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vec4 directionalFactor( in vec3 normal, in vec3 viewDir, in vec3 normal_col, in vec3 grazing_col, in float exponent, in vec4 base )
{
    float f;

    f = clamp(dot(viewDir, normal),0.0, 1.0 );
    f = 1.0 - pow(f, 1.0/exponent);
    vec3 directionalColor = mix( normal_col, grazing_col, f );

    vec4 color = base * vec4(directionalColor, 1.0);

    return color;
}