EEVEE: Volumetrics: Add Area light shape support

Previously area lights were just considered as point lights.
We now use a "most representative point" technique that make the
light shape appearant and gives more homogenous result.

This technique is quite cheap but it is not physically correct.

So I came up with a power function to have almost the same intensity
output as cycles in the general case.

2 files changed, 29 insertions, 4 deletions

diff --git a/source/blender/draw/engines/eevee/eevee_lights.c b/source/blender/draw/engines/eevee/eevee_lights.c
index 63d1cb7adc8..dff69bcdd52 100644
--- a/source/blender/draw/engines/eevee/eevee_lights.c
+++ b/source/blender/draw/engines/eevee/eevee_lights.c
@@ -72,7 +72,7 @@ static void light_shape_parameters_set(EEVEE_Light *evli, const Light *la, const
       evli->sizey = max_ff(0.003f, la->area_size * scale[1] * 0.5f);
     }
     /* For volume point lighting. */
-    evli->radius = max_ff(0.001f, hypotf(evli->sizex, evli->sizey));
+    evli->radius = max_ff(0.001f, hypotf(evli->sizex, evli->sizey) * 0.5f);
   }
   else if (la->type == LA_SUN) {
     evli->radius = max_ff(0.001f, tanf(min_ff(la->sun_angle, DEG2RADF(179.9f)) / 2.0f));
@@ -111,14 +111,22 @@ static float light_shape_power_get(const Light *la, const EEVEE_Light *evli)
   return power;
 }
 
-static float light_shape_power_volume_get(const Light *la, float area_power)
+static float light_shape_power_volume_get(const Light *la,
+                                          const EEVEE_Light *evli,
+                                          float area_power)
 {
   /* Volume light is evaluated as point lights. Remove the shape power. */
   float power = 1.0f / area_power;
-  /* Make illumination power constant */
+
   if (la->type == LA_AREA) {
     /* Match cycles. Empirical fit... must correspond to some constant. */
     power *= 0.0792f * M_PI;
+    /* This corrects for area light most representative point trick. The fit was found by
+     * reducing the average error compared to cycles. */
+    float area = evli->sizex * evli->sizey;
+    float tmp = M_PI_2 / (M_PI_2 + sqrtf(area));
+    /* Lerp between 1.0 and the limit (1 / pi). */
+    power *= tmp + (1.0f - tmp) * M_1_PI;
   }
   else if (ELEM(la->type, LA_SPOT, LA_LOCAL)) {
     /* Match cycles. Empirical fit... must correspond to some constant. */
@@ -191,7 +199,7 @@ static void eevee_light_setup(Object *ob, EEVEE_Light *evli)
   float shape_power = light_shape_power_get(la, evli);
   mul_v3_fl(evli->color, shape_power * la->energy);
 
-  evli->volume *= light_shape_power_volume_get(la, shape_power);
+  evli->volume *= light_shape_power_volume_get(la, evli, shape_power);
 
   /* No shadow by default */
   evli->shadow_id = -1.0f;
diff --git a/source/blender/draw/engines/eevee/shaders/volumetric_lib.glsl b/source/blender/draw/engines/eevee/shaders/volumetric_lib.glsl
index 8c2d1f8a5cb..6d7ddd4e56e 100644
--- a/source/blender/draw/engines/eevee/shaders/volumetric_lib.glsl
+++ b/source/blender/draw/engines/eevee/shaders/volumetric_lib.glsl
@@ -90,6 +90,23 @@ vec3 light_volume_light_vector(LightData ld, vec3 P)
   if (ld.l_type == SUN) {
     return -ld.l_forward;
   }
+  else if (ld.l_type == AREA_RECT || ld.l_type == AREA_ELLIPSE) {
+    vec3 L = P - ld.l_position;
+    vec2 closest_point = vec2(dot(ld.l_right, L), dot(ld.l_up, L));
+    vec2 max_pos = vec2(ld.l_sizex, ld.l_sizey);
+    closest_point /= max_pos;
+
+    if (ld.l_type == AREA_ELLIPSE) {
+      closest_point /= max(1.0, length(closest_point));
+    }
+    else {
+      closest_point = clamp(closest_point, -1.0, 1.0);
+    }
+    closest_point *= max_pos;
+
+    vec3 L_prime = ld.l_right * closest_point.x + ld.l_up * closest_point.y;
+    return L_prime - L;
+  }
   else {
     return ld.l_position - P;
   }