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:
>>
>> Hi Everyone,
>>
>> I am trying the matplotlib. I have to say this is a powerful package for
>> scientific 2-D plotting. However, I encountered some problems when try
>> to generate several colormaps.
>>
>> for example:
>> a=cm.get_cmap('gist_rainbow',256)(range(256))
>> will give a error:
>> Traceback (most recent call last):
>> File "<pyshell#82>", line 1, in <module>
>> a=cm.get_cmap('gist_rainbow',256)(range(256))
>> File "...\site-packages\matplotlib\colors.py", line 498, in __call__
>> if not self._isinit: self._init()
>> File "...\site-packages\matplotlib\colors.py", line 649, in _init
>> self._segmentdata['red'], self._gamma)
>> TypeError: tuple indices must be integers, not str
>>
>> but other colormaps can actually work:
>> a=cm.get_cmap('gist_stern',256)(range(256))
>> >>> a
>> array([[ 0. , 0. , 0. , 1. ],
>> [ 0.0716923 , 0.00392157, 0.00784314, 1. ],
>> [ 0.14338459, 0.00784314, 0.01568627, 1. ],
>> ...,
>> [ 0.99215686, 0.99215686, 0.97040326, 1. ],
>> [ 0.99607843, 0.99607843, 0.98520163, 1. ],
>> [ 1. , 1. , 1. , 1. ]])
>>
>> 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?
>> ###############################
>> ##matplotlib verison : '1.0.svn'
>> >>> maps=[m for m in cm.datad if not m.endswith("_r")]
>> >>> for i in maps:
>> try:
>> a=cm.get_cmap(i,256)(range(256))
>> except:
>> (type, value, traceback) = sys.exc_info()
>> print "Problems to create %s" % (i,)
>> print "The error was --> %s: %s" % (type, value)
>>
>>
>> Problems to create gist_rainbow
>> The error was --> <type 'exceptions.TypeError'>: tuple indices must be
>> integers, not str
>> Problems to create terrain
>> The error was --> <type 'exceptions.TypeError'>: tuple indices must be
>> integers, not str
>> Problems to create bwr
>> The error was --> <type 'exceptions.TypeError'>: tuple indices must be
>> integers, not str
>> Problems to create brg
>> The error was --> <type 'exceptions.TypeError'>: tuple indices must be
>> integers, not str
>> Problems to create seismic
>> The error was --> <type 'exceptions.TypeError'>: tuple indices must be
>> integers, not str
>> ##################################################################
>>
>
> Is there any particular reason why you are doing the "(range(256))"? Keep
> in mind that a colormap in matplotlib works differently than a colormap in
> Matlab. In Matlab, the colormap is a 2-D array of rgb values, while in
> matplotlib, it is an object that is used by the backends for
> color-rendering.
>
> Often times, you will not need to do anything more than specify which
> colormap you want by name e.g., pcolor(X, Y, Z, cmap='gist_rainbow'), or by
> passing in a customized or self-made colormap object to the 'cmap' keyword
> argument.
>
> What is happening in your code when you call '(range(256))' is that the
> colormap is being called for an array of values ranging from 0 to 255 and
> is determining what the color will be for each of those values. What seems
> to be happening with those few colormaps is that the call is being made
> before those maps are properly self-initialized. So, there might be some
> sort of flaw here that you have exposed, but I would suggest taking another
> look at what you are trying to accomplish to see if there is a better way.
>
> Thanks for giving matplotlib a try and I hope you continue to use it for
> your work!
>
> Ben Root
>
>
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
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. It turns out I do not
need to specify anything here. But if I do not specify anything the
colormap.N is always 256. What will happen then, if I need more color
steps? 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.
LittleBigBrain
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