HEIC / HEIF

HEIC is the default photo format on iPhones since iOS 11 (2017). It’s smaller than JPEG at equivalent quality, supports 10-bit colour and HDR, and is built on the same ISO Base Media File Format (ISOBMFF) container as MP4 video — which means the same box-walking parser handles both.

If you’ve ever AirDropped a photo and seen .heic arrive, that’s this format.

What it carries

HEIC’s metadata model is fundamentally different from JPEG’s. Instead of APP segments in a linear byte stream, HEIC has items, boxes, and references:

• meta box — the metadata container. Holds an iinf (item info) declaring what items exist, an iloc (item locations) pointing at byte offsets, and the actual item data.
• Exif item — the EXIF blob, referenced by an iinf entry with type Exif. Same TIFF wire format as JPEG.
• mime item — XMP packets and other typed payloads.
• Apple AssetIdentifier — buried inside an EXIF MakerNote tag (17). The UUID that pairs this HEIC with its companion MOV in a Live Photos pair.
• iref / iloc — boxes that describe relationships between items. The thumbnail item is referenced via thmb; the main image via pitm (primary item).
• HDR metadata — mdcv (mastering display colour volume), clli (content light level info). Decoder hints for HDR rendering.

What’s unique

The Live Photos cross-asset identifier is HEIC’s most privacy-relevant payload. When your iPhone takes a Live Photo it writes two files — a .heic still and a .mov 3-second video — and stamps both with the same identifier. Anyone with both files can prove they’re a pair, even after renames. See the MOV page for how the same identifier appears on the video side via Apple’s com.apple.quicktime.content.identifier ilst key.

HEIC’s HEVC encoding is patent-encumbered, which is why most web browsers don’t decode it natively. Tracemute doesn’t decode HEIC either — it walks the box tree, rewrites metadata items, and writes a valid HEIC file out. No libheif WASM.

What Tracemute does

The handler is native ISOBMFF box surgery:

1. Walk the box tree (ftyp → meta → iinf / iloc / item data).
2. For each metadata item (Exif, XMP, etc.), rewrite the corresponding iinf entry and zero the item payload.
3. Recompute box sizes upward — touching meta, then the top-level container.
4. The image-data items (the actual HEVC slices in mdat) are byte-identical to the input.

What survives

HDR metadata (mdcv, clli) is kept — it’s required for correct display on HDR-capable monitors. Stripping it would degrade the visual result, which is exactly the kind of trade Tracemute avoids. The image’s HEVC payload is untouched.