GeezerVison

Technical information

Introduction

High Efficiency Image File Format (HEIF, ISO/IEC 23008-12) specifies the storage of individual images, image sequences and their metadata into a container file conforming to the ISO Base Media File Format (ISO/IEC 14496-12). HEIF includes the storage specification of HEVC intra images and HEVC image sequences in which inter prediction is applied in a constrained manner. Use cases supported by HEIF include:

• Storage of still images as well as collection of images in a single container file,
• Storage of burst photos,
• Storage and efficient representation of video animations and cinemagraphs.
• Support for simultaneous capture of video and still images, i.e. storing still images and timed image sequences into the same file.
• Storage of images derived from other images of the file, either as derived images represented by non-destructive image editing operations or as pre-computed derived images.
• Storage of focal and exposure stacks into the same container file.
• Support for storing other media, such as audio and timed text, into the same container file with timed image sequences and synchronizing their playback.

As HEVC provides support for various chroma formats and sample fidelities up to lossless coding, the format can serve the whole spectrum of use cases from today's consumer devices storing images typically at 8 bits per sample to high-end professional devices with sample fidelity and dynamic range requirements going all the way up to 16 bits per sample.

Computational photography forms a new category of use cases that can benefit from the HEIF file format. Now a set of related images can be stored in a single file with associated metadata indicating relationships between different pictures.

HEIF specifies a structural format, from which codec-specific image formats can be derived. HEIF also includes the specification for encapsulating images and image sequences conforming to the High Efficiency Video Coding (HEVC, ISO/IEC 23008-2 | ITU-T Rec. H.265), Versatile Video Coding (VVC, ISO/IEC 23090-3 | ITU-T Rec. H.266), Advanced Video Coding (AVC, ISO/IEC 14496-10 | ITU-T Rec. H.264), and JPEG (ISO/IEC 10918-1 | ITU-T Rec. T.81).

In ISOBMFF, a continuous or timed media or metadata stream forms a track, whereas static media or metadata is stored as items. Consequently, HEIF has the following basic design:

• 1. Still images are stored as items. All image items are independently coded and do not depend on any other item in their decoding. Any number of image items can be included in the same file.
• 2. Image sequences are stored as tracks. An image sequence track can be indicated to be displayed either as a timed sequence or in a non-timed manner, such as a gallery of images. An image sequence track may be used instead of image items when there is coding dependency between images.

A file may contain both image items and image sequence tracks along with other media. For example, it is possible to create a file, which includes image items or image sequence tracks conforming to HEIF, and audio and timed text tracks conforming to any derivative format of the ISOBMFF.

Image I. ISOBMFF compliant file high level structure with meta box and items

Files conforming to ISOBMFF consist of a sequence of data structures called boxes, each containing a four-character code (4CC) indicating the type of the box, the size of the box in terms of bytes, and the payload of the box. Boxes may be nested, i.e. a box may contain other boxes. ISOBMFF and HEIF specify constraints on the allowed box order and hierarchy.

HEIF Related Brands and MIME Types

Files conforming to HEIF start with a FileTypeBox as defined in the ISOBMFF standard, which contains a list of brands the file complies with. As the FileTypeBox is located at the start of the file, it provides easily accessible indications of the file contents to media players. Each brand is identified by its unique four-character code. The specification of a brand can include requirements and constraints for files of the brand and for file players supporting the brand. A brand included in the FileTypeBox permits a player that supports the requirements of the brand to play the file.

The brands specified in the HEIF standard are presented in Table I. The HEIF standard specifies the 'mif1' and 'msf1' structural brands, as well as 'mif3' for the low-overhead (SlimHEIF) profile. Additionally, codec-specific brands are specified for HEVC, VVC, AVC, and JPEG as listed in Table I.

Table I. Brands, MIME subtypes, and file extensions for HEIF.

Brand	Coding format	Image or sequence?	MIME Type	MIME subtype	File extension
mif1	Any	image	image	heif	.heif
msf1	Any	sequence	image	heif-sequence	.heif
heic	HEVC (Main or Main Still Picture profile)	image	image	heic	.heic
heix	HEVC (Main 10 or format range extensions profile)	image	image	heic	.heic
hevc	HEVC (Main or Main Still Picture profile)	sequence	image	heic-sequence	.heic
hevx	HEVC (Main 10 or format range extensions profile)	sequence	image	heic-sequence	.heic
vvic	VVC	image	image	heif	.heif
vvis	VVC	sequence	image	heif-sequence	.heif
avic	AVC	image	image	avif	.avif
avis	AVC	sequence	image	avif	.avif
jpeg	JPEG	image	image	heif	.heif
mif3	Any (low-overhead profile)	image	image	heif	.heif

Internet media types, also known as MIME (Multipurpose Internet Mail Extensions) types, are used by various applications to identify the type of a resource or a file. MIME types consist of a media type ('image' in the case of HEIF files), a subtype, and zero or more optional parameters. For multi-purpose files, the selection of the subtype can be made on the basis of the primary use of the file.

An optional codecs MIME parameter can be present to indicate the used coding formats of the tracks and items present in the file. The codecs MIME parameter also includes the profile-tier-level value to which an HEVC-coded image item or an image sequence track conforms. More information about the MIME type registration and optional parameter can be found in Annex D and Annex E of the HEIF standard.

Comparison to other Image Formats

Table IX provides a comparison of the features of HEIF to other selected image formats. It can be observed that HEIF is more extensible and comprehensive than the other compared file formats. Particularly the possibility to include other media types, the advanced multi-picture features, and the support for non-destructive editing make HEIF more advanced than the other formats. The rich set of features make HEIF suitable for a broad range of devices and applications, including for example burst photography.

The references used to conclude the information in Table II are included in Table X.

Table II. Comparison of the features of some image file formats.

	.heic	JPEG/Exif	PNG	GIF (89a)	WebP	JPEG-XR / TIFF	JPEG-XR / JPX	BPG
Formats and extensibility
Base container file format	ISOBMFF	TIFF	-	-	RIFF	TIFF	- 4	-
Lossy compression	Yes (HEVC)	Yes (JPEG)	No	No	Yes (VP8)	Yes	Yes	Yes (HEVC10)
Lossless compression	Yes (HEVC)	Yes (TIFF Rev 6.0)	Yes (PNG)1	Yes (GIF)1	Yes (VP8L)	Yes	Yes	Yes (HEVC10)
Extensible to other coding formats	Yes	Yes8	No	No	No	Yes8	Yes5	No
Metadata format (on top of internal)	Exif, XMP, MPEG-7	Exif, XMP	-	-	Exif, XMP	Exif, XMP	JPX, (XMP)6	Exif, XMP
Extensible to other metadata formats	Yes	No	No	No	No	No	Yes (XML-based)	Yes
Other media types (audio, text, etc.)	Yes	Audio2	No	No	No	No	Yes7	No
Multi-picture features
Multiple images in the same file	Yes	No11	No	Yes3	Yes3	No	Yes	Yes9
Image sequences / animations	Yes	No	No	Yes	Yes	No	Yes	Yes
Image coding	Yes	No	No	No	No	No	No	Yes
Derived images
Multiple-of-90-degree rotations	Yes	Yes	No	No	No	Yes	Yes	No
Cropping	Yes	No	No	No	No	No	Yes	No
Tiling/overlaying	Yes	No	No	No	Yes	No	Yes	No
Extensible to other editing operations	Yes	No	No	No	No	No	No	No
Auxiliary picture information
Transparency (alpha plane)	Yes	No	Yes	No12	Yes	Yes	Yes	Yes
Thumbnail image	Yes	Yes	No	No	No	Yes	Yes	Yes

¹ In GIF and indexed color PNG encoding, lossy color quantization is applied while the color-quantized image is losslessly compressed.
² PCM, µ-Law PCM and ADPCM encapsulated in RIFF WAV
³ Only for animations and tiling/overlaying
⁴ JPX is a box-structured format compatible with ISOBMFF. However, only the File Type box is common in JPX and ISOBMFF.
⁵ Encapsulation of JPEG-2000 and JPEG-XR have been specified for JPX container. Mappings for other codecs could be similarly specified.
⁶ JPX (ITU-T T.800 and T.801) specifies an own metadata schema, but is capable of carrying an XML formatted metadata, such as XMP.
⁷ JPX can contain media complying with ISOBMFF (or derivatives thereof). No accurate synchronization between JPX animations and other media.
⁸ TIFF as a container format facilitates extensions to other coding formats.
⁹ Only for animations, thumbnails, and alpha planes. Non-timed image collections not supported.
¹⁰ HEVC Main 4:4:4 16 Still Picture profile, Level 8.5, with additional constraints
¹¹ Can be enabled through the MP extension
¹² A palette index for fully transparency can be specified

It is acknowledged that a summary such as that in Table II might be somewhat incomplete when it comes to features of different formats. For example, the table does not cover some of the extensions of JPEG. We welcome feedback and corrections to the table.

Table III. References for the compared image file formats

Image format	Version or date	Reference and/or URL
HEIF (.heic)	03/2015	ISO/IEC 23008-12
JPEG		ISO/IEC 10918-1 | ITU-T Rec. T.81 http://www.w3.org/Graphics/JPEG/itu-t81.pdf
Exif		http://www.cipa.jp/std/documents/e/DC-008-2012_E.pdf
PNG		http://www.w3.org/TR/PNG/
GIF	89a	http://www.w3.org/Graphics/GIF/spec-gif89a.txt
WebP		https://developers.google.com/speed/webp/docs/riff_container VP8L: https://developers.google.com/speed/webp/docs/webp_lossless_bitstream_specification
JPEG-XR		ISO/IEC 29199-2 | ITU-T Rec. T.832 ISO/IEC 15444-2 | ITU-T Rec. T.801 (for JPX)
BPG	0.9.5	http://bellard.org/bpg/bpg_spec.txt

The file size of container files are directly affected by the compression performance of the image/video codec being utilized. Such a comparison can be found in Annex B of this document.

Compression Performance

Table IV illustrates coding efficiency of HEVC intra coding with respect to well-known still picture codecs. The results indicate that JPEG would require on average 139 % higher bitrate than HEVC (i.e. 2.39 times the file size) in order to achieve the same objective picture quality. For JPEG-XR and JPEG-2000 the average increase in bitrates are 66 % and 44 %, respectively.

Table IV. HEVC intra coding performance with respect to legacy formats. Bitrate increase required to achieve the objective quality provided by HEVC intra coding is reported for each test category.

Class	Resolution	Charasteristics	JPEG	JPEG XR	JPEG 2000
Class A	2560x1600	Cropped 4Kx2K sequences for Ultra HDTV services	87 %	44 %	48 %
Class B	1920x1080	High resolution sequences for streaming and broadcast services	124 %	62 %	15 %
Class C	832x480	Medium resolution sequences for Internet/mobile video services	122 %	53 %	50 %
Class C	832x480	Medium resolution sequences for Internet/mobile video services	122 %	53 %	50 %
Class D	416x240	Low resolution sequences for services to resource constrained devices	110 %	47 %	43 %
Class E	1280x720	720p sequences for video conferencing applications	170 %	73 %	23 %
Class F	1024x768, 1280x720	Computer screen content and computer generated content	223 %	118 %	87 %
Average			139 %	66 %	44 %

Table V indicate that one can expect that for natural content the restricted inter coding can typically provide two to three times better compression than intra picture coding. In special cases like animations where majority of the scene is static the compression efficiency can significantly exceed those levels and be tens of times more efficient than intra coding.

Table V. Coding efficiency improvements provided by low latency predictive coding of the HEVC Image File Format. Bitrate impact and coding gain are reported with respect to HEVC intra coding.

Content	Type	Frames	Bitrate change	Coding gain
Class A	Image burst	8	-46 %	1.9
Class B	Image burst	8	-51 %	2.0
Class C	Image burst	8	-60 %	2.5
Class D	Image burst	8	-63 %	2.7
Class E	Image burst	8	-79 %	4.8
Class F	Image burst	8	-55 %	2.2
Memorial	Exposure stack	16	-29 %	1.4
Mersu	Focal stack	13	-25 %	1.3
Car&Tractor	Cinemagraph	48	-97 %	33.3

Images in HEIF Files

Multiple images can be stored in a HEIF file. It can be useful to differentiate between them by assigning them certain roles. The roles specified in HEIF are listed and described in Table VI. Note that a single image can be associated with more than one role.

Table VI. Roles of images.

Role	Description
cover image	A representative image of the image items and image sequence tracks of the file. The cover image should be displayed when no other information is available on the preference to display the image items of the file. The file can have only one cover image.
thumbnail image	A smaller-resolution representation of a master image.
auxiliary image	An image that complements a master image. For example, an alpha plane or a depth map. Can assist in displaying the master image but is not typically displayed as such.
master image	An image that is not a thumbnail image or an auxiliary image. Typically represents a full-resolution displayable image.
hidden image	An image that should never be displayed. Can be present in the file for example as an input image for a derived image.
pre-derived coded image	A coded image that has been derived from other images. For example, a high dynamic range image derived from an exposure-bracketed set of images.
coded image	A coded representation of an image.
derived image	An image that is represented in a file by an indicated operation to indicated input images and can be obtained by performing the indicated operation to the indicated input images.

Properties of Images

HEIF allows the storage of image properties which are shared among different image items in a compact way. These properties are stored in ItemPropertyContainerBox. There are mainly two types of properties: descriptive and transformative. Descriptive properties provide information about the image item without applying modifications on the image itself. Transformative properties provide information about the transformative modification that needs to be done on the image item. The order of application of these properties to the image items are defined in the standard. Table VII lists currently defined properties. In addition to descriptive image properties, image items can optionally be characterized with metadata items, the format of which follows Exif, XMP, or MPEG-7 metadata.

Table VII. Image Properties

Name	Type	Description
Decoder configuration and initialization	Descriptive Property	The information needed to initialize the decoder. The structure of this information is usually defined in the related image coding format specification.
Image spatial Extents (‘ispe’)	Descriptive Property	indicates the width and height of the associated image item
Pixel Aspect Ratio (‘pasp’)	Descriptive Property	has the same syntax as the PixelAspectRatioBox as defined in ISO/IEC 14496-12.
Color Information (‘colr’)	Descriptive Property	has the same syntax as the ColourInformationBox as defined in ISO/IEC 14496-12.
Pixel Information (‘pixi’)	Descriptive Property	indicates the number and bit depth of colour components in the reconstructed image of the associated image item.
Relative Location (‘rloc’)	Descriptive Property	indicates the horizontal and vertical offset of the reconstructed image item relative to the associated image item.
Image Properties for Auxiliary Images (‘auxC’)	Descriptive Property	Auxiliary images must be associated with an AuxiliaryTypeProperty.
Image Rotation (‘irot’)	Transformative	Rotation by 90, 180, or 270 degrees.
Clean Aperture (‘clap’)	Transformative	Cropping according to a given cropping rectangle.

Derived Images

Derived images enable non-destructive image editing, where the original coded images are kept in the file, while new images, called derived images, can be introduced by specifying a transformation operation that is applied to one or more input images. HEIF specifies the generic structures used for storing derived images as items as well as a few specific types of derived images. Derived images can also have descriptive or transformative image properties. Item references of type 'dimg' specify the input image(s) of the derived image. The input images can be coded images or derived images. The derived image types specified in the HEIF standard are listed in Table VIII. Other types may be specified in other documents or later versions of the HEIF standard.

Table VIII. Derived Images

Name	Description
Identity transformation (‘iden’)	Cropping and/or rotation by 90, 180, or 270 degrees, imposed through the respective transformative properties.
Image Overlay (‘iovl’)	Overlaying any number of input images in indicated order and locations onto the canvas of the output image.
Image Grid (‘grid’)	Reconstructing a grid of input images of the same width and height.

Here are some examples of Derived Images

Image Metadata

HEIF files allow the storage of the metadata which can be related to images and image sequences. Such metadata can be information related to integrity checks, EXIF or XMP data or MPEG-7 related metadata. For image items, such metadata can be stored as a metadata item and reference the related image items with a ‘cdsc’ item reference. For image sequences, timed metadata tracks can refer to an image sequence track with a ‘cdsc’ track reference.

Indicating Alternative Representations

HEIF files can contain several image items representing the same image content (i.e. alternative representations). A media player may select one of the alternative representations of the same image content for displaying. A mechanism, known as entity grouping (defined in the GroupsListBox), is used for indicating alternate groups, which can contain both image items and media tracks (image sequences). The capability to define alternate groups between tracks and image items provides a unique mechanism for display initialization based on preferences of the media player and the content creator. Examples where alternate groups can be useful are:

• The same original image is coded into two or more image items with different properties, such as spatial resolution, bit-depth, or color gamut.
• The same original image is coded into two or more image items with a different coding format or profile.
• A timed image sequence, such as a cinemagraph, is an alternative to an image item.

Image sequences

Images that have a particular correlation (e.g. image bursts or animations like cinemagraphs) can be efficiently stored in HEIF files, thanks to the inherited media track features of ISOBMFF. Such images are called image sequences and they can reside in the same file with image items. Table IX lists the roles of the image sequence tracks which are currently defined.

Table IX. Roles of image sequence tracks.

Role	Description
thumbnail image sequence	A smaller-resolution representation of a master image sequence.
auxiliary image sequence	An image sequence that complements a master image sequence. For example, a sequence of alpha plane or depth map images. Can assist in displaying the master image sequence but is not typically displayed as such.
master image sequence	An image sequence that is not a thumbnail image sequence or an auxiliary image sequence. Typically contains full-resolution displayable images.

HEIF enables the usage of Inter-picture prediction for compact storage of image sequences. Moreover, image sequence use cases may require faster access to individual images and the ability to edit individual images without affecting any other images. HEIF therefore includes the following two features:

• CodingConstraintsBox to indicate prediction constraints which applies to all the linked samples. For example, it can be indicated that all reference images used for inter-picture prediction are intra-coded images.
• DirectReferenceSamplesList to have sample-wise listing of referenced samples using the sample grouping mechanism of ISOBMFF. This feature enables decoding of only those samples that are needed for accessing a particular sample (e.g. random access) and avoids the need of sequential decoding of each sample starting from the previous intra-coded image.

Processing of image items by file players

Figure 1 illustrates how a file player processes the coded images and the derived images included in a file. The file player decodes a coded image into a reconstructed image. Similarly, the file player applies the operation of the derived image to the indicated one or more input images to obtain the respective reconstructed image. The descriptive image properties generally describe the reconstructed image, with the exception of the decoder configuration and initialization information, which is associated with the coded image. The transformative image properties, if any, are applied to the reconstructed image to obtain an output image. The output image can be displayed, when the coded image or the derived image is not a hidden image. The output image can also act as an input image to derived images.

Figure 1. Operations performed by file players

HEIF File Playback Options

The most important features that enable controlling the playback of an HEIF file are listed in Table X. Some of these features were introduced in the ISOBMFF or ISO/IEC 14496-15 and are explicitly inherited by HEIF, while other features were specifically designed for the HEIF standard.

Table X. Features controlling image sequence playback.

Feature	First appeared in	Description
non-displayable sample	ISO/IEC 14496-15	Is never displayed, but can be used as a reference for predicting other images in the track.
timed vs. non-timed playback	HEIF	In timed playback, the image sequence is played as video, whereas in non-timed playback the samples of the track are displayed by other means, such as an image gallery. Non-timed playback may be indicated e.g. when a track is used for achieving a better compression efficiency for an exposure stack.
edit list	ISOBMFF	A list of ranges of the image sequence track in their playback order. Enables modifying the playback order and pace of samples.
looping	HEIF	HEIF allows indicating edit list repetition e.g. for looping animations. The repetition can be indicated to last for a certain duration or be infinite.
cropping and rotation	ISOBMFF	Rectangular cropping and rotation by 90, 180, 270 degrees can be specified.

Storing HEVC Encoded Pictures in HEIF: HEVC Image Files

The HEIF standard includes the specification for encapsulating HEVC-coded images and image sequences into HEIF-compliant files. The specification includes the following aspects:

• The sample entry format and the sample format of HEVC image sequence tracks are identical to those of the HEVC video tracks specified in ISO/IEC 14496-15. For compatibility with HEVC-capable video players, it is therefore advisable to create both an HEVC image sequence track and an HEVC video track that refer to the same coded images. This is called a Dual-Branded File and has an insignificant impact on the file size.
• The format of the decoder configuration information in the HEVC configuration item property is identical to that in the sample entry format. Likewise, the item format is identical to the sample format of HEVC video and image sequence tracks. This enables having the same intra-coded image as an image item and as a sample in an HEVC image sequence or video track.
• The configuration information of alpha planes and depth maps follows the respective supplemental enhancement information of HEVC.

Storing VVC Encoded Pictures in HEIF

The HEIF standard specifies the encapsulation of images and image sequences coded with Versatile Video Coding (VVC, ISO/IEC 23090-3 | ITU-T Rec. H.266) into HEIF-compliant files. VVC is the successor to HEVC, offering further compression efficiency improvements particularly for high-resolution and high-dynamic-range content.

VVC-coded image items use the 'vvc1' coding type. The decoder configuration information is carried in the VVC configuration item property, whose format follows the VVC decoder configuration record defined in ISO/IEC 14496-15. The item data format is identical to the sample format of VVC video tracks, enabling reuse of the same coded picture as both an image item and a sample in a sequence track.

VVC intra coding is expected to provide significant compression gains over HEVC intra coding, with reported bitrate savings in the range of 20–40% for equivalent subjective quality depending on content class and resolution.

Low Overhead HEIF (SlimHEIF)

The low-overhead amendment to the HEIF standard (ISO/IEC 23008-12) defines a compact file-level structure identified by the 'mif3' brand. SlimHEIF reduces the container overhead by eliminating redundant metadata boxes, making it particularly effective for very small images such as icons, thumbnails, and emoji.

In a conventional HEIF file, the ISOBMFF box hierarchy introduces a fixed overhead that can be significant relative to the coded image data when the image itself is only a few hundred bytes. SlimHEIF addresses this by using a streamlined box layout that omits boxes whose information can be inferred from context.

Experimental results show that SlimHEIF can reduce file size by 30% to 80% compared to equivalent standard HEIF files for small single-image payloads. For larger images the relative savings are smaller but the absolute overhead reduction remains constant.

High Dynamic Range and Wide Colour Gamut

HEIF defines several item properties for signalling high dynamic range (HDR) and wide colour gamut (WCG) characteristics of images. These properties allow file players to correctly interpret and display HDR content, or to apply appropriate tone mapping for standard dynamic range displays.

The HDR-related item properties defined in the HEIF standard include:

• Content Light Level Information (clli) — signals the maximum content light level and maximum frame-average light level of the image, enabling display devices to apply appropriate brightness scaling.
• Mastering Display Colour Volume (mdcv) — describes the colour primaries, white point, and luminance range of the mastering display, providing a reference for colour volume mapping.
• Content Colour Volume (cclv) — specifies the colour volume of the actual content, which may be a subset of the mastering display colour volume.
• Ambient Viewing Environment (amve) — describes the intended ambient viewing conditions for the content.
• Required Video Extension (reve) — signals codec-level extension requirements for correct decoding of HDR content.
• Nominal Diffuse White (ndwt) — specifies the nominal diffuse white luminance level for the content, assisting tone-mapping operations.

Wide colour gamut is signalled through the Colour Information property (colr), which can carry an ICC colour profile or an NCLX colour description specifying the colour primaries, transfer characteristics, and matrix coefficients of the image.

Tone Mapping and Gain Maps

HEIF supports tone-map derived images through the 'tmap' item type. A tone-map item references a base image and a gain map image, enabling a single file to serve both HDR-capable and SDR-only displays. The gain map encodes the per-pixel ratio between the SDR and HDR representations, allowing the display device to reconstruct the appropriate rendition.

This approach is conceptually similar to other gain-map solutions (such as those used in JPEG-R / ISO 21496-1), but is expressed natively within the HEIF derived-image framework using standard ISOBMFF referencing mechanisms.

Entity Groups

The ISOBMFF entity grouping mechanism (GroupsListBox) provides a general facility for expressing relationships among image items and tracks. HEIF inherits and extends this mechanism for several standard-defined grouping types beyond the alternate groups described earlier.

Grouping types defined or referenced by the HEIF standard include:

• Alternate group (altr) — indicates that the grouped items are alternative representations of the same content, and a file player should select at most one for display.
• Stereo pair (ster) — groups two images representing the left-eye and right-eye views of a stereoscopic pair.

Multi-Image Application Format (MIAF)

The Multi-Image Application Format (MIAF, ISO/IEC 23000-22) is an application-level profile built on top of HEIF and ISOBMFF. MIAF defines a set of constraints and requirements intended to maximise interoperability between HEIF file producers and consumers. It specifies mandatory features, forbidden features, and recommended practices for a well-defined subset of the HEIF format.

MIAF introduces its own brand ('miaf') and defines application profiles for specific codec and colour space combinations. A file branded as MIAF-compliant signals to a player that the file conforms to a predictable, well-constrained subset of HEIF, reducing the implementation burden on decoders.