Eevee: Implement Sun area lighting and few fixes/opti.

Sun is treated as a unit distant disk like in cycles.

Opti: Since computing the diffuse contribution via LTC is the same as not using the Linear Transformation, we can bypass most of the LTC code.
This replaces the sphere analytical diffuse computation as it gives a more pleasing result very close to cycles' AND cheaper.

Lights power have been retweaked to be coherent with cycles (except sun lamp with large radius where cycles has a non-uniform light distribution).

1 files changed, 33 insertions, 30 deletions

diff --git a/source/blender/draw/engines/eevee/shaders/ltc_lib.glsl b/source/blender/draw/engines/eevee/shaders/ltc_lib.glsl
index 915e6a5cebc..5c62cb19152 100644
--- a/source/blender/draw/engines/eevee/shaders/ltc_lib.glsl
+++ b/source/blender/draw/engines/eevee/shaders/ltc_lib.glsl
@@ -15,17 +15,17 @@ uniform sampler2DArray utilTex;
 #endif /* UTIL_TEX */
 
 /* Diffuse *clipped* sphere integral. */
-float diffuse_sphere_integral_lut(vec3 avg_dir, float form_factor)
+float diffuse_sphere_integral_lut(float avg_dir_z, float form_factor)
 {
-	vec2 uv = vec2(avg_dir.z * 0.5 + 0.5, form_factor);
+	vec2 uv = vec2(avg_dir_z * 0.5 + 0.5, form_factor);
 	uv = uv * (LUT_SIZE - 1.0) / LUT_SIZE + 0.5 / LUT_SIZE;
 
 	return texture(utilTex, vec3(uv, 1.0)).w;
 }
 
-float diffuse_sphere_integral_cheap(vec3 avg_dir, float form_factor)
+float diffuse_sphere_integral_cheap(float avg_dir_z, float form_factor)
 {
-	return max((form_factor * form_factor + avg_dir.z) / (form_factor + 1.0), 0.0);
+	return max((form_factor * form_factor + avg_dir_z) / (form_factor + 1.0), 0.0);
 }
 
 /**
@@ -149,7 +149,7 @@ mat3 ltc_matrix(vec4 lut)
 	return Minv;
 }
 
-float ltc_evaluate_quad(vec3 N, vec3 V, mat3 Minv, vec3 corners[4])
+void ltc_transform_quad(vec3 N, vec3 V, mat3 Minv, inout vec3 corners[4])
 {
 	/* Avoid dot(N, V) == 1 in ortho mode, leading T1 normalize to fail. */
 	V = normalize(V + 1e-8);
@@ -167,24 +167,11 @@ float ltc_evaluate_quad(vec3 N, vec3 V, mat3 Minv, vec3 corners[4])
 	corners[1] = normalize(Minv * corners[1]);
 	corners[2] = normalize(Minv * corners[2]);
 	corners[3] = normalize(Minv * corners[3]);
-
-	/* Approximation using a sphere of the same solid angle than the quad.
-	 * Finding the clipped sphere diffuse integral is easier than clipping the quad. */
-	vec3 avg_dir;
-	avg_dir  = edge_integral_vec(corners[0], corners[1]);
-	avg_dir += edge_integral_vec(corners[1], corners[2]);
-	avg_dir += edge_integral_vec(corners[2], corners[3]);
-	avg_dir += edge_integral_vec(corners[3], corners[0]);
-
-	float form_factor = length(avg_dir);
-
-	float sphere_cosine_integral = form_factor * diffuse_sphere_integral_lut(avg_dir, form_factor);
-
-	return abs(sphere_cosine_integral);
 }
 
-/* Same as above but without the matrix transform. */
-float ltc_evaluate_quad_diffuse(vec3 corners[4])
+/* If corners have already pass through ltc_transform_quad(), then N **MUST** be vec3(0.0, 0.0, 1.0),
+ * corresponding to the Up axis of the shading basis. */
+float ltc_evaluate_quad(vec3 corners[4], vec3 N)
 {
 	/* Approximation using a sphere of the same solid angle than the quad.
 	 * Finding the clipped sphere diffuse integral is easier than clipping the quad. */
@@ -195,10 +182,27 @@ float ltc_evaluate_quad_diffuse(vec3 corners[4])
 	avg_dir += edge_integral_vec(corners[3], corners[0]);
 
 	float form_factor = length(avg_dir);
+	float avg_dir_z = dot(N, avg_dir / form_factor);
 
-	float sphere_cosine_integral = form_factor * diffuse_sphere_integral_lut(avg_dir, form_factor);
+#if 1 /* use tabulated horizon-clipped sphere */
+	return form_factor * diffuse_sphere_integral_lut(avg_dir_z, form_factor);
+#else /* Less accurate version, a bit cheaper. */
+	return form_factor * diffuse_sphere_integral_cheap(avg_dir_z, form_factor);
+#endif
+}
 
-	return abs(sphere_cosine_integral);
+/* If disk does not need to be transformed and is already front facing. */
+float ltc_evaluate_disk_simple(float disk_radius, float NL)
+{
+	float r_sqr = disk_radius * disk_radius;
+	float one_r_sqr = 1.0 + r_sqr;
+	float form_factor = r_sqr * inversesqrt(one_r_sqr * one_r_sqr);
+
+#if 1 /* use tabulated horizon-clipped sphere */
+	return form_factor * diffuse_sphere_integral_lut(NL, form_factor);
+#else /* Less accurate version, a bit cheaper. */
+	return form_factor * diffuse_sphere_integral_cheap(NL, form_factor);
+#endif
 }
 
 /* disk_points are WS vectors from the shading point to the disk "bounding domain" */
@@ -307,13 +311,12 @@ float ltc_evaluate_disk(vec3 N, vec3 V, mat3 Minv, vec3 disk_points[3])
 	float L2 = sqrt(-e2/e1);
 
 	/* Find the sphere and compute lighting. */
-	float form_factor = L1 * L2 * inversesqrt((1.0 + L1 * L1) * (1.0 + L2 * L2));
-
-	/* use tabulated horizon-clipped sphere */
-	float sphere_cosine_integral = form_factor * diffuse_sphere_integral_lut(avg_dir, form_factor);
-	/* Less accurate version, a bit cheaper. */
-	//float sphere_cosine_integral = form_factor * diffuse_sphere_integral_cheap(avg_dir, form_factor);
+	float form_factor = max(0.0, L1 * L2 * inversesqrt((1.0 + L1 * L1) * (1.0 + L2 * L2)));
 
-	return max(0.0, sphere_cosine_integral);
+#if 1 /* use tabulated horizon-clipped sphere */
+	return form_factor * diffuse_sphere_integral_lut(avg_dir.z, form_factor);
+#else /* Less accurate version, a bit cheaper. */
+	return form_factor * diffuse_sphere_integral_cheap(avg_dir.z, form_factor);
+#endif
 }