AVIF vs AVIS Brand: Key Differences Explained

The AV1 Image File Format (AVIF) uses structural identifiers called "brands" within its file header to tell decoders how to interpret image data. While the standard avif brand designates still images and item collections, the avis brand specifically identifies AV1 image sequences, such as animations. This article explains the technical purpose of the avis brand, how it functions within the container, and the precise conditions under which it is required instead of or alongside the standard avif brand.

What Are Brands in AVIF?

AVIF is built on top of the ISO Base Media File Format (ISOBMFF). Every ISOBMFF-compliant file begins with a File Type Box (ftyp), which declares a "major brand" and a list of "compatible brands." These four-character codes (FourCCs) act as compatibility flags. They inform software parsers about the exact specifications, profiles, and playback capabilities required to render the file correctly without needing to inspect the entire data stream first.

The Standard avif Brand

The standard avif brand signifies that the file contains AV1-encoded still image items. It indicates:

• The primary content consists of standalone image items.
• The file structure relies on ISOBMFF item properties and metadata (such as EXIF, XMP, and color profiles).
• The parser must be capable of decoding still frames according to the AVIF Still Image specification.

Single photos, static graphics, and multi-layer still compositions use the avif brand to ensure proper decoding across standard image viewers and web browsers.

The avis Brand

The avis brand stands for AV1 Image Sequence. Rather than storing static items, an image sequence stores temporal frames packaged as a timed media track, functionally similar to an animated GIF or animated WebP, but encoded with AV1 compression.

When an image sequence is present, the file structure shifts from isolated still image items to track-based video structures, utilizing movie boxes (moov) and track boxes (trak) to coordinate timing, looping, and synchronization.

When Is the avis Brand Required?

The avis brand is required whenever an AVIF file contains an AV1 image sequence. Specifically:

1. Animated AVIF Files: If the file contains timed, sequential frames intended to be played as an animation or a looping graphic, the file must include avis in the compatible brands list of the ftyp box.
2. Track-Based Content: Decoders optimized strictly for still images look for avif and may fail or reject files that rely solely on track structures. Declaring avis signals to decoders that the file requires media-track parsing capabilities, frame timing, and sequence decoding.
3. Specifying the Major Brand for Animations: If the file's primary presentation is an animated sequence, avis is typically set as the major brand to ensure sequence-aware renderers prioritize the temporal track.

Dual Branding: Using Both avif and avis

In many practical implementations, creators want an animated image to have a graceful fallback for parsers that only support still images.

Under the AVIF specification, a file can contain both a primary still image item and a sequence track. In such cases, both avif and avis should be listed under the compatible_brands array. This informs the reader that:

• A player capable of handling sequences can play the animation denoted by avis.
• A legacy or still-only decoder can fall back to displaying the key still frame item marked by avif.

If an animated file contains no still-item fallback and only includes temporal track data, omitting avis violates the specification and causes decoders to fail. Therefore, the avis brand is mandatory whenever animated sequence playback is intended.