Re: [matplotlib-devel] Color map gamma

[Reinier H. <re...@he...>]

Hi Eric, all,

I've attached a new patch where I also allow two extra ways to define
color maps:
- by specifying (value, color) pairs, giving a linearly interpolated map.
- by specifying functions (gnuplot default functions are included)

I've added a few colormaps: afmhot, bwr, brg, gnuplot, gnuplot2,
ocean, rainbow, seismic, terrain

I refactored a few others: flag, prism, gist_gray, gist_heat,
gist_rainbow, gist_stern, gist_yarg. This saves about ~3000 lines...
(And the differences are very minor)

You can compare them with the old scales by running
examples/pylab_examples/show_colormaps.py both with and without the
attached patch.

What do you think? If it's ok, shall I push to trunk or v99?

Regards,
Reinier

On Thu, Aug 13, 2009 at 1:16 AM, Eric Firing<ef...@ha...> wrote:
> 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,
>


-- 
Reinier Heeres
Tel: +31 6 10852639