Convert M4V to AVIF

AVIF is not a new image codec; it is an AV1 keyframe wrapped in a container. When the source video is itself AV1 the round trip is unusually kind, because the artefacts of the first pass are the shapes the second pass describes efficiently. That is a real effect and it is not available here.

An M4V holds H.264 or HEVC, both from the MPEG side of the fence, so the frame crosses families on its way out. The decode to pixels in between means this costs less than it sounds — the encoder receives an image, not a representation it has to reinterpret — but no argument about lineage applies, and the picture is judged purely as a picture.

For an HEVC recording the natural still format is HEIC: the same codec, in the same container family, which is exactly why Apple devices produce it. Converting an HEVC frame to HEIC would be the family conversion AVIF is not.

It is not on offer, and the reason is a licence rather than a limitation. Writing HEIC needs an HEVC encoder, the common one is published under terms this project’s dependency policy rules out, and shipping it anyway is not an option that exists. AVIF was designed by the Alliance for Open Media to do the same job without that problem, which is how an open format ends up being the answer to a question about an Apple one.

The registry records AVIF as supporting twelve bits and wide colour, and that is true of the format. It is not true of this path. The frame is read out of the decoder through a canvas, which hands over eight bits per channel in ordinary sRGB, and the encoder receives exactly that.

The consequence matters on modern Apple footage specifically, because recent iPhones and Final Cut exports carry HDR as a matter of course. That grade does not survive: highlights are compressed into the eight-bit range and the result looks like the same shot handled by an ordinary camera. For a web page that is the right outcome, since almost nothing downstream would have used the extra depth. For anything where the grade is the work, this is not the tool.

The encoding cost is charged per image and it is not small. AV1 earns its efficiency through an exhaustive search over ways to describe each block, and on a full-resolution frame from a 4K recording that search takes real seconds on a good machine and rather more on a modest one.

That fixes what this conversion is for. One large picture at the top of a page, served a great many times, is worth the wait many times over. A catalogue of course thumbnails is not — the saving per image is small at thumbnail sizes and the waiting is multiplied by the number of videos. Use WebP for the grid and AVIF for the header, which is the split most performance work arrives at anyway.

The timestamp accepts up to 86,400 seconds, so any moment in a long recording is reachable by typing the number of seconds. What the number should be is decided by the video encoder rather than by you: a held shot, a slide or a speaker between gestures was expensive to encode and comes out detailed, while a pan or a cut was cheap and comes out soft.

On this pair that choice outweighs the format by a wide margin. A still taken at a still moment and saved as JPG beats one taken mid-motion and saved as AVIF on every measure including size, because a smeared frame is full of detail an encoder has to spend bits describing. Moving the timestamp half a second is free; a better codec is not.

Serve it from a `<picture>` element with a WebP source behind it and a JPG as the final `<img>`. Three lines, no JavaScript, and the browser takes the first thing it understands — which matters because AVIF support arrived years after WebP and devices that no longer receive updates persist.

Check the other end of the pipeline too. A number of content management systems and static-site build steps still refuse `.avif` on upload or fail to generate derivatives from it, which is a more common obstacle in practice than a visitor without a decoder. Finding that out before you encode thirty images is worth two minutes.

Anything that is not a web page. Email clients do not render it, office software largely does not, print workflows do not, and a colleague sent one will usually reply asking what it is. Upload forms on other people’s sites reject the extension often enough to be a planning consideration rather than an edge case.

The library case is worth naming separately, because it is close by and different. Artwork for a Plex, Jellyfin or Emby collection goes to a server that expects JPG or PNG and matches it by file name, and the JPG route on this site is written for that job. AVIF outside a browser produces a smaller file nobody can open, which is not an optimisation.

Films and episodes bought or rented through the iTunes Store or the Apple TV app carry FairPlay encryption. Nothing in a browser can decode them, so there is no frame to encode and the conversion reports that nothing in the file could be converted before it starts.

Everything else is fine: QuickTime, iMovie, Final Cut and Compressor exports, camera recordings, conference and course downloads. If a batch works apart from a handful, the handful were purchases rather than damaged files.

AVIF encoding is demanding enough that services offering it tend to queue it, cap it or charge for it. Here it runs in the tab on your own processor, so there is no queue to wait in and no daily allowance to spend — a drop takes up to 100 files at once, and the only cost is your own time, which you spent knowingly when you decided the trade was worth making.

It also means the video stays where it is. The free tier accepts input files up to 100 MB, and nothing carrying the recording is transmitted at any point, which on this pair is as much about proportion as about privacy: uploading a large video to receive a forty-kilobyte image is a strange amount of network for the result.