surface slicePoint
(
float Kd = 1.0;
float Ks = 1;
float roughness = 0.1;

float near = 1;
float far = 10000;
float nearFadeWidth = 0.1;
float farFadeWidth = 1;
){

    normal nn = normalize(faceforward(N,I));
    vector i = normalize(-I);

    color diff = diffuse(nn);
    color spec = specular(nn, i, roughness);

    diff *= Kd;
    spec *= Ks;

    float z = P[2];

    float opacity = 0;
    float outNear = near - nearFadeWidth;
    float outFar = far + farFadeWidth;

    opacity = smoothstep(outNear, near, z);
    opacity *= 1 - smoothstep(far, outFar, z);

    Oi = opacity * Os;
    Ci = Cs * (diff* Kd + spec* Ks) * opacity;

}