On Sun, Jul 8, 2012 at 9:04 PM, Brendan Barnwell <brenb...@brenbarn.net>wrote:
> I just spent some time debugging an odd problem. Take this code:
>
> x,y = np.arange(-10,10), np.arange(-10,10)
> x,y = np.meshgrid(x,y)
> z = x**2+y**2
> cs = pyplot.contourf(x,y,z,levels=np.arange(50, 220, 20),
> cmap=pyplot.cm.jet, extend='both')
> cs.cmap.set_under('k')
> cb = pyplot.colorbar(cs)
>
> I was expecting the set_under call to mean that contours outside
> the
> passed level range would be painted in that color. This seems to be
> what the documentation says will happen:
>
> "Unless this [extend argument] is ‘neither’, contour levels are
> automatically added to one or both ends of the range so that all data
> are included. These added ranges are then mapped to the special
> colormap values which default to the ends of the colormap range, but
> can be set via matplotlib.colors.Colormap.set_under() and
> matplotlib.colors.Colormap.set_over() methods."
>
> . . .but it doesn't work. Instead, the levels outside my specified
> range show up colored the same as the lowest selected level (i.e., 50).
>
> I poked around in the code and found that the culprit is this
> section
> in matplotlib.contour.ContourSet._process_levels():
>
> if self.extend in ('both', 'min'):
> self.vmin = 2 * self.levels[0] - self.levels[1]
> if self.extend in ('both', 'max'):
> self.vmax = 2 * self.levels[-1] - self.levels[-2]
>
> Here, if the "extend" argument is given, the code sets the data limits
> to some odd extension of the actual data limits. I can fix it and get
> the correct output by resetting the data limits after plotting my
> contours:
>
> cs = pyplot.contourf(x,y,z,levels=np.arange(50, 220, 20),
> cmap=pyplot.cm.jet, extend='both')
> cs.cmap.set_under('k')
> cs.set_clim(50, 210)
> cb = pyplot.colorbar(cs)
>
> But why do I have to do this? The whole reason I passed in my
> specified levels was because I wanted THOSE to be the data limits.
> Why is matplotlib expanding the data limits, and thus preventing me
> from specifying the "out of range" color using the normal set_under
> and set_over methods?
>
>
You are right, that behavior is very inconsistent with the documentation.
Looking over the QuadContourSet._process_levels() method, I doubt the
problem lies there. While testing, I noticed that whatever color I set for
set_under() was always being ignored in the colorbar. I suspect the
problem lies there.
Ben Root
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