Eevee: LTC area lights

Using Linear Transform Cosines to compute area lighting. This is far more accurate than other techniques but also slower.

We use rotating quad to mimic sphere area light. For a better approximation, we use a rotating octogon.

1 files changed, 65 insertions, 8 deletions

diff --git a/source/blender/draw/engines/eevee/shaders/bsdf_direct_lib.glsl b/source/blender/draw/engines/eevee/shaders/bsdf_direct_lib.glsl
index bf854f6eee5..afa87dfb2c8 100644
--- a/source/blender/draw/engines/eevee/shaders/bsdf_direct_lib.glsl
+++ b/source/blender/draw/engines/eevee/shaders/bsdf_direct_lib.glsl
@@ -42,7 +42,7 @@ float direct_diffuse_sphere(LightData ld, ShadingData sd)
 	}
 
 	bsdf = max(bsdf, 0.0);
-	bsdf *= M_1_PI * M_1_PI;
+	bsdf *= M_1_PI2;
 
 	return bsdf;
 }
@@ -51,6 +51,10 @@ float direct_diffuse_sphere(LightData ld, ShadingData sd)
  * http://www.frostbite.com/wp-content/uploads/2014/11/course_notes_moving_frostbite_to_pbr.pdf */
 float direct_diffuse_rectangle(LightData ld, ShadingData sd)
 {
+#ifdef USE_LTC
+	float bsdf = ltc_evaluate(sd.N, sd.V, mat3(1.0), sd.area_data.corner);
+	bsdf *= M_1_2PI;
+#else
 	float bsdf = sd.area_data.solid_angle * 0.2 * (
 		max(0.0, dot(normalize(sd.area_data.corner[0]), sd.N)) +
 		max(0.0, dot(normalize(sd.area_data.corner[1]), sd.N)) +
@@ -58,7 +62,8 @@ float direct_diffuse_rectangle(LightData ld, ShadingData sd)
 		max(0.0, dot(normalize(sd.area_data.corner[3]), sd.N)) +
 		max(0.0, dot(sd.L, sd.N))
 	);
-
+	bsdf *= M_1_PI;
+#endif
 	return bsdf;
 }
 
@@ -87,24 +92,76 @@ float direct_ggx_point(ShadingData sd, float roughness)
 
 float direct_ggx_sphere(LightData ld, ShadingData sd, float roughness)
 {
+#ifdef USE_LTC
+	vec3 P = line_aligned_plane_intersect(vec3(0.0), sd.spec_dominant_dir, sd.l_vector);
+
+	vec3 Px = normalize(P - sd.l_vector) * ld.l_radius;
+	vec3 Py = cross(Px, sd.L);
+
+	float NV = max(dot(sd.N, sd.V), 1e-8);
+	vec2 uv = ltc_coords(NV, sqrt(roughness));
+	mat3 ltcmat = ltc_matrix(uv);
+
+// #define HIGHEST_QUALITY
+#ifdef HIGHEST_QUALITY
+	vec3 Pxy1 = normalize( Px + Py) * ld.l_radius;
+	vec3 Pxy2 = normalize(-Px + Py) * ld.l_radius;
+
+	/* counter clockwise */
+	vec3 points[8];
+	points[0] = sd.l_vector + Px;
+	points[1] = sd.l_vector - Pxy2;
+	points[2] = sd.l_vector - Py;
+	points[3] = sd.l_vector - Pxy1;
+	points[4] = sd.l_vector - Px;
+	points[5] = sd.l_vector + Pxy2;
+	points[6] = sd.l_vector + Py;
+	points[7] = sd.l_vector + Pxy1;
+	float bsdf = ltc_evaluate_circle(sd.N, sd.V, ltcmat, points);
+#else
+	vec3 points[4];
+	points[0] = sd.l_vector + Px;
+	points[1] = sd.l_vector - Py;
+	points[2] = sd.l_vector - Px;
+	points[3] = sd.l_vector + Py;
+	float bsdf = ltc_evaluate(sd.N, sd.V, ltcmat, points);
+	/* sqrt(pi/2) difference between square and disk area */
+	bsdf *= 1.25331413731;
+#endif
+
+	bsdf *= texture(ltcMag, uv).r; /* Bsdf intensity */
+	bsdf *= M_1_2PI * M_1_PI;
+#else
 	float energy_conservation;
 	vec3 L = mrp_sphere(ld, sd, sd.spec_dominant_dir, roughness, energy_conservation);
 	float bsdf = bsdf_ggx(sd.N, L, sd.V, roughness);
 
 	bsdf *= energy_conservation / (sd.l_distance * sd.l_distance);
 	bsdf *= max(ld.l_radius * ld.l_radius, 1e-16); /* radius is already inside energy_conservation */
-
+#endif
 	return bsdf;
 }
 
 float direct_ggx_rectangle(LightData ld, ShadingData sd, float roughness)
 {
-	vec3 L = mrp_area(ld, sd, sd.spec_dominant_dir);
-
-	float bsdf = bsdf_ggx(sd.N, L, sd.V, roughness) * sd.area_data.solid_angle;
-
-	bsdf *= max(0.0, dot(-sd.spec_dominant_dir, ld.l_forward)); /* fade mrp artifacts */
+#ifdef USE_LTC
+	float NV = max(dot(sd.N, sd.V), 1e-8);
+	vec2 uv = ltc_coords(NV, sqrt(roughness));
+	mat3 ltcmat = ltc_matrix(uv);
+
+	float bsdf = ltc_evaluate(sd.N, sd.V, ltcmat, sd.area_data.corner);
+	bsdf *= texture(ltcMag, uv).r; /* Bsdf intensity */
+	bsdf *= M_1_2PI;
+#else
+	float energy_conservation;
+	vec3 L = mrp_area(ld, sd, sd.spec_dominant_dir, roughness, energy_conservation);
+	float bsdf = bsdf_ggx(sd.N, L, sd.V, roughness);
 
+	bsdf *= energy_conservation;
+	/* fade mrp artifacts */
+	bsdf *= max(0.0, dot(-sd.spec_dominant_dir, ld.l_forward));
+	bsdf *= max(0.0, -dot(L, ld.l_forward));
+#endif
 	return bsdf;
 }