+		*lamp= ~0;
 
-	if(pdf == 0.0f)
+	if(ls.pdf == 0.0f)
 		return false;
 
 	/* evaluate closure */
@@ -112,13 +114,13 @@ __device bool direct_emission(KernelGlobals *kg, ShaderData *sd, int lindex,
 
 	shader_bsdf_eval(kg, sd, ls.D, eval, &bsdf_pdf);
 
-	if(ls.prim != ~0 || ls.type == LIGHT_BACKGROUND) {
+	if(ls.use_mis) {
 		/* multiple importance sampling */
-		float mis_weight = power_heuristic(pdf, bsdf_pdf);
+		float mis_weight = power_heuristic(ls.pdf, bsdf_pdf);
 		light_eval *= mis_weight;
 	}
 	
-	bsdf_eval_mul(eval, light_eval*(ls.eval_fac/pdf));
+	bsdf_eval_mul(eval, light_eval/ls.pdf);
 
 	if(bsdf_eval_is_zero(eval))
 		return false;
@@ -144,14 +146,12 @@ __device bool direct_emission(KernelGlobals *kg, ShaderData *sd, int lindex,
 		ray->t = 0.0f;
 	}
 
-	*is_lamp = (ls.prim == ~0);
-
 	return true;
 }
 
-/* Indirect Emission */
+/* Indirect Primitive Emission */
 
-__device float3 indirect_emission(KernelGlobals *kg, ShaderData *sd, float t, int path_flag, float bsdf_pdf)
+__device float3 indirect_primitive_emission(KernelGlobals *kg, ShaderData *sd, float t, int path_flag, float bsdf_pdf)
 {
 	/* evaluate emissive closure */
 	float3 L = shader_emissive_eval(kg, sd);
@@ -172,6 +172,35 @@ __device float3 indirect_emission(KernelGlobals *kg, ShaderData *sd, float t, in
 	return L;
 }
 
+/* Indirect Lamp Emission */
+
+__device bool indirect_lamp_emission(KernelGlobals *kg, Ray *ray, int path_flag, float bsdf_pdf, float randt, float3 *emission)
+{
+	LightSample ls;
+	int lamp = lamp_light_eval_sample(kg, randt);
+
+	if(lamp == ~0)
+		return false;
+
+	if(!lamp_light_eval(kg, lamp, ray->P, ray->D, ray->t, &ls))
+		return false;
+	
+	/* todo: missing texture coordinates */
+	float u = 0.0f;
+	float v = 0.0f;
+	float3 L = direct_emissive_eval(kg, 0.0f, &ls, u, v, -ray->D, ls.t, ray->time);
+
+	if(!(path_flag & PATH_RAY_MIS_SKIP)) {
+		/* multiple importance sampling, get regular light pdf,
+		 * and compute weight with respect to BSDF pdf */
+		float mis_weight = power_heuristic(bsdf_pdf, ls.pdf);
+		L *= mis_weight;
+	}
+
+	*emission = L;
+	return true;
+}
+
 /* Indirect Background */
 
 __device float3 indirect_background(KernelGlobals *kg, Ray *ray, int path_flag, float bsdf_pdf)