Re: [matplotlib-devel] Color map gamma

[Eric F. <ef...@ha...>]

Reinier Heeres wrote:
> Hi all,
> 
> I would like to propose the attached patch to be able to use a gamma
> value for color maps. This will make it simple to make your color
> scale more 'sensitive' at the bottom or at the top, as can be seen in
> the attached example. This could in principle also be solved by adding
> a gamma normalizer, but I think that applying it to a color map is
> quite coming practice, so in this case the preferred way.

Your patch looks reasonable to me.
> 
> I'd also like to add a few extra color maps (at least one plain
> blue-white-red and one with darker shades at the high and low ends, as

Good.

> in the attachment). I also remember a particular one ('terrain') in a
> measurement program called 'Igor' that would be nice.

Is there any potential licensing problem?  I hope not.  I presume you 
would copy the effect, not any particular set of numbers extracted from 
Igor.

> 
> Looking at _cm.py, I would guess that could be done a bit more
> efficient than the current 5880 lines as well by just specifying a few
> colors and using LinearSegmentedColormap.from_list(). Is it ok if I
> try to refactor that?

You mean take the colormaps that have a huge number of color dictionary 
entries in _cm.py, and subsample them down to something reasonable? 
Please do!  I always hated those blocks of numbers, but never enough to 
motivate me to do something about them other than a little reformatting.

It sounds like you are talking about going farther than that, which 
might be fine but might make things more complicated.  As it is now, all 
the built-in colormaps are associated with color dictionaries for direct 
use in LinearSegmentedColormap.  If you make two styles, one based on 
the dictionaries (which allows discontinuities) and one based on 
from_list (which does not), then you need to keep track of which is 
which.  Is it worth it?  I am inclined to stick with the cdict approach.

It looks like an obvious addition would a function that takes a list of
breakpoints (starting with 0 and ending with 1) and a matching list of 
colors and generates the corresponding cdict for continuous mapping.

Eric

> 
> Let me know what you think.
> 
> Cheers,