Fix base color and reflectivity calculations for PBR materials

CHANGES.md

@@ -14,6 +14,7 @@
 
 ##### Fixes :wrench:
 
+- Fixed diffuse color calculation for PBR materials. Many models will now appear slightly brighter. This also fixes an [issue affecting the KHR_materials_specular extension](https://github.com/CesiumGS/cesium/issues/12041). [#12043](https://github.com/CesiumGS/cesium/pull/12043)
 - Fixed issue where Entities would not use a custom ellipsoid. [#3543](https://github.com/CesiumGS/cesium/issues/3543)
 
 ##### Breaking Changes :mega:

packages/engine/Source/Shaders/Builtin/Structs/modelMaterial.glsl

@@ -10,6 +10,7 @@
  * @name czm_modelMaterial
  * @glslStruct
  *
+ * @property {vec4} baseColor The base color of the material.
  * @property {vec3} diffuse Incoming light that scatters evenly in all directions.
  * @property {float} alpha Alpha of this material. 0.0 is completely transparent; 1.0 is completely opaque.
  * @property {vec3} specular Color of reflected light at normal incidence in PBR materials. This is sometimes referred to as f0 in the literature.
@@ -19,6 +20,7 @@
  * @property {vec3} emissive Light emitted by the material equally in all directions. The default is vec3(0.0), which emits no light.
  */
 struct czm_modelMaterial {
+    vec4 baseColor;
     vec3 diffuse;
     float alpha;
     vec3 specular;

packages/engine/Source/Shaders/Model/MaterialStageFS.glsl

@@ -296,12 +296,12 @@ float setMetallicRoughness(inout czm_modelMaterial material)
 
     // dielectrics use f0 = 0.04, metals use albedo as f0
     const vec3 REFLECTANCE_DIELECTRIC = vec3(0.04);
-    vec3 f0 = mix(REFLECTANCE_DIELECTRIC, material.diffuse, metalness);
+    vec3 f0 = mix(REFLECTANCE_DIELECTRIC, material.baseColor.rgb, metalness);
 
     material.specular = f0;
 
     // diffuse only applies to dielectrics.
-    material.diffuse = material.diffuse * (1.0 - f0) * (1.0 - metalness);
+    material.diffuse = mix(material.baseColor.rgb, vec3(0), metalness);
 
     // roughness is authored as perceptual roughness
     // square it to get material roughness
@@ -349,7 +349,7 @@ void setSpecular(inout czm_modelMaterial material, in float metalness)
     material.specularWeight = specularWeight;
     vec3 f0 = material.specular;
     vec3 dielectricSpecularF0 = min(f0 * specularColorFactor, vec3(1.0));
-    material.specular = mix(dielectricSpecularF0, material.diffuse, metalness);
+    material.specular = mix(dielectricSpecularF0, material.baseColor.rgb, metalness);
 }
 #endif
 #ifdef USE_ANISOTROPY
@@ -458,6 +458,7 @@ void materialStage(inout czm_modelMaterial material, ProcessedAttributes attribu
         baseColorWithAlpha *= color;
     #endif
 
+    material.baseColor = baseColorWithAlpha;
     material.diffuse = baseColorWithAlpha.rgb;
     material.alpha = baseColorWithAlpha.a;