Fixes palette-based PNG uploads failing original full-size AVIF/WebP conversion under GD #2024

b1ink0 · 2025-05-27T12:23:03Z

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 · 2025-05-27T12:32:11Z

Codecov Report

Attention: Patch coverage is 88.46154% with 3 lines in your changes missing coverage. Please review.

Project coverage is 67.25%. Comparing base (e147007) to head (37b893b).

Files with missing lines	Patch %	Lines
plugins/webp-uploads/hooks.php	88.46%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##            trunk    #2024      +/-   ##
==========================================
+ Coverage   67.18%   67.25%   +0.07%     
==========================================
  Files          93       93              
  Lines        7750     7776      +26     
==========================================
+ Hits         5207     5230      +23     
- Misses       2543     2546       +3

Flag	Coverage Δ
multisite	`67.25% <88.46%> (+0.07%)`	⬆️
single	`36.49% <0.00%> (-0.13%)`	⬇️

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…te PNGs to truecolor

github-actions · 2025-05-30T12:39:08Z

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Co-authored-by: b1ink0 <[email protected]>
Co-authored-by: adamsilverstein <[email protected]>
Co-authored-by: felixarntz <[email protected]>
Co-authored-by: westonruter <[email protected]>
Co-authored-by: swissspidy <[email protected]>
Co-authored-by: mukeshpanchal27 <[email protected]>
Co-authored-by: benniledl <[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.

plugins/webp-uploads/hooks.php

… hooks

plugins/webp-uploads/hooks.php

westonruter · 2025-06-04T21:23:59Z

plugins/webp-uploads/hooks.php

+		return $file;
+	}
+
+	if ( 'png' !== strtolower( pathinfo( $file['name'], PATHINFO_EXTENSION ) ) ) {


Should this look at the attachment metadata instead to see if the MIME type is image/png?

Yes, we can just check if $file['type'] is image/png.

Done in 7443e68.

Co-authored-by: Weston Ruter <[email protected]>

adamsilverstein · 2025-06-10T12:08:28Z

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.

plugins/webp-uploads/hooks.php

adamsilverstein · 2025-06-10T12:16:42Z

plugins/webp-uploads/hooks.php

+
+	// Check if required GD functions exist.
+	if (
+		! function_exists( 'imagecreatefrompng' ) ||


+1 to defensive programming; however all of these functions have been around since PHP4/5 as as far as I know should be available to all WordPress installs with GD. So maybe we can remove this check?

That would address the code coverage issue as well, if it is impossible to create a scenario where any of these functions don't exist.

Oh, but actually will these functions be available if GD is not installed? If PHP is not compiled with --with-gd then I don't think they will.

If PHP is not compiled with GD, then these functions will not be available. I compiled PHP with and without GD and tested it; the version without GD did not have these functions.

If PHP is not compiled with GD,

Just to clarify for my own understanding - is GD bundled by default and would need to be disabled for the build?

I still think we can remove these - if GD is the chosen image editor we can be confident PHP supports GD. See https://github.com/WordPress/performance/pull/2024/files#r2217413978

If PHP is not compiled with GD,

Just to clarify for my own understanding - is GD bundled by default and would need to be disabled for the build?

GD is bundled with PHP source code by default, but it must be explicitly enabled during compilation and can be omitted from the final build.

https://www.php.net/manual/en/image.installation.php

I still think we can remove these - if GD is the chosen image editor we can be confident PHP supports GD. See https://github.com/WordPress/performance/pull/2024/files#r2217413978

You're right, I'll remove these redundant function checks.

Done in ca8599c.

adamsilverstein · 2025-06-10T12:21:14Z

plugins/webp-uploads/hooks.php

+	}
+
+	$image = imagecreatefrompng( $file['tmp_name'] );
+	if ( false === $image || imageistruecolor( $image ) ) {


I'm confused by this check. If $image is false, the conversion failed. In that case, don't we want to continue to the processing below?

When imagecreatefrompng() returns false, it means the creation of the PNG failed. In that case, we can't convert it to truecolor because we need a valid GDImage resource for imagepalettetotruecolor() to work properly. Returning early in this scenario is the best choice since there's no valid image resource to operate on.

I also combined two different checks in that condition, which I realize makes the logic less clear. Separate those out in 7f757c7 to improve readability.

thanks for clarifying

plugins/webp-uploads/hooks.php

plugins/webp-uploads/tests/test-load.php

Co-authored-by: Adam Silverstein <[email protected]>

…and teardown parameters

b1ink0 · 2025-06-17T09:53:26Z

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.

I ran some tests on a few indexed PNGs. As expected, converting them to truecolor PNGs increased the file size up to 2.5x in some cases. However, when these converted truecolor PNGs were then encoded to AVIF or WebP, the resulting file sizes were consistently smaller than the original indexed PNGs encoded directly to AVIF/WebP.

Indexed PNG Size	Truecolor PNG Size	AVIF Size (from Truecolor PNG)	WebP Size (from Truecolor PNG)
7 KB	15 KB	3 KB	2 KB
65 KB	116 KB	4 KB	3 KB
41 KB	87 KB	26 KB	16 KB
1.4 MB	3.8 MB	674 KB	628 KB
177 KB	426 KB	149 KB	107 KB

cc: @adamsilverstein

adamsilverstein · 2025-07-19T01:21:40Z

Thanks for the PR and all the work here @b1ink0. I will try testing with the Bedrock container you linked to earlier.

adamsilverstein · 2025-07-19T16:35:16Z

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.

plugins/webp-uploads/hooks.php

adamsilverstein · 2025-07-19T18:39:59Z

plugins/webp-uploads/hooks.php

+		return $file;
+	}
+	if ( isset( $file['type'] ) && is_string( $file['type'] ) ) {
+		if ( 'image/png' !== strtolower( $file['type'] ) ) {


do we need this check? it seems duplicated by the following check with wp_check_filetype

The checks aren't duplicated they handle different scenarios. I added the first check for $file['type'] per this #2024 (comment) feedback to use the MIME type from attachment metadata (this is available when the file is uploaded and present in $_FILES for ref.) instead of just relying on the file extension. The elseif ensures we fall back to wp_check_filetype() when $file['type'] isn't available.

Ok, makes sense. We also have wp_check_filetype_and_ext in core that might be useful here.

Done in e39b2c7.

plugins/webp-uploads/hooks.php

adamsilverstein · 2025-07-19T18:54:22Z

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

Co-authored-by: Adam Silverstein <[email protected]>

Add function to convert palette PNGs to true color during upload

4d54810

b1ink0 added this to the webp-uploads n.e.x.t milestone May 27, 2025

b1ink0 added [Type] Bug An existing feature is broken [Plugin] Modern Image Formats Issues for the Modern Image Formats plugin (formerly WebP Uploads) labels May 27, 2025

This was linked to issues May 27, 2025

Fatal error: Paletter image not supported by webp #1561

Open

Transparent .pngs with colour type Palette cannot be converted #1864

Open

b1ink0 added 2 commits May 27, 2025 18:29

Refactor to use imagepalettetotruecolor() to convert uploaded palet…

82cf154

…te PNGs to truecolor

Add test for converting palette PNG to truecolor

cf97730

b1ink0 marked this pull request as ready for review May 30, 2025 12:38

b1ink0 requested review from adamsilverstein, felixarntz and westonruter as code owners May 30, 2025 12:38

felixarntz reviewed May 30, 2025

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b1ink0 added 2 commits May 30, 2025 23:35

Add hooks for palette PNG to truecolor conversion and tests for those…

8ad675c

… hooks

Merge branch 'trunk' into fix/palette-pngs-to-avif-webp-conversion

85482bc

westonruter reviewed Jun 4, 2025

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b1ink0 and others added 2 commits June 6, 2025 00:25

Add type safety guard for palette PNG conversion filter

ef9a9f9

Co-authored-by: Weston Ruter <[email protected]>

Improve PNG type checking

7443e68

adamsilverstein reviewed Jun 10, 2025

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Fix typo in function description "true color" to "truecolor"

731d3b0

Co-authored-by: Adam Silverstein <[email protected]>

b1ink0 added 2 commits June 11, 2025 16:20

Separate image creation failure and truecolor checks for clarity

7f757c7

Refactor test for palette PNG conversion to remove unnecessary setup …

e646941

…and teardown parameters

Add test to assert converted image filesize does not exceed original

56e8dac

b1ink0 requested review from westonruter, adamsilverstein and felixarntz June 17, 2025 10:21

b1ink0 added 2 commits June 24, 2025 10:13

Remove unused import

9ff40f2

Improve test code coverage

d930ea9

This was referenced Jul 19, 2025

Transparent .pngs with colour type Palette cannot be converted #1864

Open

Fatal error: Paletter image not supported by webp #1561

Open

Merge branch 'trunk' into fix/palette-pngs-to-avif-webp-conversion

2b1d612

adamsilverstein reviewed Jul 19, 2025

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adamsilverstein reviewed Jul 19, 2025

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adamsilverstein mentioned this pull request Jul 20, 2025

Transparent PNG's loses transparency with the Modern Image Formats #1576

Open

b1ink0 and others added 5 commits July 22, 2025 00:36

Remove redundant GD function checks

ca8599c

Return non-array file parameter unchanged

8244cae

Co-authored-by: Adam Silverstein <[email protected]>

Fix PHPStan and test failures caused by returning a mixed value

62f767c

Improve file type check by using wp_check_filetype_and_ext

e39b2c7

Merge branch 'trunk' into fix/palette-pngs-to-avif-webp-conversion

37b893b

b1ink0 requested a review from adamsilverstein July 24, 2025 07:12

Fixes palette-based PNG uploads failing original full-size AVIF/WebP conversion under GD #2024

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Fixes palette-based PNG uploads failing original full-size AVIF/WebP conversion under GD #2024

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