Re: [PyX-user] Ellipse in PyX

[Joerg L. <jo...@us...>]

On 14.09.05, Andre Wobst wrote:
> On 14.09.05, Joerg Lehmann wrote:
> > > On 13.09.05, Dani Marti wrote:
> > > > What is the most straightforward way to plot an ellipse (or something
> > > > that looks like an ellipse, like a deformed circle)? Is any predefined
> > > > path for an ellipse?
> > > 
> > > def elipse(x, y, a, b, angle):
> > >     t = trafo.scale(unit.topt(a), unit.topt(b)).rotated(angle).translated(x, y)
> > >     return path.circle_pt(0, 0, 1).transformed(t)
> > 
> > Btw, this makes another class/factory function for our future paths
> > module...
> 
> Right.
> 
> > > Its fine as long as a and b are not close to zero. For this case two
> > > problems occur: First scale does not work as expected (for a or b
> > > being exactly zero). Jörg, do you remember what the idea was? 
> > 
> > Then the transformation matrix becomes degenerate, which we somehow
> > wanted to exclude at that time. Maybe, we shouldn't be too strict about
> > that, at least if PostScript doesn't complain in such a case (I didn't
> > check).
> 
> I didn't either. Maybe that's the point. Otherwise I don't see any
> reason for this restriction. Beside that the checks are strange,
> they're even wrong for the case of b=0, which lead to a circle instead
> of a line. Again, a marker issue here ... ;-)

According to PLRM, p. 189, transformation with determinant zero are "not
very useful" and can lead to undefinedresult errors "during the
execution of graphics and font operators". Oh well, not a very useful
statement for a reference either.

> Well, the ellipse should just work. Suppose you're using it in a graph
> style with flexible size given by the data values you put in. Like for
> circles in the first graphstyle example. It just shouldn't break. It
> doesn't even break for negative values so why should it for zero or
> something close to zero ...

I really don't know. But I would tend to attempt preventing PS code
which is not leading to errors.

        Jörg