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From Odwiki

The VEX Plastic SHOP is a Shader Operator which uses the VEX Plastic SurfaceShader to compute the color of the surface.

The shader itself is quite simple

surface
plastic(
        vector amb=1;
        vector diff=1;
        vector spec=1;
        float rough=0.1;
        )
{
    vector      nml; 

    nml = frontface(normalize(N), I);
    Cf  = amb  * ambient();
    Cf += diff * diffuse(nml);
    Cf += spec * phong(nml, -normalize(I), 1.0/rough);
}

The inputs to the shader (which show up as parameters in the SHOP) are

• amb - The ambient response of the surface (tint for ambient light)
• diff - The diffuse response of the surface (tint for diffuse light)
• spec - The specular response of the surface (tint for specular light)
• rough - The surface roughness

The shader first computes a front-facing normal which is used for the lighting computations.

Each type of light (ambient, diffuse and specular) is then added to the produce the final output color.