1 files changed, 55 insertions, 2 deletions

diff --git a/intern/cycles/kernel/kernel_light_common.h b/intern/cycles/kernel/kernel_light_common.h
index 7efd1e74202..1f4aecc0b68 100644
--- a/intern/cycles/kernel/kernel_light_common.h
+++ b/intern/cycles/kernel/kernel_light_common.h
@@ -147,16 +147,69 @@ ccl_device float spot_light_attenuation(float3 dir, float spot_angle, float spot
 }
 
 ccl_device float light_spread_attenuation(const float3 D,
-                                          const float3 Ng,
+                                          const float3 lightNg,
                                           const float tan_spread,
                                           const float normalize_spread)
 {
-  const float cos_a = -dot(D, Ng);
+  /* Model a softbox grid, computing the ratio of light not hidden by the
+   * slats of the grid at a given angle. (seee D10594). */
+  const float cos_a = -dot(D, lightNg);
   const float sin_a = safe_sqrtf(1.0f - sqr(cos_a));
   const float tan_a = sin_a / cos_a;
   return max((1.0f - (tan_spread * tan_a)) * normalize_spread, 0.0f);
 }
 
+/* Compute subset of area light that actually has an influence on the shading point, to
+ * reduce noise with low spread. */
+ccl_device bool light_spread_clamp_area_light(const float3 P,
+                                              const float3 lightNg,
+                                              float3 *lightP,
+                                              float3 *axisu,
+                                              float3 *axisv,
+                                              const float tan_spread)
+{
+  /* Closest point in area light plane and distance to that plane. */
+  const float3 closest_P = P - dot(lightNg, P - *lightP) * lightNg;
+  const float t = len(closest_P - P);
+
+  /* Radius of circle on area light that actually affects the shading point. */
+  const float radius = t / tan_spread;
+
+  /* TODO: would be faster to store as normalized vector + length, also in rect_light_sample. */
+  float len_u, len_v;
+  const float3 u = normalize_len(*axisu, &len_u);
+  const float3 v = normalize_len(*axisv, &len_v);
+
+  /* Local uv coordinates of closest point. */
+  const float closest_u = dot(u, closest_P - *lightP);
+  const float closest_v = dot(v, closest_P - *lightP);
+
+  /* Compute rectangle encompassing the circle that affects the shading point,
+   * clamped to the bounds of the area light. */
+  const float min_u = max(closest_u - radius, -len_u * 0.5f);
+  const float max_u = min(closest_u + radius, len_u * 0.5f);
+  const float min_v = max(closest_v - radius, -len_v * 0.5f);
+  const float max_v = min(closest_v + radius, len_v * 0.5f);
+
+  /* Skip if rectangle is empty. */
+  if (min_u >= max_u || min_v >= max_v) {
+    return false;
+  }
+
+  /* Compute new area light center position and axes from rectangle in local
+   * uv coordinates. */
+  const float new_center_u = 0.5f * (min_u + max_u);
+  const float new_center_v = 0.5f * (min_v + max_v);
+  const float new_len_u = 0.5f * (max_u - min_u);
+  const float new_len_v = 0.5f * (max_v - min_v);
+
+  *lightP = *lightP + new_center_u * u + new_center_v * v;
+  *axisu = u * new_len_u * 2.0f;
+  *axisv = v * new_len_v * 2.0f;
+
+  return true;
+}
+
 ccl_device float lamp_light_pdf(KernelGlobals *kg, const float3 Ng, const float3 I, float t)
 {
   float cos_pi = dot(Ng, I);