Anisotropic Reflection

anisotropic.osl

/*
 * https://en.wikibooks.org/wiki/GLSL_Programming/Unity/Brushed_Metal
 */
float anisotropy(
    vector varyingNormalDirection,
    vector varyingTangentDirection,
    vector lightDirection,
    vector viewDirection,
    float alphaX,
    float alphaY
) {
    vector normalDirection = normalize(varyingNormalDirection);
    vector tangentDirection = normalize(varyingTangentDirection);
    float attenuation = 1.0;
    vector halfwayVector = normalize(lightDirection + viewDirection);
    vector binormalDirection = cross(normalDirection, tangentDirection);
    float dotLN = dot(lightDirection, normalDirection); 
    float dotHN = dot(halfwayVector, normalDirection);
    float dotVN = dot(viewDirection, normalDirection);
    float dotHTAlphaX = dot(halfwayVector, tangentDirection) / alphaX;
    float dotHBAlphaY = dot(halfwayVector, binormalDirection) / alphaY;

    float specularReflection = attenuation
        * sqrt(max(0.0, dotLN / dotVN)) 
        * exp(-2.0 * (dotHTAlphaX * dotHTAlphaX + dotHBAlphaY * dotHBAlphaY) / (1.0 + dotHN));

    return (1.0 - abs((specularReflection - 1.0))) / 2.0;
}

shader anisotropy(
    normal n = (0.0),
    vector tangent = (0.0),
    vector viewDir = (0.0),
    vector lightDir = (0.0),
    float alphaX = 0.0,
    float alphaY = 0.0,
    output float anisotropic_reflection = (0.0),
) {
    anisotropic_reflection = anisotropy(n, tangent, lightDir, viewDir, alphaX, alphaY);
}