Media: Add HEIC canvas fallback for client-side processing#76731
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adamsilverstein wants to merge 9 commits intotrunkfrom
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Media: Add HEIC canvas fallback for client-side processing#76731adamsilverstein wants to merge 9 commits intotrunkfrom
adamsilverstein wants to merge 9 commits intotrunkfrom
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When users upload HEIC images, try server processing first. If the server lacks HEIC support, fall back to the browser's native createImageBitmap() + OffscreenCanvas to decode and convert to JPEG. The original HEIC is sideloaded as the original, the JPEG as the scaled version, and sub-sizes are generated from the JPEG via the existing vips resize pipeline. PHP: bypass MIME type check for HEIC/HEIF uploads so files can be stored even when the server can't process them. JS: add HEIC detection in prepareItem(), canvas conversion utility, and HEIC-aware generateThumbnails() fallback.
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Size Change: +3.18 kB (+0.04%) Total Size: 7.66 MB
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adamsilverstein
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[Type] Enhancement
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Flaky tests detected in 781b9f5. 🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/23373218279
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Links core PR wordpress-develop#11323 to Gutenberg PR #76731.
Three issues prevented the HEIC canvas fallback from working: 1. generate_sub_sizes was set to true for HEIC uploads, causing the server to attempt (and silently fail) sub-size generation instead of letting the client handle it. 2. The server can't read HEIC files, so it always returned an empty missing_image_sizes array. The client now derives missing sizes from registered image sizes in settings. 3. createImageBitmap() doesn't support HEIC in Chrome. Added a fallback using <img> element + HTMLCanvasElement, which works on macOS because Chrome exposes OS-level HEIC decoding through the img rendering pipeline.
Chrome doesn't expose macOS's native HEIC decoder through image APIs (createImageBitmap, ImageDecoder, <img>). However, Chrome 107+ supports HEVC video decoding via the WebCodecs VideoDecoder API using macOS VideoToolbox. This adds a new decoding strategy that bridges the gap: 1. Parse the HEIC/ISOBMFF container in pure JS to extract the HEVC decoder configuration (hvcC) and compressed image data 2. Feed the HEVC frames through VideoDecoder (hardware-accelerated) 3. Composite decoded tiles onto a canvas and convert to JPEG Handles both single-image and grid/tiled HEIC files (the common iPhone format where photos are split into multiple HEVC tiles). No WASM decoder or patented codec is shipped — only the standard container format is parsed, and decoding uses Chrome's already-licensed platform decoder. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
- Register HEIC/HEIF as allowed upload MIME types so macOS file picker doesn't silently convert them to JPEG - Handle idat construction method in HEIC container parser for grid descriptor data stored inside the meta box - Relax grid tile count validation to allow extra iref entries (alpha planes, thumbnails) - Disable server-side sub-size generation and format conversion for HEIC uploads (client handles these) - Add debug logging throughout HEIC upload pipeline
The initial upload already saves the HEIC file on the server. The scaled sideload handler records it as original_image in metadata, so a separate original sideload is unnecessary and creates a duplicate file.
Don't update blocks with HEIC attachment URLs since browsers can't display them. The scaled JPEG sideload will provide a usable URL once client-side conversion completes. Also removes all HEIC debug logging added during development.
Tests cover: - Bit reversal for HEVC codec string construction - Single-image HEIC parsing with synthetic binary fixtures - Codec string generation (e.g. hvc1.1.6.L93.B0) - HEVCDecoderConfigurationRecord extraction - Error cases: empty buffer, missing meta/pitm/iloc boxes
Validate grid descriptor data length before accessing fields to avoid out-of-bounds reads on malformed files.
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Summary
original, the JPEG becomes thescaledversion, and sub-sizes are generated from the JPEG via the existing vips resize pipelineNo HEIC decoder or patented codec is shipped — only standard container parsing is done client-side, and decoding uses the browser's already-licensed platform decoders.
Client-side decoding strategies
The canvas conversion tries three strategies in order:
createImageBitmap()+OffscreenCanvas— Works natively in Safari (all platforms). The simplest path.ImageDecoderAPI — Uses platform codecs exposed via the WebCodecs image pipeline. May work in future Chrome versions if HEIC is added.VideoDecoder— For Chrome 107+ on macOS. Parses the HEIC/ISOBMFF container in pure JS to extract the HEVC bitstream and decoder configuration (hvcC), then feeds the HEVC frames through Chrome's hardware-acceleratedVideoDecoder(backed by macOS VideoToolbox). Handles both single-image and grid/tiled HEIC files (the common iPhone format where photos are split into multiple HEVC tiles).Upload flow
Server has HEIC support: Upload → server processes everything → done.
Server lacks HEIC support, browser can decode: Upload → sideload original HEIC → canvas converts HEIC → JPEG (using strategies above) → sideload JPEG as scaled → generate sub-sizes from JPEG via vips → sideload each → finalize.
Neither supports HEIC: Upload → all client-side decode strategies fail → error shown to user.
Browser support matrix
createImageBitmapScreencast
screencast.2026-03-20.22-50-56.mp4
Test instructions