Babylon.js/src/Shaders/ShadersInclude/pbrBlockNormalFinal.fx

#if defined(FORCENORMALFORWARD) && defined(NORMAL)
    vec3 faceNormal = normalize(cross(dFdx(vPositionW), dFdy(vPositionW))) * vEyePosition.w;
    #if defined(TWOSIDEDLIGHTING)
        faceNormal = gl_FrontFacing ? faceNormal : -faceNormal;
    #endif

    normalW *= sign(dot(normalW, faceNormal));
#endif

#if defined(TWOSIDEDLIGHTING) && defined(NORMAL)
    normalW = gl_FrontFacing ? normalW : -normalW;
#endif