>> On Tuesday, February 28, 2012, Andreas H. wrote: >> >>> Good morning, >>> >>> I'm creating the attached plot using pcolormesh(). What I would like to >>> do now is draw contour lines at +/- 2.5%, which follow the grid edges. >>> >>> The problem is that when I use contour(), the lines drawn do not follow >>> the grid edges but seem to be interpolated somehow. >>> >>> Do you have an idea how to draw the contour lines following the grid >>> edges? >>> >>> Your insight is very much appreciated :) >>> >>> Cheers, >>> Andreas. >>> >> >> This is because of a subtle difference in how pcolor-like functions and >> contour-like functions work. I always forget which is which, but one >> assumes that the z value lies on the vertices of the grid while the >> other >> assumes that it lies in the middle of each grid point. This is why you >> see >> them slightly offset from each other. > > Thanks, Ben! > > To `pcolormesh`, I pass the *edges* of the grid: > > xbin = linspace(0, 12, nxbin + 1) > ybin = np.linspace(-90, 90, nybin + 1) > > pl = spl.pcolormesh(xbin, ybin, pdata.T, cmap=cmap, edgecolors='None', > vmin=-5, vmax=20) > > `contour`, however, wants the coordinates themselves. So I do > > spl.contour((xbin[:-1]+xbin[1:])/2., (ybin[:-1]+ybin[1:])/2, pdata.T, > [-2.5, 2.5]) > > Still, the outcome is, well, unexpected to me. Actually, no matter if > contour wants centres or edges, the actual behaviour seems strange. There > is some interpolation going on, apparently. The input `pdata` has shape > (12, 72) (or 72,12), and I definitely wouldn't expect this sub-grid > movement in the x-direction. > > Any ideas?
Okay, after some diving into matplotlib sources, I guess the interpolation comes within the function `QuadContourSet._get_allsegs_and_allkinds`. So there seems to be no way to accomplish what I actually want with the current matplotlib API. Correct? If I wanted to do something about this, I would need to * implement a class `GriddedContourSet`, derived from `ContourSet`, where I implement the `_get_allsegs_and_allkinds` method appropriately. * add an additional keyword argument to `contour()` to make this gridded contourset an option when calling `contour()`. Is this all correct? If yes, I might start working on this if I get the time ... Cheers, Andreas. ------------------------------------------------------------------------------ Keep Your Developer Skills Current with LearnDevNow! The most comprehensive online learning library for Microsoft developers is just $99.99! Visual Studio, SharePoint, SQL - plus HTML5, CSS3, MVC3, Metro Style Apps, more. Free future releases when you subscribe now! http://p.sf.net/sfu/learndevnow-d2d _______________________________________________ Matplotlib-users mailing list Matplotlib-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/matplotlib-users
