Fixes palette-based PNG uploads failing original full-size AVIF/WebP conversion under GD

[b1ink0]

Summary

Fixes #2018, #1561, #1864

Relevant technical choices

This change hooks into both wp_handle_upload_prefilter and wp_handle_sideload_prefilter to detect palette-based (indexed-color) PNGs and convert them in-place to truecolor while preserving transparency before any AVIF/WebP encoding takes place. It preserves all existing upload metadata and only affects PNGs handled by the GD image editor.

[codecov]

Codecov Report

❌ Patch coverage is 85.18519% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.61%. Comparing base (0b9d8f0) to head (15fd0ea).
⚠️ Report is 27 commits behind head on trunk.

Files with missing lines	Patch %	Lines
plugins/webp-uploads/hooks.php	85.18%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##            trunk    #2024      +/-   ##
==========================================
+ Coverage   68.54%   68.61%   +0.06%     
==========================================
  Files          91       91              
  Lines        7930     7956      +26     
==========================================
+ Hits         5436     5459      +23     
- Misses       2494     2497       +3     

Flag	Coverage Δ
multisite	68.61% <85.18%> (+0.06%)	⬆️
single	35.68% <0.00%> (-0.12%)	⬇️

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[github-actions]

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Co-authored-by: adamsilverstein <[email protected]>
Co-authored-by: westonruter <[email protected]>
Co-authored-by: felixarntz <[email protected]>
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[felixarntz]

@b1ink0 Thank you, this looks great to me so far, just one comment.

It would be great to get @adamsilverstein's review on this as well.

[adamsilverstein]

Hi @b1ink0 and thanks for the PR.

detect palette-based (indexed-color) PNGs and convert them in-place to truecolor while preserving transparency before any AVIF/WebP encoding takes place

Good catch! I understand the technical requirement for GD here and agree we need to address them to resolve this issue.

My only concern is that by converting a palette image (up to 8 bit) to truecolor (eg. 24 bit), we might inadvertently bloat the image size. We tried to address this for Imagick in https://core.trac.wordpress.org/ticket/36477. I'd love to see some before/after size numbers for indexed PNG uploads converted to WebP and AVIF to confirm that isn't the case. I suspect we are ok given the lossy compression of WebP/AVIF vs the lossless PNG compression, but it would be worth checking and perhaps adding a unit test for this aspect.

[adamsilverstein]

I was able to verify the issue with the Bedrock container and some code to force using GD. I also verified the PR fixes the issue. This should be good to go.

We should probably port this fix to WordPress core since this will also affect anyone using the core output filter directly with the same configuration.

[adamsilverstein]

@b1ink0 - this looks very close, thanks! I left a few additional comments, let me know what you think.

[adamsilverstein]

Looks good, nice work!

[adamsilverstein]

I opened a ticket to move this fix upstream to core since I expect it is a bug there: https://core.trac.wordpress.org/ticket/63773#ticket

[adamsilverstein]

@b1ink0 if you have a moment, can you please review my fix and test for core: WordPress/wordpress-develop#9368. The tests would benefit from being expanded to be more through, I wanted to capture the simplest case first to validate the test fails before and is fixed after the change.

[b1ink0]

@adamsilverstein Yes, the patch looks good and is working all tests pass with your patch. When the logic for imagepalettetotruecolor is commented out, the test fails with the imagewebp(): Palette image not supported by webp message.

I think also adding a test to ensure the size of the converted WebP from truecolor is smaller than the palette PNG would be good, as you suggested in this PR.

Also tested Modern Image Formats version 2.5.1 installed through the WordPress plugin directory with the core patched there is no error when uploading a palette-based PNG.

[westonruter]

Just a couple tiny suggestions.

[westonruter]

Here's a build of the plugin in this branch's current state: webp-uploads.zip

[westonruter]

Fix confirmed: #1576 (comment)