HEIC Image Sequences and Live Photos Explained

A single HEIC file can contain dozens of images, audio tracks, and auxiliary data. This capability separates the HEIF container from legacy formats like JPG and PNG, which store exactly one image per file. Live Photos, burst shots, depth maps, HDR gain maps, and thumbnails all coexist inside one container.

This guide explains the technical mechanisms behind HEIC's multi-image architecture and what happens to that data during conversion.

How the HEIF Container Stores Multiple Images

HEIF uses an ISO Base Media File Format (ISOBMFF) container to organize image items. This is the same box-based structure used by MP4 video files. Each piece of data is an "item" with a unique ID, a type declaration, and defined relationships to other items.

The container has 3 core organizational concepts:

• Image items -- individual encoded images stored as discrete data blocks
• Image sequences -- ordered series of image frames sharing a timeline
• Item references -- relationships linking items together (thumbnail-of, auxiliary-to, derived-from)

A simple iPhone photo contains at minimum 3 items: the primary image, a thumbnail, and EXIF metadata. A Live Photo HEIC can contain 30+ items. A burst sequence can contain hundreds.

Item references connect items through typed relationships. A thumbnail references the primary image with a thmb link. A depth map references it with auxl. The container maps every relationship explicitly.

Live Photos: Still Frame, Motion, and Audio in One File

A Live Photo HEIC packages a full-resolution still image with approximately 1.5 seconds of motion frames and a synchronized audio track. Apple introduced Live Photos with the iPhone 6s in 2015 and moved their storage to HEIF with iOS 11 in 2017.

What a Live Photo Contains

A Live Photo HEIC contains 3 distinct data streams:

1. Primary still image -- a single HEVC-compressed frame at full sensor resolution (12 MP or 48 MP). This is the key frame displayed in photo galleries.
2. Image sequence track -- approximately 45 HEVC-compressed frames at 30 fps, spanning roughly 1.5 seconds before and after the shutter press. Each frame encodes only the differences from its neighbors.
3. Audio track -- AAC-encoded audio synchronized to the frame timeline.

How Inter-Frame Prediction Reduces Size

Live Photo sequences achieve 70-80% compression efficiency through inter-frame prediction. The encoder designates keyframes (I-frames) at intervals and encodes remaining frames as differences (P-frames and B-frames).

A typical 12 MP HEIC still occupies approximately 1.8 MB. Without inter-frame prediction, 45 additional frames would add approximately 81 MB. With HEVC prediction, those 45 frames add only 1-2 MB. The total Live Photo file typically measures 3-4 MB including audio.

Apple uses a ContentIdentifier metadata field to bind the still image and the motion sequence. Both the still frame and the sequence track share an identical UUID string, ensuring decoders know they belong to the same capture event.

Burst Photos: Derived Image Items and Shared Data

Burst shots use HEIF's derived image items to share redundant data between frames, reducing total file size by 30-50%. When an iPhone fires at up to 10 frames per second, consecutive images share most of their pixel data.

How Derived Image Items Work

A derived image item references another image and stores only the difference. In a burst sequence:

1. The first frame is encoded as a complete, independent I-frame.
2. Subsequent frames are encoded as derived items referencing the previous frame.
3. Each derived item stores only the pixel differences (delta data).
4. The decoder reconstructs each frame by applying the delta to its reference frame.

A 10-frame burst sequence at 12 MP resolution illustrates the savings:

| Storage Method | Total Size (approx.) | Per-Frame Average | | --- | --- | --- | | 10 separate HEIC files | 18 MB | 1.8 MB | | Single HEIC with derived items | 9-12 MB | 0.9-1.2 MB | | 10 separate JPG files | 35 MB | 3.5 MB |

Key Photo Selection

The HEIF container marks one frame in a burst as the "primary item." Apple Photos analyzes each frame for sharpness, exposure, and facial expressions, then sets the best frame as primary. Users can override this selection. The primary item flag lives in container metadata, not in the image data.

Image Derivations: Thumbnails, Depth Maps, and Gain Maps

Every HEIC file from a modern iPhone contains 3-7 auxiliary image items alongside the primary photo. These items declare their relationship to the primary image through typed references.

Thumbnail Images

The container stores at least one pre-rendered thumbnail, typically at 320x240 pixels. This thumbnail is a separate HEVC-compressed image item linked to the primary image via a thmb reference. Gallery applications read thumbnails directly instead of decoding full-resolution images, which is why scrolling through thousands of photos feels fast.

Depth Maps

Portrait Mode photos include a depth map stored as an auxiliary image item. The depth map is a grayscale image at approximately 768x576 pixels. Each pixel value represents the estimated distance from the camera to that point in the scene. Apple's rendering engine uses this map to apply variable background blur.

HDR Gain Maps

iPhones since the iPhone 12 embed an HDR gain map in every HEIC photo. The gain map encodes the difference between the SDR rendition and the full HDR rendition. On an HDR display, the system combines the base image with the gain map to produce extended brightness and contrast. The gain map is typically one-quarter resolution of the primary image, adding approximately 200-400 KB to total file size.

Auxiliary Image Types Summary

| Auxiliary Type | Typical Resolution | Purpose | Linked Via | | --- | --- | --- | --- | | Thumbnail | 320x240 px | Fast gallery preview | thmb reference | | Depth map | 768x576 px | Portrait Mode blur control | auxl reference | | HDR gain map | Quarter resolution | SDR-to-HDR tone mapping | auxl reference | | Alpha mask | Matches primary | Transparency / segmentation | auxl reference | | Semantic mask | Varies | Subject/background separation | auxl reference |

What Happens During Conversion to JPG or PNG

Converting a multi-image HEIC file to JPG or PNG extracts only the primary image item. Every other item -- sequences, audio, depth maps, gain maps, thumbnails, derived frames -- is discarded. JPG and PNG have no container structure for auxiliary data.

Conversion drops these elements:

• Live Photo motion frames -- all 45 sequence frames and the audio track are lost
• Burst derived frames -- only the primary (key) frame is extracted; all other burst frames are discarded
• Depth maps -- Portrait Mode blur becomes baked into the pixel data and can no longer be adjusted
• HDR gain maps -- the output becomes an SDR-only image with no HDR enhancement data
• Non-destructive edit history -- Apple Photos edit instructions stored in the container are flattened into the final pixel output

The primary image pixel data is fully preserved at maximum quality settings. What is lost is everything that made the HEIC file more than a single photograph.

Apple's Implementation on iPhone

Apple's Camera app uses HEIF's multi-image capabilities more extensively than any other device manufacturer. Every photo type produces a different item count inside the container:

| Photo Type | Container Contents | Total Items | | --- | --- | --- | | Standard photo | Primary image, thumbnail, EXIF, HDR gain map, segmentation mask | 4-5 | | Live Photo | Standard items + ~45 motion frames + AAC audio + ContentIdentifier | 6-7 | | Portrait Mode | Standard items + depth map + matte/segmentation mask | 6-7 | | 10-frame burst | 1 key frame + 9 derived frames + 10 thumbnails + group metadata | ~21 |

The HEIF container handles this complexity transparently. Users see one file in their photo library regardless of how many items it holds.

Extracting the Primary Frame with HEICify

When you need a universally compatible version of a HEIC file, HEICify's converter extracts the primary image item and converts it to JPG or PNG. The conversion runs entirely in your browser using Web Workers and libheif-js. No files leave your device.

For Live Photos, HEICify extracts the full-resolution still frame. For burst sequences, it extracts the primary frame selected by Apple's algorithm. For Portrait Mode shots, the rendered image with blur applied is converted.

To preserve a Live Photo's motion component, export it as a video from Apple Photos before converting. The HEIC-to-JPG path extracts the still image only.

Key Takeaways

The HEIF container gives HEIC files capabilities no legacy image format can match. A single file stores still images, motion sequences, audio, depth data, HDR metadata, and thumbnails through typed image items and explicit references. Conversion to JPG or PNG collapses all of this into a single flat image. The primary frame survives intact. Everything else is lost.

For more on the HEIF container architecture, see HEIF vs HEIC: What Is the Difference?. For a complete overview of iPhone camera output formats, read the iPhone Photo Formats Guide. For foundational knowledge about the format, start with What Is HEIC?.