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GeezerVison is showing github.com — Remove fallback code for glyph indices by QuLogic · Pull Request #30168 · matplotlib/matplotlib

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@QuLogic

@QuLogic QuLogic commented Jun 12, 2025

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PR summary

Glyph indices are specific to each font. It does not make sense to fall back based on glyph index to another font.

This could only really be populated by calling FT2Font.set_text, but even that was fragile. If a fallback font was used for a character with the same glyph index as a previous character in the main font, then these lookups could be overwritten to the fallback instead of the main font, with a completely different character!

Fortunately, nothing actually uses or requires a fallback through glyph indices, and this is all internal to the extension.

Note to reviewers: the second commit here is just the last commit from the text-overhaul-images branch, but is not actually relevant here. Please only review the first commit as the second will be dropped before merging. If you prefer, I can target main, but I thought this was relevant to the text-overhaul branch.

PR checklist

  • [n/a] "closes #0000" is in the body of the PR description to link the related issue
  • new and changed code is tested
  • [n/a] Plotting related features are demonstrated in an example
  • [n/a] New Features and API Changes are noted with a directive and release note
  • [n/a] Documentation complies with general and docstring guidelines

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@QuLogic
Remove fallback code for glyph indices
41cc933 
Glyph indices are specific to each font. It does not make sense to fall
back based on glyph index to another font.

This could only really be populated by calling `FT2Font.set_text`, but
even that was fragile. If a fallback font was used for a character with
the same glyph index as a previous character in the main font, then
these lookups could be overwritten to the fallback instead of the main
font, with a completely different character!

Fortunately, nothing actually uses or requires a fallback through glyph
indices.
@QuLogic QuLogic added this to the v3.11.0 milestone Jun 12, 2025
@QuLogic QuLogic added this to Font and text overhaul Jun 12, 2025
@QuLogic QuLogic added the Maintenance label Jun 12, 2025
@github-project-automation github-project-automation Bot moved this to Waiting for other PR in Font and text overhaul Jun 12, 2025
@github-actions github-actions Bot added topic: mplot3d topic: text/fonts labels Jun 12, 2025
@QuLogic QuLogic moved this from Waiting for other PR to Ready for Review in Font and text overhaul Jun 12, 2025
@QuLogic

QuLogic commented Jun 13, 2025 •
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As an example, in DejaVu Sans, the letter A is glyph index 36:

>>> dejavu = FT2Font('lib/matplotlib/mpl-data/fonts/ttf/DejaVuSans.ttf')
>>> dejavu.get_char_index(ord('A'))
36

which has metrics like:

>>> g = dejavu.load_glyph(36)
>>> g.bbox
(6, 0, 519, 576)

In the Last Resort font, the Tibetan block is glyph index 36:

>>> lastresort = FT2Font('lib/matplotlib/mpl-data/fonts/ttf/LastResortHE-Regular.ttf')
>>> lastresort.set_charmap(0)
>>> lastresort.get_char_index(ord('\U00000F00'))
36
>>> g = lastresort.load_glyph(36)
>>> g.bbox
(72, -167, 808, 578)

So if we have a string with both using fallbacks, then the last glyph 'wins':

>>> font = FT2Font('lib/matplotlib/mpl-data/fonts/ttf/DejaVuSans.ttf', _fallback_list=[lastresort])
>>> font.set_text('A\U00000F00')
array([[  0.,   0.],
       [528.,   0.]])
>>> g = font.load_glyph(36)
>>> g.bbox
(72, -167, 808, 578)

If we had multiple fonts in the fallbacks, and multiple characters in those fonts that used the same glyph index, then lookup would depend on the order of characters in the text!
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anntzer
anntzer approved these changes Jun 15, 2025
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@tacaswell

tacaswell commented Jun 19, 2025

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looks good!

Can you please drop the last commit and self-merge?

The windows failures are a know issue with a new numpy release that is fixed on main.
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tacaswell
tacaswell approved these changes Jun 19, 2025
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@QuLogic
QuLogic force-pushed the no-glyph-fallback branch from 1c722e0 to 41cc933 Compare June 19, 2025 18:50
@github-actions github-actions Bot removed the topic: mplot3d label Jun 19, 2025
@QuLogic
QuLogic merged commit 67d1a02 into matplotlib:text-overhaul Jun 19, 2025
@github-project-automation github-project-automation Bot moved this from Ready for Review to Done in Font and text overhaul Jun 19, 2025
@QuLogic

QuLogic commented Jun 19, 2025

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Not going to wait for CI, because it's just going to fail, and then run again when merged, and fail there too...
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@QuLogic
QuLogic deleted the no-glyph-fallback branch June 19, 2025 19:01
wavebyrd pushed a commit to wavebyrd/matplotlib that referenced this pull request Mar 13, 2026
@QuLogic
Merge pull request matplotlib#30168 from QuLogic/no-glyph-fallback
9fe600c 
Remove fallback code for glyph indices
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@tacaswell tacaswell tacaswell approved these changes
@anntzer anntzer anntzer approved these changes

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Maintenance topic: text/fonts

Projects

Font and text overhaul
Status: Done

Milestone

v3.11.0

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3 participants

@QuLogic @tacaswell @anntzer
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