Formats

Reading and writing

This site reads more formats than it writes. Reading means decoding something that already exists, which is a solved problem for almost any format ever shipped. Writing means having an encoder for it, and for a few formats no browser-side encoder exists at all.

Each format page below says which side of that line it falls on, along with what the format is good at, where it falls short, and the specific thing that usually goes wrong with it.

The formats worth knowing

• PNG keeps every pixel exactly and handles transparency.
• JPG makes photographs small, at the cost of some detail.
• WEBP and AVIF do both jobs better, on newer software.
• MP4 is the video container that plays absolutely everywhere.
• MP3 is the audio format nothing refuses.
• FLAC compresses audio without discarding a single sample.

Questions

What is a file format?

A format is an agreement about how information is arranged inside a file. The same photograph stored as PNG and as JPG contains the same picture written down two different ways, one of which is exact and large and the other approximate and small.

What is the difference between a codec and a container?

A container is the box; a codec is how the contents were compressed. MP4 is a container that usually holds H.264 video and AAC audio. This is why two files with the same extension can behave completely differently, and why one MP4 plays where another does not.

What does lossless mean?

A lossless format stores the original exactly, so decoding it gives back precisely what went in. A lossy format throws some of it away to save space, and that part cannot be recovered. PNG and FLAC are lossless; JPG and MP3 are not.

Why will a site not accept my file?

Usually because the bytes do not match the extension. Renaming a WEBP to .jpg changes nothing inside the file, and upload validators read the bytes. Converting it properly is what fixes it.