normal getBumpedNormal(color centerColor, color uColor, color vColor, float amount)
{
    vector avg = vector(0.333333);
    float c = dot(avg, vector(centerColor));
    float ddu = c - dot(avg, vector(uColor));
    float ddv = c - dot(avg, vector(vColor));
    return normalize(normal("shader", ddu, ddv, (1.000000 / amount)));
}

surface
carbon
    [[ string description = "Carbon fiber material" ]]
(
    string diffuseTexture = "diffuse.png",
    string specularTexture = "spec.png",
    string anisoTexture = "aniso.png",
    string bumpTexture = "bump.png",
    float diffuseWeight = 0.5,
    float specularWeight = 0.2,
    float specularGlossiness = 0.75,
    float bumpAmount = 2.0
)
{
    color diffuseColor = texture(diffuseTexture, u, v);
    color specularColor = texture(specularTexture, u, v);

    color anisoColor = texture(anisoTexture, u, v);
    float anisotropy = 0.5 - anisoColor[0] * 4.0;

    color center = texture(bumpTexture, u, v);
    color uColor = texture(bumpTexture, u + 0.004, v);
    color vColor = texture(bumpTexture, u, v + 0.004);
    normal n = getBumpedNormal(center, uColor, vColor, bumpAmount);

    Ci = diffuseWeight * diffuseColor * diffuse(n) + specularWeight * specularColor * ward(n, dPdu, specularGlossiness, specularGlossiness, "anisotropy", anisotropy);
}