[metapost] reverse clip operation

[Andreas Matthias]

lfinsto1 at gwdg.de wrote:

Probably the term `reverse clipping' is wrong. Maybe it should
be named `inverse clipping'? I'm not sure.

[ picture: q0...red ellipse, q1...black line]

> beginfig(4);
>   undraw frame;
>   currentpicture := q1;
>   addto currentpicture also q0 withcolor white;
> endfig;

Your are missing the red ellipse here. Let's suppose the
red ellipse had be added to this fig as a first operation.
Then you are clipping the black line q1, but with the
penalty that your are painting (in white) over the red ellipse.

My aim would be to:
   1) draw the red ellipse
   2) draw (and clip) the black line

That should be all. A third operation like:
   3) repaint red ellipse
should be unnecessary.


Ciao
Andreas