[BREAKING] Use linear space for vertex colors in gLTF

src/scene/materials/standard-material-options-builder.js

@@ -289,6 +289,8 @@ class StandardMaterialOptionsBuilder {
         options.litOptions.useDynamicRefraction = stdMat.useDynamicRefraction;
         options.litOptions.dispersion = stdMat.dispersion > 0;
         options.litOptions.shadowCatcher = stdMat.shadowCatcher;
+
+        options.litOptions.useVertexColorGamma = stdMat.vertexColorGamma;
     }
 
     _updateEnvOptions(options, stdMat, scene, cameraShaderParams) {

src/scene/materials/standard-material-options.js

@@ -47,6 +47,8 @@ class StandardMaterialOptions {
 
     lightMapEncoding = 'linear';
 
+    vertexColorGamma = false;
+
     /**
      * If normal map contains X in RGB, Y in Alpha, and Z must be reconstructed.
      *

src/scene/materials/standard-material-parameters.js

@@ -33,6 +33,8 @@ const standardMaterialParameterTypes = {
     ..._textureParameter('diffuseDetail', true, false),
     diffuseDetailMode: 'string',
 
+    vertexColorGamma: 'boolean',
+
     specular: 'rgb',
     specularTint: 'boolean',
     ..._textureParameter('specular'),

src/scene/materials/standard-material.js

@@ -523,6 +523,11 @@ const _tempColor = new Color();
  * backfaces.
  * @property {boolean} shadowCatcher When enabled, the material will output accumulated directional
  * shadow value in linear space as the color.
+ * @property {boolean} vertexColorGamma When set to true, the vertex shader converts vertex colors
+ * from gamma to linear space to ensure correct interpolation in the fragment shader. This flag is
+ * provided for backwards compatibility, allowing users to mark their materials to handle vertex
+ * colors in gamma space. Defaults to false, which indicates that vertex colors are stored in
+ * linear space.
  *
  * @category Graphics
  */
@@ -1194,6 +1199,7 @@ function _defineMaterialProps() {
     _defineFlag('opacityDither', DITHER_NONE);
     _defineFlag('opacityShadowDither', DITHER_NONE);
     _defineFlag('shadowCatcher', false);
+    _defineFlag('vertexColorGamma', false);
 
     _defineTex2D('diffuse');
     _defineTex2D('specular');

src/scene/shader-lib/glsl/chunks/lit/vert/litMain.js

@@ -63,6 +63,15 @@ mat4 dModelMatrix;
 
 #include  "litUserCodeVS"
 
+#ifdef VERTEX_COLOR
+    vec3 decodeGamma(vec3 raw) {
+        return pow(raw, vec3(2.2));
+    }
+    vec4 gammaCorrectInput(vec4 color) {
+        return vec4(decodeGamma(color.xyz), color.w);
+    }
+#endif
+
 void main(void) {
 
     #include "litUserMainStartVS"
@@ -102,7 +111,11 @@ void main(void) {
     #include "uvTransformVS, UV_TRANSFORMS_COUNT"
 
     #ifdef VERTEX_COLOR
-        vVertexColor = vertex_color;
+        #ifdef STD_VERTEX_COLOR_GAMMA
+            vVertexColor = gammaCorrectInput(vertex_color);
+        #else
+            vVertexColor = vertex_color;
+        #endif
     #endif
 
     #ifdef LINEAR_DEPTH

src/scene/shader-lib/glsl/chunks/standard/frag/diffuse.js

@@ -20,7 +20,7 @@ void getAlbedo() {
     #endif
 
     #ifdef STD_DIFFUSE_VERTEX
-        dAlbedo *= gammaCorrectInput(saturate(vVertexColor.{STD_DIFFUSE_VERTEX_CHANNEL}));
+        dAlbedo *= saturate(vVertexColor.{STD_DIFFUSE_VERTEX_CHANNEL});
     #endif
 }
 `;

src/scene/shader-lib/glsl/chunks/standard/frag/emissive.js

@@ -10,7 +10,7 @@ void getEmission() {
     #endif
 
     #ifdef STD_EMISSIVE_VERTEX
-    dEmission *= gammaCorrectInput(saturate(vVertexColor.{STD_EMISSIVE_VERTEX_CHANNEL}));
+    dEmission *= saturate(vVertexColor.{STD_EMISSIVE_VERTEX_CHANNEL});
     #endif
 }
 `;

src/scene/shader-lib/programs/lit-shader-options.js

@@ -83,6 +83,8 @@ class LitShaderOptions {
 
     vertexColors = false;
 
+    useVertexColorGamma = false;
+
     lightMapEnabled = false;
 
     dirLightMapEnabled = false;

src/scene/shader-lib/programs/lit-shader.js

@@ -284,6 +284,9 @@ class LitShader {
             attributes.vertex_color = SEMANTIC_COLOR;
             vDefines.set('VERTEX_COLOR', true);
             varyings.set('vVertexColor', 'vec4');
+            if (options.useVertexColorGamma) {
+                vDefines.set('STD_VERTEX_COLOR_GAMMA', '');
+            }
         }
 
         if (options.useMsdf && options.msdfTextAttribute) {

src/scene/shader-lib/programs/standard.js

@@ -88,11 +88,8 @@ class ShaderGeneratorStandard extends ShaderGenerator {
         return expression;
     }
 
-    _validateMapChunk(propName, chunkName, chunks) {
+    _validateMapChunk(code, propName, chunkName, chunks) {
         Debug.call(() => {
-            // strip comments from the chunk
-            const code = chunks.get(chunkName).replace(/\/\*[\s\S]*?\*\/|([^\\:]|^)\/\/.*$/gm, '$1');
-
             const requiredChangeStrings = [];
 
             // Helper function to add a formatted change string if the old syntax is found
@@ -170,7 +167,16 @@ class ShaderGeneratorStandard extends ShaderGenerator {
         // log errors if the chunk format is deprecated (format changed in engine 2.7)
         Debug.call(() => {
             if (chunkCode) {
-                this._validateMapChunk(propNameCaps, chunkName, chunks);
+                // strip comments from the chunk
+                const code = chunks.get(chunkName).replace(/\/\*[\s\S]*?\*\/|([^\\:]|^)\/\/.*$/gm, '$1');
+
+                this._validateMapChunk(code, propNameCaps, chunkName, chunks);
+
+                if (vertexColorOption) {
+                    if (code.includes('gammaCorrectInputVec3(saturate3(vVertexColor.') || code.includes('gammaCorrectInput(saturate(vVertexColor.')) {
+                        Debug.errorOnce(`Shader chunk ${chunkName} contains gamma correction code which is incompatible with vertexColorGamma=true. Please remove gamma correction calls from the chunk.`, { code: code });
+                    }
+                }
             }
         });
 

src/scene/shader-lib/wgsl/chunks/lit/vert/litMain.js

@@ -66,6 +66,15 @@ var<private> dModelMatrix: mat4x4f;
 
 #include  "litUserCodeVS"
 
+#ifdef VERTEX_COLOR
+    fn decodeGamma3(raw: vec3f) -> vec3f {
+        return pow(raw, vec3f(2.2));
+    }
+    fn gammaCorrectInputVec4(color: vec4f) -> vec4f {
+        return vec4f(decodeGamma3(color.xyz), color.w);
+    }
+#endif
+
 @vertex
 fn vertexMain(input : VertexInput) -> VertexOutput {
 
@@ -104,7 +113,11 @@ fn vertexMain(input : VertexInput) -> VertexOutput {
     #include "uvTransformVS, UV_TRANSFORMS_COUNT"
 
     #ifdef VERTEX_COLOR
-        output.vVertexColor = vertex_color;
+        #ifdef STD_VERTEX_COLOR_GAMMA
+            output.vVertexColor = gammaCorrectInputVec4(vertex_color);
+        #else
+            output.vVertexColor = vertex_color;
+        #endif
     #endif
 
     #ifdef LINEAR_DEPTH

src/scene/shader-lib/wgsl/chunks/standard/frag/diffuse.js

@@ -20,7 +20,7 @@ fn getAlbedo() {
     #endif
 
     #ifdef STD_DIFFUSE_VERTEX
-        dAlbedo = dAlbedo * gammaCorrectInputVec3(saturate3(vVertexColor.{STD_DIFFUSE_VERTEX_CHANNEL}));
+        dAlbedo = dAlbedo * saturate3(vVertexColor.{STD_DIFFUSE_VERTEX_CHANNEL});
     #endif
 }
 `;

src/scene/shader-lib/wgsl/chunks/standard/frag/emissive.js

@@ -10,7 +10,7 @@ fn getEmission() {
     #endif
 
     #ifdef STD_EMISSIVE_VERTEX
-    dEmission = dEmission * gammaCorrectInputVec3(saturate3(vVertexColor.{STD_EMISSIVE_VERTEX_CHANNEL}));
+    dEmission = dEmission * saturate3(vVertexColor.{STD_EMISSIVE_VERTEX_CHANNEL});
     #endif
 }
 `;

test/scene/materials/standard-material.test.mjs

@@ -291,6 +291,7 @@ describe('StandardMaterial', function () {
         expect(material.useMetalness).to.equal(false);
         expect(material.useMetalnessSpecularColor).to.equal(false);
         expect(material.useSkybox).to.equal(true);
+        expect(material.vertexColorGamma).to.equal(false);
     }
 
     describe('#constructor()', function () {