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Improve handling of alpha when saving to jpeg. by anntzer · Pull Request #15437 · matplotlib/matplotlib · GitHub

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Improve handling of alpha when saving to jpeg. - #15437

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QuLogic merged 1 commit into
matplotlib:masterfrom
anntzer:jpegalpha
Mar 24, 2020
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Improve handling of alpha when saving to jpeg.#15437
QuLogic merged 1 commit into
matplotlib:masterfrom
anntzer:jpegalpha

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anntzer commented Oct 18, 2019 •
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We can compose the figure facecolor against a white background before
rendering the image, rather than after. This saves an image composition
step.

PR Summary

PR Checklist

  • Has Pytest style unit tests
  • Code is Flake 8 compliant
  • New features are documented, with examples if plot related
  • Documentation is sphinx and numpydoc compliant
  • Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
  • Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way

timhoffm reviewed Oct 18, 2019
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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

Temporarily changing the figure background and manually calculating the alpha-shading towards white is more low-level. It's harder to understand what's going on and it might be more susceptible of introducing bugs in the future.

Is the performance gain worth it?

Comment thread lib/matplotlib/backends/backend_agg.py Outdated
FigureCanvasAgg.draw(self)
if dryrun:
return
# The image is "pasted" onto a white background image to safely

timhoffm Oct 18, 2019

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

The comment needs to be adapted.

anntzer Oct 18, 2019

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

well, this is still pasting the figure over a white background.

anntzer commented Oct 18, 2019 •
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Actually, another motivation for the change is to make buffer_rgba() return an array that can be directly passed to imsave() -- essentially doing the same thing as in #15435, but for jpeg.
Also, I think using more mpl/numpy APIs and fewer Pillow APIs (Image.paste) is actually clearer, not lower-level.

timhoffm reviewed Oct 18, 2019
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Comment thread lib/matplotlib/backends/backend_agg.py Outdated
over *quality*, *optimize* and *progressive*.
"""
FigureCanvasAgg.draw(self)
# The image is "pasted" onto a white background to handle transparency.

timhoffm Oct 18, 2019

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

I think the term "paste" is a bit misleading

Suggested change
# The image is "pasted" onto a white background to handle transparency.
# Remove transparency by alpha-blending on an assumed white background

anntzer Oct 18, 2019 •
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sure, sounds good, edited accordingly

Improve handling of alpha when saving to jpeg.
757fe20 
We can compose the figure facecolor against a white background before
rendering the image, rather than after.  This saves an image composition
step.

QuLogic commented Mar 23, 2020

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Will this cause weird flickering on interactive views (maybe not regular figures that composite on white, but embedded/notebook/something else)?

anntzer commented Mar 24, 2020

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I'm not sure why it would?

QuLogic commented Mar 24, 2020

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I was thinking of stuff like changing the DPI, but I see setting the canvas manager is handled in the call stack above this one.

QuLogic merged commit bba7763 into matplotlib:master Mar 24, 2020
anntzer deleted the jpegalpha branch March 24, 2020 20:03
QuLogic added this to the v3.3.0 milestone Mar 24, 2020
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