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Issues with `imshow` and RGBA values · Issue #3343 · matplotlib/matplotlib · GitHub

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Issues with imshow and RGBA values #3343

Closed
Closed
Issues with imshow and RGBA values#3343
Labels
backend: aggtopic: color/alpha
Milestone
v2.0.0

Description

tacaswell
opened on Aug 4, 2014

Raised at https://stackoverflow.com/questions/25089068/how-does-imshow-handle-the-alpha-channel-with-an-m-x-n-x-4-input by @mwaskom

import numpy as np
import matplotlib.pyplot as plt

d = np.ones((100, 100, 4), dtype=np.uint8)*255
d[:, :, 3] = np.linspace(0, 255, num=100)
plt.imshow(d, interpolation='none')
plt.show()

One might expect the image to be completely white (as semi-transparent white over white should still be white), but is instead gray in the middle due to pre-multiplying of the alpha (1, 1, 1, .5) -> (.5, .5, .5) which is then composited with the background as an opaque layer, hence the gray.

Previously (related) discussion:

Activity

  1. tacaswell
    added this to the milestone on Aug 4, 2014
  2. tacaswell
    added
    topic: color/alpha
    backend: agg
    on Nov 26, 2014

  3. gcr commented on Mar 13, 2015

    gcr
    on Mar 13, 2015

    This issue also affects me. Looks like matplotlib is doing some double-multiplication of the alpha value.

    Matplotlib's current incorrect behavior is also confusing existing users on stackoverflow, which is in fact how I found this issue: http://stackoverflow.com/questions/25089068/how-does-imshow-handle-the-alpha-channel-with-an-m-x-n-x-4-input

  4. tacaswell
    modified the milestones: , on Mar 13, 2015
  5. tacaswell
    modified the milestones: , on Jul 17, 2015

  6. hitzg commented on Sep 29, 2015

    hitzg
    on Sep 29, 2015

    I believe that this is also related to this issue: http://stackoverflow.com/questions/32828445/matplotlib-colormap-with-transparency-looks-different-in-colorbar-and-imshow

  7. goerz commented on Oct 11, 2015

    goerz
    on Oct 11, 2015

    I'm also running into this. Here's an interactive notebook that should illustrate the issue nicely: http://nbviewer.ipython.org/gist/goerz/d6543e0878c1a10ed0da

  8. QuLogic commented on Sep 30, 2016

    QuLogic
    on Sep 30, 2016
    Member

    @tacaswell This was fixed by #5718 and the one I meant to close instead of #5949.

  9. QuLogic
    closed this as completedon Sep 30, 2016
  10. QuLogic
    modified the milestones: , on Sep 30, 2016

  11. tacaswell commented on Oct 3, 2016

    tacaswell
    on Oct 3, 2016
    MemberAuthor

    @QuLogic Ah, that makes more sense.

    Thanks for all of your work keeping the issues tidy!

  12. thomasmansencal commented on Nov 28, 2017

    thomasmansencal
    on Nov 28, 2017
    Contributor

    I that issue fixed? I'm running 2.1.0 and I still see the same issue:

    What I get using imshow:

    What I would expect (composited in The Foundry Nuke):

    Original image is here: https://drive.google.com/file/d/1S09dwNcsO5sWvfBB2mMs29O0ZSAcM1pk/view?usp=sharing

  13. tacaswell commented on Dec 1, 2017

    tacaswell
    on Dec 1, 2017
    MemberAuthor

    @kelsolaar Can you open a new issue for this with a minimal example?

  14. thomasmansencal commented on Dec 1, 2017

    thomasmansencal
    on Dec 1, 2017
    Contributor

    @tacaswell : I will!

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