The color of the shading can be in any color space except for pattern.
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
(see ps_setcolor() for their meaning).
x0, x1, y0, y1
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.
c1, c2, c3, c4
See ps_setcolor() for their meaning.
If the shading is of type radial the
optlist must also contain the parameters
r0 and r1 with the radius of the
start and end circle.