.AVIF

Introduction

AVIF is an image format from the Alliance for Open Media, published in 2019. It stores images compressed with AV1, a royalty-free video codec, inside the same HEIF container that HEIC uses.

It supports lossy and lossless compression, alpha transparency, high bit depths, wide colour and HDR, and animated image sequences. All the major web browsers can display it, and it is used on the web as a smaller alternative to JPEG and WebP.

Structure

An AVIF file is a HEIF file made of ISO base media boxes, each a 4-byte big-endian size, a 4-byte type and its contents. It starts with an ftyp box (the text ftyp at offset 4) whose brand is avif for still images or avis for image sequences.

A meta box lists the image items and their properties, including an av1C box with the AV1 codec settings. Transparency is a separate greyscale image linked to the main one as an auxiliary item. The coded AV1 data sits in an mdat box.

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