On Fri, Feb 13, 2009 at 12:08 PM, <jason-s...@creativetrax.com> wrote:
>
> A student of mine recently noticed that sometimes, quiver plots were
> coming up empty (using the plot_vector_field function from Sage, which
> passes everything on to quiver). Upon investigation, we saw that some
> of the array entries passed in were infinity because of where we
> happened to evaluate the function. It was relatively easy to correct in
> our case (change the evaluation to miss the bad point), but is there a
> better way to handle this? Can this be considered a bug in quiver (i.e.,
> returning a blank plot when one of the vectors has an infinite
> coordinate?).
>
> Here is some example code illustrating the problem:
>
>
> import pylab
> import numpy
> step=1
> X,Y = numpy.meshgrid( numpy.arange(-1,1.1,step),numpy.arange(-1,1.1,step) )
> U = 1/X
> V = Y
> pylab.figure()
> Q = pylab.quiver( X,Y,U, V)
> pylab.savefig("test.png")
>
> When you change step to something that avoids an evaluation at x=0 (say,
> step=0.13), you get a nice plot.
>
> Is this something that we should be preprocessing in Sage before calling
> quiver, masking those "bad" points or something? I haven't used masking
> before, but I'd like to fix Sage's plot_vector_field function to return
> something sensible, even when the function happens to be infinite at one
> of the points.
>
I'm not sure why quiver does not plot any arrows in that case, but it's also
easy enough to mask out the values yourself:
U = 1/X
U = numpy.ma.array(U, mask=numpy.isinf(U))
V = Y
V = numpy.ma.array(V, mask=numpy.isinf(V))
You can also catch NaN values by using ~numpy.isfinite() instead of
numpy.isinf().
Ryan
--
Ryan May
Graduate Research Assistant
School of Meteorology
University of Oklahoma
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