Fix incorrect MIS weights in Cycles with multiple lights.

This causes some difference in the classroom scene, where ray visibility
tricks are used and break the MIS balance. Otherwise there doesn't seem
to be much effect, but better to use the right formulas. Problem originally
identified by Lukas.

1 files changed, 7 insertions, 5 deletions

diff --git a/intern/cycles/kernel/kernel_light.h b/intern/cycles/kernel/kernel_light.h
index c806deee8e7..dfa3150dc92 100644
--- a/intern/cycles/kernel/kernel_light.h
+++ b/intern/cycles/kernel/kernel_light.h
@@ -547,7 +547,7 @@ ccl_device_inline bool lamp_light_sample(KernelGlobals *kg,
 
 		float costheta = dot(lightD, D);
 		ls->pdf = invarea/(costheta*costheta*costheta);
-		ls->eval_fac = ls->pdf*kernel_data.integrator.inv_pdf_lights;
+		ls->eval_fac = ls->pdf;
 	}
 #ifdef __BACKGROUND_MIS__
 	else if(type == LIGHT_BACKGROUND) {
@@ -559,7 +559,6 @@ ccl_device_inline bool lamp_light_sample(KernelGlobals *kg,
 		ls->D = -D;
 		ls->t = FLT_MAX;
 		ls->eval_fac = 1.0f;
-		ls->pdf *= kernel_data.integrator.pdf_lights;
 	}
 #endif
 	else {
@@ -622,10 +621,10 @@ ccl_device_inline bool lamp_light_sample(KernelGlobals *kg,
 			float invarea = data2.x;
 			ls->eval_fac = 0.25f*invarea;
 		}
-
-		ls->eval_fac *= kernel_data.integrator.inv_pdf_lights;
 	}
 
+	ls->pdf *= kernel_data.integrator.pdf_lights;
+
 	return (ls->pdf > 0.0f);
 }
 
@@ -757,8 +756,11 @@ ccl_device bool lamp_light_eval(KernelGlobals *kg, int lamp, float3 P, float3 D,
 		ls->pdf = area_light_sample(P, &light_P, axisu, axisv, 0, 0, false);
 		ls->eval_fac = 0.25f*invarea;
 	}
-	else
+	else {
 		return false;
+	}
+
+	ls->pdf *= kernel_data.integrator.pdf_lights;
 
 	return true;
 }