The color of the shading can be in any color space except for
Resource identifier of the postscript file
as returned by ps_new().
The type of shading can be either radial or
axial. Each shading starts with the current fill
color and ends with the given color values passed in the parameters
c1 to c4
(see ps_setcolor() for their meaning).
x0, x1, y0, y1
The coordinates x0, y0,
x1, y1 are the start and
end point of the shading. If the type of shading is
radial the two points are the middle points of
a starting and ending circle.