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

[Benjamin R. <ben...@ou...>]

On Thursday, February 16, 2012, Kathleen M Tacina wrote:

> **
> 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
>


 Kathy,

It is great that you have found mpl to be useful, and better still that you
found a way to improve it.

The recommended way to submit changes is through github.  The developers'
guide at matplotlib.sourceforge.net will describe how to do that.

At the very least, diff patches are needed so that we can be sure we make
exactly the right changes.

Welcome!
Ben Root