Re: [Matplotlib-users] quiver and Inf values

[<jas...@cr...>]

Eric Firing wrote:
> Ryan May wrote:
>> On Fri, Feb 13, 2009 at 12:08 PM, <jas...@cr... 
>> <mailto:jas...@cr...>> 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().
>
> This is a good use case for numpy.ma.masked_invalid:
>
> In [2]:numpy.ma.masked_invalid?
> Type:        function
> Base Class:    <type 'function'>
> String Form:    <function masked_invalid at 0xb62bccdc>
> Namespace:    Interactive
> File:        /usr/local/lib/python2.5/site-packages/numpy/ma/core.py
> Definition:    numpy.ma.masked_invalid(a, copy=True)
> Docstring:
>     Mask the array for invalid values (NaNs or infs).
>     Any preexisting mask is conserved.
>

Thanks for both of your replies.  So I tried the following:

import pylab
import numpy
step=1
X,Y = numpy.meshgrid( numpy.arange(-1,1.1,step),numpy.arange(-1,1.1,step) )
U = numpy.ma.masked_invalid(1/X)
V = numpy.ma.masked_invalid(Y)
pylab.figure()
Q = pylab.quiver( X,Y,U, V)
pylab.savefig("test.png")

and I still didn't get a plot.  I noticed two things:

1. The unmasked portion of each array might be different; I hope quiver 
can handle that.

2. Even when I called pylab.quiver(X,Y,U,U) (so that the masks lined 
up), I still didn't get a plot.

Does quiver handle masks properly, or did I just do something wrong?

Jason