[Matplotlib-users] Confused about rgb_to_hsv and hsv_to_rgb

[Matteo N. <ma...@my...>]

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?

Thanks
Matteo