[pstricks] clipping in 3d

[Zbigniew Nitecki]

I tried clipping, but there's something I don't understand.  In the  
example below, I first want to plot the surface
z=-xy, but only inside the cylinder x^2 +y^2 \leq 2.25.  I used the  
circle centered at the origin with radius 1.5 as the clipping curve.   
Then I plotted what should be the boundary curve for this surface,  
using \parametricplot3D.
The output is attached.  Why the discrepancy?

Example based on Herbert's example from 11/24/2008:

			\begin{pspicture}(-5,-9.5)(5,3)
			\psset{unit=2}
			\pstThreeDCoor[xMin=-2, xMax=2, yMin=-2,yMax=2, zMin=0, zMax=2]
			\begin{psclip}{%
				\pstThreeDCircle[linestyle=none](0,0,0)(1.5,0,0)(0,1.5,0)
			}
				\psplotThreeD[drawStyle=xyLines](-2,2)(-2,2){%
					x y mul neg%
				}
			\end{psclip}
			
			\parametricplotThreeD[linewidth=1.2pt, xPlotpoints=200](0,360){%
				1.5 t cos mul %
				1.5 t sin mul %
				t cos t sin mul 1.5 mul neg%
			}
				
			\end{pspicture}




Zbigniew Nitecki
Department of Mathematics
Tufts University
Medford, MA 02155

telephones:
Office    (617)627-3843
Dept.    (617)627-3234
Dept. fax    (617)627-3966
http://www.tufts.edu/~znitecki/

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