Re: [matplotlib-devel] new commit: changes in color handling, masked arrays

Eric Firing wrote:
> John,
>
> I have committed to CVS a set of changes that I think will address 
> recent requests by Jordan and Gerald, and will be of more general use 
> as well.  From the CHANGELOG:
>
> 2005-11-27 Multiple changes in cm.py, colors.py, figure.py, image.py,
>            contour.py, contour_demo.py; new _cm.py, 
> examples/image_masked.py.
>            1) Separated the color table data from cm.py out into
>            a new file, _cm.py, to make it easier to find the actual
>            code in cm.py and to add new colormaps.  Also added
>            some line breaks to the color data dictionaries. Everything
>            from _cm.py is imported by cm.py, so the split should be
>            transparent.
>            2) Enabled automatic generation of a colormap from
>            a list of colors in contour; see modified
>            examples/contour_demo.py.
>            3) Support for imshow of a masked array, with the
>            ability to specify colors (or no color at all) for
>            masked regions, and for regions that are above or
>            below the normally mapped region.  See
>            examples/image_masked.py.
>            4) In support of the above, added two new classes,
>            ListedColormap, and no_norm, to colors.py, and modified
>            the Colormap class to include common functionality. Added
>            a clip kwarg to the normalize class.  Reworked color
>            handling in contour.py, especially in the ContourLabeller
>            mixin.
>
> With one very subtle exception, I don't think any default behaviors 
> have been changed, so the changes should be entirely non-disruptive.
>
> That one exception is that in the original color mapping scheme, the 
> last color was essentially never used.  Consider an extreme case: a 
> colormap with two entries.  Suppose the image data ranged from 0 to 1. 
> All values except exactly 1.0 were mapped to the first color, and only 
> that upper limit value was mapped to the second color.  As I have 
> changed colors.py, values from 0.0 up to 0.5 will get the first color, 
> and values from 0.5 through 1.0 will get the second.  I think this is 
> what a user would expect, and the only reason it hasn't mattered is 
> that with 256 colors in a continuous range, one can't see the difference.
>
> The request from Jordan that is addressed here is the desire for 
> precise color control in filled contouring.  Now one can specify a 
> list of colors in the "colors" kwarg, and they will be used to 
> generate a colormap, which will then be used by colorbar.  This is 
> shown in a second plot added to examples/contourf_demo.py.  (Jordan 
> suggested a more extensive refactoring, which may indeed be a good 
> idea; but I wanted to make these simpler changes before trying to 
> think about anything more drastic.)
>
> The request from Gerald was for easy handling of masked arrays in 
> imshow.  It was in the context of basemap, which I have not tested 
> yet; but at least for pylab.imshow, the situation is now easy to 
> handle.  If changes to basemap are needed, they should be very simple, 
> and I can do them later as needed.  I think that the changes to 
> colormapping that I made to support this will be useful much more 
> widely, but I have made no attempt to track down the places where 
> changes will be in order.  I may make additional changes to contour, 
> and I know I will need to change colorbar to fully support this.  I 
> think colorbar needs some more refactoring anyway, but I can't do it 
> immediately, and I did not want to delay getting the other changes out 
> for testing and, hopefully, productive use.
>
> Eric
>
Eric:  This is fantastic - thanks!  I'll try imshow with masked arrays 
and let you know if there are any problems.  Should be very useful for 
projections which have some parts hidden, such as orthographic.

-Jeff

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