Cycles: add a spread setting for area lights

This simulates the effect of a honeycomb or grid placed in front of a softbox.
In practice, it works by attenuating rays coming off-angle as a function of the
provided spread angle parameter.

Setting the parameter to 180 degrees poses no restrictions to the rays, making
the light behave the same way as before this patch.

The total light power is normalized based on the spread angle, so that the
light strength remains the same.

Differential Revision: https://developer.blender.org/D10594

1 files changed, 16 insertions, 0 deletions

diff --git a/intern/cycles/kernel/kernel_light.h b/intern/cycles/kernel/kernel_light.h
index 9650b85a5c2..93b05f0ffce 100644
--- a/intern/cycles/kernel/kernel_light.h
+++ b/intern/cycles/kernel/kernel_light.h
@@ -147,6 +147,13 @@ ccl_device_inline bool lamp_light_sample(
       ls->D = normalize_len(ls->P - P, &ls->t);
 
       ls->eval_fac = 0.25f * invarea;
+
+      if (klight->area.tan_spread > 0.0f) {
+        /* Area Light spread angle attenuation */
+        ls->eval_fac *= light_spread_attenuation(
+            ls->D, ls->Ng, klight->area.tan_spread, klight->area.normalize_spread);
+      }
+
       if (is_round) {
         ls->pdf *= lamp_light_pdf(kg, D, -ls->D, ls->t);
       }
@@ -286,6 +293,15 @@ ccl_device bool lamp_light_eval(
       ls->pdf = rect_light_sample(P, &light_P, axisu, axisv, 0, 0, false);
     }
     ls->eval_fac = 0.25f * invarea;
+
+    if (klight->area.tan_spread > 0.0f) {
+      /* Area Light spread angle attenuation */
+      ls->eval_fac *= light_spread_attenuation(
+          ls->D, ls->Ng, klight->area.tan_spread, klight->area.normalize_spread);
+      if (ls->eval_fac == 0.0f) {
+        return false;
+      }
+    }
   }
   else {
     return false;