Re: [Matplotlib-users] Confused about rgb_to_hsv and hsv_to_rgb

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

On 2015/05/21 5:50 AM, Matteo Niccoli wrote:
> I posted a question on stackoverflow about creating with making my own
> shading effect (I want to use horizontal gradient for the shading).
> http://stackoverflow.com/questions/30310002/issue-creating-map-shading-in-matplotlib-imshow-by-setting-opacity-to-data-gradi
>
>
> Unfortunately I cannot share the data because I am using it for a
> manuscripts, but my notebook with full code listing and plots, here:
> http://nbviewer.ipython.org/urls/dl.dropbox.com/s/2pfhla9rn66lsbv/surface_shading.ipynb/%3Fdl%3D0
>
> The shading using gradient is implemented in two ways as suggested in the
> answer. What I do not understand is why the last plot comes out with a
> rainbow-like colors, when I did specify cubehelix as colormap.
>
> hsv = cl.rgb_to_hsv(img_array[:, :, :3])
> hsv[:, :, 2] = tdx_n
> rgb = cl.hsv_to_rgb(hsv)
> plt.imshow(rgb[4:-3,4:-3], cmap='cubehelix')
> plt.show()
>
>
> Am I doing something wrong or is this unexpected behavior; is there a
> workaround?

Colormapping occurs only when you give imshow a 2-D array of numbers to 
be mapped; when you feed it a 3-D array of RGB values, it simply shows 
those colors.  For colormapping to occur, it must be done on a 2-D array 
as a step leading up to the generation of your img_array.

Eric

>
> Thanks
> Matteo
>