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

intern/cycles/blender/addon/ui.py

@@ -1409,15 +1409,15 @@ class CYCLES_LIGHT_PT_nodes(CyclesButtonsPanel, Panel):
         panel_node_draw(layout, light, 'OUTPUT_LIGHT', 'Surface')
 
 
-class CYCLES_LIGHT_PT_spot(CyclesButtonsPanel, Panel):
-    bl_label = "Spot Shape"
+class CYCLES_LIGHT_PT_beam_shape(CyclesButtonsPanel, Panel):
+    bl_label = "Beam Shape"
     bl_parent_id = "CYCLES_LIGHT_PT_light"
     bl_context = "data"
 
     @classmethod
     def poll(cls, context):
-        light = context.light
-        return (light and light.type == 'SPOT') and CyclesButtonsPanel.poll(context)
+        if context.light.type in {'SPOT', 'AREA'}:
+            return context.light and CyclesButtonsPanel.poll(context)
 
     def draw(self, context):
         layout = self.layout
@@ -1425,9 +1425,12 @@ class CYCLES_LIGHT_PT_spot(CyclesButtonsPanel, Panel):
         layout.use_property_split = True
 
         col = layout.column()
-        col.prop(light, "spot_size", text="Size")
-        col.prop(light, "spot_blend", text="Blend", slider=True)
-        col.prop(light, "show_cone")
+        if light.type == 'SPOT':
+            col.prop(light, "spot_size", text="Spot Size")
+            col.prop(light, "spot_blend", text="Blend", slider=True)
+            col.prop(light, "show_cone")
+        elif light.type == 'AREA':
+            col.prop(light, "spread", text="Spread")
 
 
 class CYCLES_WORLD_PT_preview(CyclesButtonsPanel, Panel):
@@ -2284,7 +2287,7 @@ classes = (
     CYCLES_LIGHT_PT_preview,
     CYCLES_LIGHT_PT_light,
     CYCLES_LIGHT_PT_nodes,
-    CYCLES_LIGHT_PT_spot,
+    CYCLES_LIGHT_PT_beam_shape,
     CYCLES_WORLD_PT_preview,
     CYCLES_WORLD_PT_surface,
     CYCLES_WORLD_PT_volume,
@@ -2314,7 +2317,7 @@ classes = (
     node_panel(CYCLES_WORLD_PT_settings_surface),
     node_panel(CYCLES_WORLD_PT_settings_volume),
     node_panel(CYCLES_LIGHT_PT_light),
-    node_panel(CYCLES_LIGHT_PT_spot),
+    node_panel(CYCLES_LIGHT_PT_beam_shape)
 )
 
 

intern/cycles/blender/blender_light.cpp

@@ -82,6 +82,7 @@ void BlenderSync::sync_light(BL::Object &b_parent,
       light->set_axisu(transform_get_column(&tfm, 0));
       light->set_axisv(transform_get_column(&tfm, 1));
       light->set_sizeu(b_area_light.size());
+      light->set_spread(b_area_light.spread());
       switch (b_area_light.shape()) {
         case BL::AreaLight::shape_SQUARE:
           light->set_sizev(light->get_sizeu());

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;

intern/cycles/kernel/kernel_light_common.h

@@ -146,6 +146,17 @@ ccl_device float spot_light_attenuation(float3 dir, float spot_angle, float spot
   return attenuation;
 }
 
+ccl_device float light_spread_attenuation(const float3 D,
+                                          const float3 Ng,
+                                          const float tan_spread,
+                                          const float normalize_spread)
+{
+  const float cos_a = -dot(D, Ng);
+  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);
+}
+
 ccl_device float lamp_light_pdf(KernelGlobals *kg, const float3 Ng, const float3 I, float t)
 {
   float cos_pi = dot(Ng, I);

intern/cycles/kernel/kernel_types.h

@@ -1501,9 +1501,9 @@ typedef struct KernelAreaLight {
   float axisu[3];
   float invarea;
   float axisv[3];
-  float pad1;
+  float tan_spread;
   float dir[3];
-  float pad2;
+  float normalize_spread;
 } KernelAreaLight;
 
 typedef struct KernelDistantLight {

intern/cycles/render/light.cpp

@@ -129,6 +129,7 @@ NODE_DEFINE(Light)
   SOCKET_VECTOR(axisv, "Axis V", zero_float3());
   SOCKET_FLOAT(sizev, "Size V", 1.0f);
   SOCKET_BOOLEAN(round, "Round", false);
+  SOCKET_FLOAT(spread, "Spread", M_PI_F);
 
   SOCKET_INT(map_resolution, "Map Resolution", 0);
 
@@ -858,6 +859,15 @@ void LightManager::device_update_points(Device *, DeviceScene *dscene, Scene *sc
       float invarea = (area != 0.0f) ? 1.0f / area : 1.0f;
       float3 dir = light->dir;
 
+      /* Convert from spread angle 0..180 to 90..0, clamping to a minimum
+       * angle to avoid excessive noise. */
+      const float min_spread_angle = 1.0f * M_PI_F / 180.0f;
+      const float spread_angle = 0.5f * (M_PI_F - max(light->spread, min_spread_angle));
+      /* Normalization computed using:
+       * integrate cos(x) (1 - tan(x) * tan(a)) * sin(x) from x = a to pi/2. */
+      const float tan_spread = tanf(spread_angle);
+      const float normalize_spread = 2.0f / (2.0f + (2.0f * spread_angle - M_PI_F) * tan_spread);
+
       dir = safe_normalize(dir);
 
       if (light->use_mis && area != 0.0f)
@@ -877,6 +887,8 @@ void LightManager::device_update_points(Device *, DeviceScene *dscene, Scene *sc
       klights[light_index].area.dir[0] = dir.x;
       klights[light_index].area.dir[1] = dir.y;
       klights[light_index].area.dir[2] = dir.z;
+      klights[light_index].area.tan_spread = tan_spread;
+      klights[light_index].area.normalize_spread = normalize_spread;
     }
     else if (light->light_type == LIGHT_SPOT) {
       shader_id &= ~SHADER_AREA_LIGHT;

intern/cycles/render/light.h

@@ -58,6 +58,7 @@ class Light : public Node {
   NODE_SOCKET_API(float3, axisv)
   NODE_SOCKET_API(float, sizev)
   NODE_SOCKET_API(bool, round)
+  NODE_SOCKET_API(float, spread)
 
   NODE_SOCKET_API(Transform, tfm)
 

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);
+      }
+    }
   }
 }

source/blender/makesdna/DNA_light_defaults.h

@@ -68,6 +68,7 @@
     .volume_fac = 1.0f, \
     .att_dist = 40.0f, \
     .sun_angle = DEG2RADF(0.526f), \
+    .area_spread = DEG2RADF(180.0f), \
   }
 
 /** \} */

source/blender/makesdna/DNA_light_types.h

@@ -70,9 +70,9 @@ typedef struct Light {
 
   short area_shape;
   float area_size, area_sizey, area_sizez;
+  float area_spread;
 
   float sun_angle;
-  char _pad3[4];
 
   /* texact is for buttons */
   short texact, shadhalostep;

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)