A shading dictionary specifies details of a particular gradient fill, including the
type of shading to be used, the geometry of the area to be shaded, and the geometry of the gradient fill. Various shading types are available, depending on the value of the dictionary’s ShadingType entry:

• Coons patch meshes (type 6) construct a shading from one or more color
  patches, each bounded by four cubic Bézier curves.