qt-everywhere-src-5.15.1/qtquick3d/src/runtimerender/res/effectlib/doubleSided.glsllib

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// Inverts N when rendering a backface
vec3 adjustNormalForFace(in vec3 N, in vec3 position)
{
    vec3 fdx = dFdx(position);
    vec3 fdy = dFdy(position);
    vec3 faceNormal = normalize(cross(fdx, fdy));
    if (dot (N, faceNormal) < 0.0) {
        N *= -1.0;
    }
    return N;
}