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, 7 insertions, 0 deletions

diff --git a/source/blender/blenloader/intern/versioning_290.c b/source/blender/blenloader/intern/versioning_290.c
index d8f798a11cd..0c6817542d1 100644
--- a/source/blender/blenloader/intern/versioning_290.c
+++ b/source/blender/blenloader/intern/versioning_290.c
@@ -1963,5 +1963,12 @@ void blo_do_versions_290(FileData *fd, Library *UNUSED(lib), Main *bmain)
         arm->axes_position = 1.0;
       }
     }
+
+    /* Initialize the spread parameter for area lights*/
+    if (!DNA_struct_elem_find(fd->filesdna, "Lamp", "float", "area_spread")) {
+      LISTBASE_FOREACH (Light *, la, &bmain->lights) {
+        la->area_spread = DEG2RADF(180.0f);
+      }
+    }
   }
 }