[Matplotlib-users] Suggested enhancement to imshow shading (with possible code)

[Kathleen M T. <Kat...@na...>]

I've found matplotlib to be a very nice tool.  In particular, I've been
using the LightSource class in colors.py.

However, LightSource.shade always uses the array min and max to set the
minimum and maximum colormap values.  I'd like to be able to manually
set minimum and maximum values used in the colormap, and I'd also be
like to limit the elevation for the shading algorithm. I have three use
cases:
(1) comparing related images, which is much easier to do when the
colormaps are constant
(2) cases where I have very high or very low values 
(3) cases where I've used the set_over or set_under methods of the
colormap because low and/or high values are significant.

Since it was fairly easy to modify this code myself, I did so.  This
modification might be useful to other people, too, so I was wondering if
it would be possible to modify LightSource.shade similar to the code
below.  

    def shade(self,data,cmap,vmin=None,vmax=None,limit_elevation=False):
        """
        Take the input data array, convert to HSV values in the
        given colormap, then adjust those color values
        to given the impression of a shaded relief map with a
        specified light source.
        RGBA values are returned, which can then be used to
        plot the shaded image with imshow.

        Parameters
        ----------
        data            Input data array
        vmin            Minimum data value for colormap.  Default
min(data).
        vmax            Maximum data value for colormap.  Default
max(data).
        limit_elevation Limit the elevation in the shading routine?
Default False.
                        If true, the elevation will be limited by vmin
and vmax.

        Returns 
        -------
        rgb             Shaded RGBA values, suitable for use with
imshow.

        """

        if (vmin is not None) or (vmax is not None):
            limitschanged = True
        else:
            limitschanged = False
        if vmin is None:
            vmin = np.min(data)
        if vmax is None:
            vmax = np.max(data)
        rgb0 = cmap((data-vmin)/(np.float(vmax-vmin)))
        #avoid using extra memory if copy of array not needed
        if limitschanged and limit_elevation:
            d = data.copy()
            d[d<vmin] = vmin
            d[d>vmax] = vmax
            rgb1 = self.shade_rgb(rgb0, elevation=d)
        else:
            rgb1 = self.shade_rgb(rgb0, elevation=data)
        rgb0[:,:,0:3] = rgb1
        return rgb0

I've attached a colors.py with a modified LightSource.shade and some
examples (shading3.py and shading4.py) that use the modifications to
shade.  

This is the first time I've suggested a change to matplotlib, so please
let me know if there was a better way to make this suggestion.

Regards,
Kathy