Merge pull request #8494 from AnalyticalGraphicsInc/fix-mac-front-facing

Fix lighting on Mac/Intel

Author: IanLilleyT

CHANGES.md

@@ -12,6 +12,7 @@ Change Log
 * Fixed `OrientedBoundingBox.fromRectangle` for rectangles with width greater than 180 degrees. [#8475](https://github.com/AnalyticalGraphicsInc/cesium/pull/8475)
 * Fixed globe picking so that it returns the closest intersecting triangle instead of the first intersecting triangle. [#8390](https://github.com/AnalyticalGraphicsInc/cesium/pull/8390)
 * Fixed horizon culling issues with large root tiles. [#8487](https://github.com/AnalyticalGraphicsInc/cesium/pull/8487)
+* Fixed a lighting bug affecting Macs with Intel integrated graphics where glTF 2.0 PBR models with double sided materials would have flipped normals. [#8494](https://github.com/AnalyticalGraphicsInc/cesium/pull/8494)
 
 ##### Additions :tada:
 * Added `Globe.backFaceCulling` to support viewing terrain from below the surface. [#8470](https://github.com/AnalyticalGraphicsInc/cesium/pull/8470)

Source/Scene/processModelMaterialsCommon.js

@@ -543,6 +543,7 @@ import ModelUtility from './ModelUtility.js';
         if (hasNormals) {
             fragmentShader += '  vec3 normal = normalize(v_normal);\n';
             if (khrMaterialsCommon.doubleSided) {
+                // !gl_FrontFacing doesn't work as expected on Mac/Intel so use the more verbose form instead. See https://github.com/AnalyticalGraphicsInc/cesium/pull/8494.
                 fragmentShader += '  if (gl_FrontFacing == false)\n';
                 fragmentShader += '  {\n';
                 fragmentShader += '    normal = -normal;\n';

Source/Scene/processPbrMaterials.js

@@ -602,7 +602,8 @@ import ModelUtility from './ModelUtility.js';
                 fragmentShader += '    vec3 n = ng;\n';
             }
             if (material.doubleSided) {
-                fragmentShader += '    if (!gl_FrontFacing)\n';
+                // !gl_FrontFacing doesn't work as expected on Mac/Intel so use the more verbose form instead. See https://github.com/AnalyticalGraphicsInc/cesium/pull/8494.
+                fragmentShader += '    if (gl_FrontFacing == false)\n';
                 fragmentShader += '    {\n';
                 fragmentShader += '        n = -n;\n';
                 fragmentShader += '    }\n';