Properly remap roughness when reading from radiance map

This ensures that we consistently use perceptual roughness which matches the behaviour of most other PBR renderers like Blender, Ue4 and Godot 3

drivers/gles3/shaders/scene.glsl

@@ -1100,7 +1100,7 @@ void main() {
 		ref_vec = mix(ref_vec, normal, roughness * roughness);
 		float horizon = min(1.0 + dot(ref_vec, normal), 1.0);
 		ref_vec = scene_data.radiance_inverse_xform * ref_vec;
-		specular_light = textureLod(radiance_map, ref_vec, roughness * RADIANCE_MAX_LOD).rgb;
+		specular_light = textureLod(radiance_map, ref_vec, sqrt(roughness) * RADIANCE_MAX_LOD).rgb;
 		specular_light = srgb_to_linear(specular_light);
 		specular_light *= horizon * horizon;
 		specular_light *= scene_data.ambient_light_color_energy.a;

servers/rendering/renderer_rd/effects/copy_effects.cpp

@@ -1153,7 +1153,8 @@ void CopyEffects::cubemap_roughness(RID p_source_rd_texture, RID p_dest_texture,
 	memset(&roughness.push_constant, 0, sizeof(CubemapRoughnessPushConstant));
 
 	roughness.push_constant.face_id = p_face_id > 9 ? 0 : p_face_id;
-	roughness.push_constant.roughness = p_roughness * p_roughness; // Shader expects roughness, not perceptual roughness, so multiply before passing in.
+	// Remap to perceptual-roughness^2 to create more detail in lower mips and match the mapping of cubemap_filter.
+	roughness.push_constant.roughness = p_roughness * p_roughness;
 	roughness.push_constant.sample_count = p_sample_count;
 	roughness.push_constant.use_direct_write = p_roughness == 0.0;
 	roughness.push_constant.face_size = p_size;

servers/rendering/renderer_rd/shaders/effects/cubemap_roughness_inc.glsl

@@ -70,17 +70,6 @@ float DistributionGGX(float NdotH, float roughness4) {
 	return roughness4 / denom;
 }
 
-// https://graphicrants.blogspot.com.au/2013/08/specular-brdf-reference.html
-float GGX(float NdotV, float a) {
-	float k = a / 2.0;
-	return NdotV / (NdotV * (1.0 - k) + k);
-}
-
-// https://graphicrants.blogspot.com.au/2013/08/specular-brdf-reference.html
-float G_Smith(float a, float nDotV, float nDotL) {
-	return GGX(nDotL, a * a) * GGX(nDotV, a * a);
-}
-
 float radicalInverse_VdC(uint bits) {
 	bits = (bits << 16u) | (bits >> 16u);
 	bits = ((bits & 0x55555555u) << 1u) | ((bits & 0xAAAAAAAAu) >> 1u);

servers/rendering/renderer_rd/shaders/forward_clustered/scene_forward_clustered.glsl

@@ -1084,12 +1084,13 @@ void fragment_shader(in SceneData scene_data) {
 #ifdef USE_RADIANCE_CUBEMAP_ARRAY
 
 		float lod, blend;
-		blend = modf(roughness * MAX_ROUGHNESS_LOD, lod);
+
+		blend = modf(sqrt(roughness) * MAX_ROUGHNESS_LOD, lod);
 		specular_light = texture(samplerCubeArray(radiance_cubemap, material_samplers[SAMPLER_LINEAR_WITH_MIPMAPS_CLAMP]), vec4(ref_vec, lod)).rgb;
 		specular_light = mix(specular_light, texture(samplerCubeArray(radiance_cubemap, material_samplers[SAMPLER_LINEAR_WITH_MIPMAPS_CLAMP]), vec4(ref_vec, lod + 1)).rgb, blend);
 
 #else
-		specular_light = textureLod(samplerCube(radiance_cubemap, material_samplers[SAMPLER_LINEAR_WITH_MIPMAPS_CLAMP]), ref_vec, roughness * MAX_ROUGHNESS_LOD).rgb;
+		specular_light = textureLod(samplerCube(radiance_cubemap, material_samplers[SAMPLER_LINEAR_WITH_MIPMAPS_CLAMP]), ref_vec, sqrt(roughness) * MAX_ROUGHNESS_LOD).rgb;
 
 #endif //USE_RADIANCE_CUBEMAP_ARRAY
 		specular_light *= scene_data.IBL_exposure_normalization;
@@ -1137,7 +1138,7 @@ void fragment_shader(in SceneData scene_data) {
 		ref_vec = mix(ref_vec, n, clearcoat_roughness * clearcoat_roughness);
 		float horizon = min(1.0 + dot(ref_vec, normal), 1.0);
 		ref_vec = scene_data.radiance_inverse_xform * ref_vec;
-		float roughness_lod = mix(0.001, 0.1, clearcoat_roughness) * MAX_ROUGHNESS_LOD;
+		float roughness_lod = mix(0.001, 0.1, sqrt(clearcoat_roughness)) * MAX_ROUGHNESS_LOD;
 #ifdef USE_RADIANCE_CUBEMAP_ARRAY
 
 		float lod, blend;

servers/rendering/renderer_rd/shaders/forward_mobile/scene_forward_mobile.glsl

@@ -987,12 +987,12 @@ void main() {
 #ifdef USE_RADIANCE_CUBEMAP_ARRAY
 
 		float lod, blend;
-		blend = modf(roughness * MAX_ROUGHNESS_LOD, lod);
+		blend = modf(sqrt(roughness) * MAX_ROUGHNESS_LOD, lod);
 		specular_light = texture(samplerCubeArray(radiance_cubemap, material_samplers[SAMPLER_LINEAR_WITH_MIPMAPS_CLAMP]), vec4(ref_vec, lod)).rgb;
 		specular_light = mix(specular_light, texture(samplerCubeArray(radiance_cubemap, material_samplers[SAMPLER_LINEAR_WITH_MIPMAPS_CLAMP]), vec4(ref_vec, lod + 1)).rgb, blend);
 
 #else // USE_RADIANCE_CUBEMAP_ARRAY
-		specular_light = textureLod(samplerCube(radiance_cubemap, material_samplers[SAMPLER_LINEAR_WITH_MIPMAPS_CLAMP]), ref_vec, roughness * MAX_ROUGHNESS_LOD).rgb;
+		specular_light = textureLod(samplerCube(radiance_cubemap, material_samplers[SAMPLER_LINEAR_WITH_MIPMAPS_CLAMP]), ref_vec, sqrt(roughness) * MAX_ROUGHNESS_LOD).rgb;
 
 #endif //USE_RADIANCE_CUBEMAP_ARRAY
 		specular_light *= sc_luminance_multiplier;
@@ -1042,7 +1042,7 @@ void main() {
 
 		float horizon = min(1.0 + dot(ref_vec, normal), 1.0);
 		ref_vec = scene_data.radiance_inverse_xform * ref_vec;
-		float roughness_lod = mix(0.001, 0.1, clearcoat_roughness) * MAX_ROUGHNESS_LOD;
+		float roughness_lod = mix(0.001, 0.1, sqrt(clearcoat_roughness)) * MAX_ROUGHNESS_LOD;
 #ifdef USE_RADIANCE_CUBEMAP_ARRAY
 
 		float lod, blend;