Fix Cycles MNEE wrong results with area light spread

When the solve is successful, the light sample needs to be updated since the
effective shading point is now on the last refractive interface. Spread was
not taken into account, creating false caustics.

Differential Revision: https://developer.blender.org/D15449

1 files changed, 7 insertions, 1 deletions

diff --git a/intern/cycles/kernel/integrator/mnee.h b/intern/cycles/kernel/integrator/mnee.h
index 67505b9b612..70b009d3b48 100644
--- a/intern/cycles/kernel/integrator/mnee.h
+++ b/intern/cycles/kernel/integrator/mnee.h
@@ -137,8 +137,14 @@ ccl_device_forceinline void mnee_update_light_sample(KernelGlobals kg,
     }
   }
   else if (ls->type == LIGHT_AREA) {
+    float invarea = fabsf(klight->area.invarea);
     ls->D = normalize_len(ls->P - P, &ls->t);
-    ls->pdf = fabsf(klight->area.invarea);
+    ls->pdf = invarea;
+    if (klight->area.tan_spread > 0.f) {
+      ls->eval_fac = 0.25f * invarea;
+      ls->eval_fac *= light_spread_attenuation(
+          ls->D, ls->Ng, klight->area.tan_spread, klight->area.normalize_spread);
+    }
   }
 
   ls->pdf *= kernel_data.integrator.pdf_lights;