2010/10/18 脑关(BrainGateway)生命科学仪器 <braingate...@gmail.com>:
> On Mon, Oct 18, 2010 at 12:54 AM, Benjamin Root <ben.r...@ou.edu> wrote:
>> On Sun, Oct 17, 2010 at 5:35 PM, braingateway <braingate...@gmail.com>
>> wrote:
>>> I tried all possible colormaps and found out: gist_rainbow, terrain,
>>> bwr, brg, and seismic will generate the same error, all other colormaps
>>> are OK. I wonder is this a bug or expected behavior?
> Hi Ben,
> Thanks a lot for answering my question! I am a newbie to matplotlib,
> so please forgive me, if the question is stupid ;p
Questions are never stupid ...
> In my program I wanna know the exact RGBA value of a data point in the
> figure, in order to plot a corresponding line with the same color in
> another figure. That is why I need to call a(z) to get RGBA value of a
> point at (x,y,z) (z is represented by color). If I understood it
> correctly, you said I do not need to specify the 'lut' in
> cm.get_cmap(name,lut)? I thought the colormap object is actually a
> lookup table with a length specified by lut.
Kind of, but it's initialised from a linear segmentation dictionary,
so you can get cmaps with a precision you want.
> It turns out I do not
> need to specify anything here.
Default param is 256.
> But if I do not specify anything the
> colormap.N is always 256. What will happen then, if I need more color
> steps?
cm = get_cmap('...', 1024)
> I do notice a(2) and a(2.2) returns different values. So I am
> very confused about the principle how the RGBA value is generated by
> the colormaps.
It's the "magic" to distinguish between integer (in the LUT range) and
float (in [0.0, 1.0]). 2 gives the LUT entry 2, 2.0 will give the
upper value since it's > 1, as will 2.2 do. Try 0.5, and 0.6, or 0.0
and 0.1.
Actually I cannot reproduce your error on a recently (some weeks ago)
checked-out GitHub repo version of mpl 1.0.0. Please provide
mpl.__version__ so that we check if that's the reason - as simple as
it might be.
MacBook-Pro-Friedrich:Report Friedrich$ python
Python 2.6.5 (r265:79063, Jul 18 2010, 12:14:53)
[GCC 4.2.1 (Apple Inc. build 5659)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import matplotlib.cm
>>> cm = matplotlib.cm.get_cmap('gist_rainbow')
>>> cm(range(256))
array([[ 1. , 0. , 0.16 , 1. ],
[ 1. , 0. , 0.13908497, 1. ],
[ 1. , 0. , 0.11816993, 1. ],
...,
[ 1. , 0. , 0.79262575, 1. ],
[ 1. , 0. , 0.77131287, 1. ],
[ 1. , 0. , 0.75 , 1. ]])
>>> print matplotlib.__version__
1.0.0
The attribute of the cm where the error occurs on your machine is set
to a valid value for me. I also cannot find any code path leading to
a wrong initialisation of the attribute. Might be that the data the
cm is init'ed from changed. There is a code path initialising the cm
from a tuple (your attribute was a tuple used like a dict), but this
works too: (terrain is such an example):
>>> cm = matplotlib.cm.get_cmap('terrain')
>>> cm(range(256))
array([[ 0.2 , 0.2 , 0.6 , 1. ],
[ 0.19477124, 0.21045752, 0.61045752, 1. ],
[ 0.18954248, 0.22091503, 0.62091503, 1. ],
...,
[ 0.98431373, 0.97992157, 0.97898039, 1. ],
[ 0.99215686, 0.98996078, 0.9894902 , 1. ],
[ 1. , 1. , 1. , 1. ]])
Might be that there was 'red' misspelled in gist_rainbow in you mpl
version, this may explain the behaviour if we track it down. Can you
do the following to verify this:
import matplotlib._cm
print matplotlib._cm._gist_stern_data
? Thx,
Friedrich
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