Explained
What AVIF actually is
AVIF (AV1 Image File Format) stores a still picture — or a short sequence — inside a HEIF-style container, compressed with the AV1 video codec. It is built for the web: small files, sharp edges, and optional transparency or HDR.
The container and the codec
An .avif file is an ISO BMFF (the same family as MP4). The brand in the ftyp box is usually avif or avis. Inside, AV1 encodes the pixels. That is why a file can look like “video plumbing” if you inspect the first bytes, and why AvifUndo sniffs forftyp + avif instead of trusting the extension alone.
AV1 is a royalty-free codec from the Alliance for Open Media. AVIF is the still-image profile of that work — the counterpart to how HEIC wraps HEVC, or how WebP wrapped VP8/VP8L.
What AVIF is good at
- Smaller than JPEG at a similar visual quality, often by a wide margin on screenshots and illustrations.
- Alpha — unlike JPEG, AVIF can keep transparency.
- Wide color and HDR — when the encoder and the display pipeline support it.
- Film grain and textures — AV1 handles noisy photos without the blocky JPEG look at low bitrates.
What AVIF is bad at
- Everywhere-support. Email clients, printers, Word, older Photoshop, and many CMSs still want JPG or PNG.
- Encode time. Creating an AVIF can be slower than JPEG. Decode is usually fine on a modern phone.
- Tooling. Some desktop viewers show a blank frame or a generic icon. That is a viewer gap, not a corrupt file.
- Editing. Layers, EXIF workflows, and print RIPs are still JPEG/TIFF country.
Browser decode — the line AvifUndo cares about
AvifUndo never re-encodes AVIF on a server. It asks this browser to decode the file. IfcreateImageBitmap (or an Image element) cannot produce pixels, conversion stops and you see an error.
Practical decode support:
- Chrome 85+ and other Chromium browsers
- Firefox 93+
- Safari 16+ (macOS Ventura, iOS 16)
- Edge 121+ (Chromium Edge with AVIF enabled)
Very old phones, locked-down enterprise browsers, and some in-app WebViews still fail. In those cases the honest answer is “this tab cannot decode AVIF” — not a silent empty JPG.
When to convert AVIF to JPG
Keep AVIF for your own site if you can serve it with a JPEG fallback. Convert to JPG when thenext tool in the chain cannot open AVIF: a client’s email, a print shop, a government upload form, Slack on an old desktop, or a CMS media library that only indexes JPEG/PNG.
Use AvifUndo for that hop. Choose JPG quality 92 unless you need a smaller attachment. Choose PNG if the AVIF has transparency you must keep.
AVIF is not HEIC
iPhone photos saved as HEIC are HEVC in a HEIF container. The extension might even look similar, and both start with an ftyp box — but the brand is heic, not avif. AvifUndo will refuse those files on purpose and tell you so. Convert HEIC with a tool that understands HEVC, or export JPEG from the Photos app.