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, 9 insertions, 0 deletions

diff --git a/source/blender/makesrna/intern/rna_light.c b/source/blender/makesrna/intern/rna_light.c
index bb99e5c6c1d..0593db0dd56 100644
--- a/source/blender/makesrna/intern/rna_light.c
+++ b/source/blender/makesrna/intern/rna_light.c
@@ -480,6 +480,15 @@ static void rna_def_area_light(BlenderRNA *brna)
       "Size Y",
       "Size of the area of the area light in the Y direction for rectangle shapes");
   RNA_def_property_update(prop, 0, "rna_Light_draw_update");
+
+  prop = RNA_def_property(srna, "spread", PROP_FLOAT, PROP_ANGLE);
+  RNA_def_property_float_sdna(prop, NULL, "area_spread");
+  RNA_def_property_range(prop, DEG2RADF(1.0f), DEG2RADF(180.0f));
+  RNA_def_property_ui_text(
+      prop,
+      "Spread",
+      "How widely the emitted light fans out, as in the case of a gridded softbox");
+  RNA_def_property_update(prop, 0, "rna_Light_draw_update");
 }
 
 static void rna_def_spot_light(BlenderRNA *brna)