Thanks for the report, Ethan, Ethan Swint, on 2012-01-10 16:34, wrote: > Can anyone else reproduce?
Confirming, this is a bug. We need to change the matplotlib code, but I'm not sure how to proceed. I've filed it as the ominously numbered matplotlib issue #666 https://github.com/matplotlib/matplotlib/issues/666 > It seems that 'xi,yi = np.meshgrid(xi,yi)' from line 2766 of mlab.py > doesn't always produce uniformly spaced data, as the test for uniform > data fails at line 2779. Ignoring the error, however, results in > satisfactory evaluation of interp() on 2783. That's not quite the issue here, as meshgrid simply reshapes and repeats the data that you hand it. At first I was going to blame linspace, because a similar issue came up on this list late last year, and in that example, simply using arange instead of linspace resolved the issue: http://old.nabble.com/constant-spacing-with-griddata-td33007330.html#a33007330 There, I considered the possibility that either (or both) the numpy and mpl code should be changed. With your example, I'm convinced now the MPL code needs to change, since here, linspace and arange produce identical results. The reason I'm not sure how to proceed is that the difference between dx.max() and dx.min() can be an order of magnitude greater than the epsx value it is compared against. Here's a modification to your example that does this In [155]: xi = np.linspace(0,603,100);dx =np.diff(xi); dx.ptp() Out[155]: 0.00000000000011368684 In [157]: zi = griddata(x,y,z,xi,yi,interp='linear') --error-- In [158]: debug > /home/pi/usr/local/lib/python2.6/site-packages/matplotlib/mlab.py(2780)griddata() 2779 if dx.max()-dx.min() > epsx or dy.max()-dy.min() > epsy: -> 2780 raise ValueError("output grid must have constant spacing" 2781 " when using interp='linear'") ipdb> dx.ptp() 1.1368683772161603e-13 ipdb> epsx 1.0000000000000001e-15 ipdb> I believe that this has to do with the limited precision and non-uniformity of the possible numbers represented by the floating point standard. The check for constant spacing likely should take into account and compare dx.ptp() to the average dx itself, or something like that. best, -- Paul Ivanov 314 address only used for lists, off-list direct email at: http://pirsquared.org | GPG/PGP key id: 0x0F3E28F7 ------------------------------------------------------------------------------ Write once. Port to many. Get the SDK and tools to simplify cross-platform app development. Create new or port existing apps to sell to consumers worldwide. Explore the Intel AppUpSM program developer opportunity. appdeveloper.intel.com/join http://p.sf.net/sfu/intel-appdev _______________________________________________ Matplotlib-users mailing list Matplotlib-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/matplotlib-users
