Fix #112963: EEVEE: Wrong specular from lights

This was caused by the wrong LUT being sampled.

This cleans-up the utilTex layer define usage
and implement the small optimization marked
as todo.

source/blender/draw/engines/eevee/eevee_materials.cc

@@ -180,10 +180,14 @@ static void eevee_init_util_texture()
    * Copy ltc_mag_ggx into blue and alpha channel. */
   for (int x = 0; x < 64; x++) {
     for (int y = 0; y < 64; y++) {
+      float ltc_sum = blender::eevee::lut::ltc_mag_ggx[y][x][0] +
+                      blender::eevee::lut::ltc_mag_ggx[y][x][1];
+      float brdf_sum = blender::eevee::lut::brdf_ggx[y][x][0] +
+                       blender::eevee::lut::brdf_ggx[y][x][1];
       texels_layer[y * 64 + x][0] = blender::eevee::lut::ltc_disk_integral[y][x][0];
-      texels_layer[y * 64 + x][1] = 0.0; /* UNUSED */
-      texels_layer[y * 64 + x][2] = blender::eevee::lut::ltc_mag_ggx[y][x][0];
-      texels_layer[y * 64 + x][3] = blender::eevee::lut::ltc_mag_ggx[y][x][1];
+      texels_layer[y * 64 + x][1] = ltc_sum / brdf_sum;
+      texels_layer[y * 64 + x][2] = 0.0f; /* UNUSED */
+      texels_layer[y * 64 + x][3] = 0.0f; /* UNUSED */
     }
   }
   texels_layer += 64 * 64;

source/blender/draw/engines/eevee/shaders/closure_eval_glossy_lib.glsl

@@ -78,10 +78,7 @@ ClosureEvalGlossy closure_Glossy_eval_init(inout ClosureInputGlossy cl_in,
 
   /* The BRDF split sum LUT is applied after the radiance accumulation.
    * Correct the LTC so that its energy is constant. */
-  /* TODO(@fclem): Optimize this so that only one scale factor is stored. */
-  vec4 ltc_brdf = texture(utilTex, vec3(lut_uv, LTC_BRDF_LAYER)).barg;
-  vec2 split_sum_brdf = ltc_brdf.zw;
-  cl_eval.ltc_brdf_scale = (ltc_brdf.x + ltc_brdf.y) / (split_sum_brdf.x + split_sum_brdf.y);
+  cl_eval.ltc_brdf_scale = texture(utilTex, vec3(lut_uv, LTC_BRDF_LAYER)).g;
   return cl_eval;
 }
 

source/blender/draw/engines/eevee/shaders/common_utiltex_lib.glsl

@@ -21,9 +21,9 @@ uniform sampler2DArray utilTex;
 
 #define LTC_MAT_LAYER 0
 #define LTC_BRDF_LAYER 3
+#define LTC_DISK_LAYER 3
 #define BRDF_LUT_LAYER 1
 #define NOISE_LAYER 2
-#define LTC_DISK_LAYER 3 /* UNUSED */
 
 /* Layers 4 to 20 are for BTDF LUT. */
 #define lut_btdf_layer_first 4.0

source/blender/draw/engines/eevee/shaders/ltc_lib.glsl

@@ -23,7 +23,7 @@ float diffuse_sphere_integral(float avg_dir_z, float 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, 3.0)).x;
+  return texture(utilTex, vec3(uv, LTC_DISK_LAYER)).x;
 #else
   /* Cheap approximation. Less smooth and have energy issues. */
   return max((form_factor * form_factor + avg_dir_z) / (form_factor + 1.0), 0.0);