Media: Generate animated image sub-sizes by default.

[adamsilverstein]

Updated to match the change of direction in WordPress/gutenberg#80385 (comment): animated sub-sizes are now the default, and the filter is an opt-out.

Claude rewrote the description to follow along:

Core backport of the server-side changes in WordPress/gutenberg#80385 (design discussion in WordPress/gutenberg#80383).

Resized versions of animated GIFs have always lost their animation - the subject of the 12-year-old Trac #28474, which stalled server-side because GD cannot preserve frames and Imagick is only available on a subset of hosts. The client-side media processing path decodes and re-encodes every frame in the uploading user's browser, independent of host configuration, so sub-sizes can keep their animation.

This adds the wp_generate_animated_image_subsizes filter (boolean, default true) and exposes its value on the REST API index as generate_animated_image_subsizes, inside the existing client-side media processing block in WP_REST_Server::get_index() (alongside image_size_threshold, only for users who can upload_files). By default the client pipeline keeps the animation in every sub-size of an animated GIF or WebP, cropped sizes such as thumbnail included. Returning false generates static first-frame sub-sizes, matching the server-side path.

The field is also added to the block editor root-index preload field lists in wp-admin/edit-form-blocks.php and wp-admin/site-editor.php - these must match packages/core-data/src/entities.js exactly for preloading to work, and the matching entities.js change is part of the Gutenberg PR.

The client-side consumption (editor settings plumbing, @wordpress/upload-media, and the @wordpress/vips worker that loads every frame, crops each frame with one rectangle, and falls back to a static sub-size when an animation is too large to decode) ships via the Gutenberg package updates in WordPress/gutenberg#80385.

Trac ticket: https://core.trac.wordpress.org/ticket/65656
Trac ticket: https://core.trac.wordpress.org/ticket/28474

Proposed commit message

Media: Generate animated image sub-sizes by default.

Fix an issue where sub-sizes of animated image uploads are static first-frame images, as GD and Imagick flatten animated images when resizing. Generated image now retain animations, leveraging `libvips` and client-side media processing.

Add a `wp_generate_animated_image_subsizes` filter (boolean, default true) so the client pipeline can keep or drop the animation in sub-sizes. The client-side consumption ships via the Gutenberg package updates in https://github.com/WordPress/gutenberg/pull/80385.

Fixes #65656, #28474.

AI Use

The code and this description came out of a session with 🤖 Claude Code. I will review and test.

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[adamsilverstein]

Just noting we have decided to land this in 7.2.

[t-hamano]

Pre-approving this prior to WordPress/gutenberg#80385.

Just in case, it might be better to wait to commit this PR until WordPress/gutenberg#80385 is merged and confirmed to be shipped in 7.2. Otherwise, a mismatch may occur between the preload list and entities.js.

[mahdialikhanusiya]

Looks good. Clean opt-in approach for preserving animated GIF sub-sizes via the client-side pipeline.

Happy to test once both sides are ready.

[adamsilverstein]

Looks good. Clean opt-in approach for preserving animated GIF sub-sizes via the client-side pipeline.

Happy to test once both sides are ready.

This is ready for testing @mahdialikhanusiya

[mahdialikhanusiya]

@adamsilverstein Thanks I will test it soon. Hopefully by tomorrow.

[adamsilverstein]

Pre-approving this prior to WordPress/gutenberg#80385.

Just in case, it might be better to wait to commit this PR until WordPress/gutenberg#80385 is merged and confirmed to be shipped in 7.2. Otherwise, a mismatch may occur between the preload list and entities.js.

WordPress/gutenberg#80385 is merged and I plan to merge this PR ahead of beta 1.

[mahdialikhanusiya]

Tested on the Playground.

Uploaded an animated GIF via the block editor.

• Full-size: animates ✅
• Medium (300×169) and thumbnail (150×150): static (first frame only) ❌

Also tested after changing the resolution in the block editor — sub-sizes still remained static.

Looks like the client-side path may not be generating animated sub-sizes in this environment. Let me know if additional testing steps are needed.

[adamsilverstein]

@mahdialikhanusiya thanks for testing!

Claude dug into why the sub-sizes came out static, here is what it found:

That result is expected on this Playground build. This PR is only the server half: it adds the wp_generate_animated_image_subsizes filter and exposes it on the REST index. The part that actually keeps the frames lives in the editor packages from WordPress/gutenberg#80385, which merged on October 16.

wordpress-develop builds the editor from a pinned Gutenberg commit (gutenberg.sha in package.json), currently 954fe7f from September 30, two weeks before that merge. So the Playground for this PR runs the old upload pipeline, which ignores the new field and generates static sub-sizes the same way trunk does.

Animated sub-sizes will show up end to end once the Gutenberg pin in core is bumped past the #80385 merge. Until then, a way to test the full behavior is the Gutenberg plugin from trunk (or a recent nightly) on top of this branch: upload an animated GIF in the block editor, then open the medium and thumbnail files directly, and they should animate.

[mahdialikhanusiya]

@adamsilverstein Thanks for the clear explanation! 🙌