// pattern for use in texture/pigment
// this pattern can only be used when applied to an object
// value is influenced by the slope of the object
  slope
    y,              // direction vector for slope
    //y,            // optional: dependency from altitude
    //<0.5,1.0>,    // optional: range of slope values
    //<0.75,1.0>    // optional: range of altitude values

