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Media: Convert palette images to true color before saving as WebP or AVIF - #15908

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Media: Convert palette images to true color before saving as WebP or AVIF#15908
JDeepD wants to merge 1 commit into
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JDeepD:fix-59339/gd-palette-webp-avif

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@JDeepD

@JDeepD JDeepD commented Nov 11, 2026

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When GD is the image editor and palette (indexed colour) images are converted to WebP or AVIF, for example a PNG-8 with 'image/png' => 'image/webp' set through image_editor_output_format, the full-size image is saved as a 0-byte file and PHP logs imagewebp(): Palette image not supported by webp. GD's WebP and AVIF encoders only accept truecolor images. Sub-sizes are not affected, because _resize() creates them with wp_imagecreatetruecolor().

This PR converts palette images to truecolor with imagepalettetotruecolor() right before encoding, when the output format is WebP or AVIF:

  • WP_Image_Editor_GD::_save(): covers uploads, sub-size regeneration, and saves after editing.
  • WP_Image_Editor_GD::stream(): covers the image editor preview, which otherwise returns true with empty output.

The conversion is lossless and keeps transparency. Truecolor images, other output formats, and the Imagick editor are unchanged.

Trac ticket: https://core.trac.wordpress.org/ticket/59339

Use of AI Tools

AI assistance: Yes
Tool(s): Claude Code
Model(s): Claude Opus 5.5
Used for: Reproducing the issue, drafting the fix and the unit tests; the final implementation and tests were reviewed and tested by me.

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