Glossary of HEIC and image format terms
Short, precise definitions of the terms used across this site, in the sense they actually carry when you are dealing with iPhone photos.
Terms are defined here in the sense they carry in practice, not the broadest possible sense. Where a distinction causes real confusion, the definition says so.
- HEIC
- High Efficiency Image Container. Apple's file extension for a HEIF file whose image is compressed with HEVC. Default photo format on iPhone and iPad since iOS 11.
- HEIF
- High Efficiency Image File Format, ISO/IEC 23008-12. The container standard itself, which can hold images compressed by several different codecs.
- HEVC
- High Efficiency Video Coding, also called H.265. The codec that compresses the image inside a HEIC file. Patent-licensed, which is why support is uneven.
- HIF
- Canon's file extension for HEIF images from EOS R series cameras. The same container as HEIC, usually holding 10-bit data.
- AVIF
- AV1 Image File Format. Uses the HEIF container design with the royalty-free AV1 codec instead of HEVC, which is why browsers support it and HEIC never got support.
- JPEG
- The 1992 image standard behind .jpg and .jpeg files. Lossy, 8-bit, no transparency, and readable by effectively every device made since.
- Lossy compression
- Compression that permanently discards image information judged least noticeable. JPEG, HEIC, WebP and AVIF are all lossy by default.
- Lossless compression
- Compression that reproduces the original pixels exactly. PNG is always lossless; WebP and HEIC each have an optional lossless mode.
- EXIF
- Exchangeable Image File Format. The metadata block carrying camera model, exposure settings, capture time and, often, GPS coordinates.
- Display P3
- A wide colour space used by recent iPhones, roughly 25% larger than sRGB. Converting to sRGB without translating the values is what makes photos look flat.
- sRGB
- The standard colour space of the web. The safe target for anything shared, uploaded or printed.
- Alpha channel
- Per-pixel transparency data. Supported by PNG, WebP, AVIF and HEIC, but not by JPEG, which fills transparent areas with white.
- Orientation flag
- An EXIF value from 1 to 8 telling software how to rotate a photo before display. Ignoring it is why converted photos come out sideways.
- Live Photo
- An iPhone capture stored as two files: a HEIC still and a short MOV video. Image converters only ever see the still.
- Depth map
- An auxiliary image recording distance from the lens, stored alongside the photo in a HEIC container and used for Portrait mode blur.
- libheif
- The open-source reference decoder for HEIF and HEIC. Compiled to WebAssembly, it is what lets a browser read HEIC without any system codec.
- WebAssembly
- A binary format that runs compiled code at near-native speed inside a browser tab. It is what makes local, upload-free conversion possible.
- HEVC Video Extensions
- The Microsoft Store package that lets Windows decode HEIC. Paid, though a free device-manufacturer version exists for many PCs.
- Container format
- A file structure that holds one or more compressed images plus metadata. HEIF is a container; HEVC is the codec used inside it.
- Chroma subsampling
- Storing colour detail at lower resolution than brightness detail, since the eye is less sensitive to it. Used by JPEG and HEIC alike.
The three confusions worth clearing up
- HEIC is not a codec. It is a file extension. The codec inside is HEVC, and the container is HEIF. Software that says it cannot open HEIC usually means it has no HEVC decoder.
- AVIF is not HEIC. They share a container design but use different codecs, which is why one works in browsers and the other does not.
- Lossless does not mean better. Converting an already-lossy HEIC to PNG preserves what is there and prevents further loss. It cannot recover what the original encoder discarded.
Common questions
- What is the difference between HEIC and HEVC?
- HEIC is the file extension and HEIF is the container standard. HEVC is the codec that compresses the picture stored inside that container.
- Is AVIF the same as HEIC?
- No. The two formats are cousins rather than twins: AVIF borrows the container layout but swaps HEVC for AV1, which is why browsers read one and not the other.
- What does lossy mean for my photos?
- Information is permanently discarded during compression to make the file smaller. It is invisible at high quality settings and becomes obvious as the setting drops.