MPEG_accessor_timed

Contributors

  • ISO/IEC SC29 WG3 (MPEG Systems) - Scene Description Breakout Group
  • Contacts
    • Thomas Stockhammer (MPEG-I Scene Description BoG Chair, tsto@qti.qualcomm.com)

Status

Based on ISO/IEC FDIS 23090-14

Dependencies

Written against the glTF 2.0 spec.

Overview

An accessor in glTF defines the types and layout of the data as stored in a buffer that is viewed through a bufferView. When timed data is read from a buffer, the data in the buffer could change dynamically with time.

A scene that contains timed media and/or metadata shall utilize the timed accessor extension to describe access to the dynamically changing data. The MPEG_accessor_timed is an extension to regular accessors to indicate that the underlying data buffer is dynamic. Note that a timed accessor has two bufferViews, one inherited from the containing accessor and a second in the MPEG_accessor_timed extension. The former shall be used to reference the timed media data. The latter may be used to point to a dynamic buffer header, which may or may not be present. When present, both bufferViews shall point into the same circular buffer.

The accessor.bufferView field, in an accessor that has the MPEG_accessor_timed extension, as well as the timed accessor information header fields apply to the data of each frame within the circular buffer.

The timed accessor extension is identified by MPEG_accessor_timed.

glTF Schema Updates

glTF extension to specify timed accessor format formats

MPEG_accessor_timed Properties

  Type Description Required
immutable boolean This flag indicates if the accessor information may change over time No, default: true
bufferView integer This property provides the index in the bufferViews array to a bufferView element that points to the timed accessor information header. No
suggestedUpdateRate number This property provides the frequency at which the renderer is recommended to poll the underlying buffer for new data. No, default: 25
extensions object JSON object with extension-specific objects. No
extras any Application-specific data. No

Additional properties are allowed.

MPEG_accessor_timed.immutable

This flag equal to false indicates the accessor information componentType, type, and normalized may change over time. The changing values of componentType, type and normalized are provided through accessor information header. This flag equal to true indicates the accessor information componentType, type, and normalized do not change over time and are not present in the accessor information header. Note that count, max, min, and byteOffset are expected to change and are always included as part of the timed accessor information header.

  • Type: boolean
  • Required: No, default: true

MPEG_accessor_timed.bufferView

This property provides the index in the bufferViews array to a bufferView element that points to the timed accessor information header as described in ISO/IEC 23090-14:Table 8. byteLength field of the bufferView element indicates the size of the timed accessor information header. The buffer properties in the bufferView element shall point to the same buffer as the bufferView in the containing accessor object. In the absence of the bufferView attribute, it shall be assumed that the buffer has no dynamic header. In that case, the immutable flag shall be present and shall be set to true.

  • Type: integer
  • Required: No
  • Minimum: ` >= 0`

MPEG_accessor_timed.suggestedUpdateRate

The suggestedUpdateRate provides the frequency at which the renderer is recommended to poll the underlying buffer for new data. The rate is provided in number of changes per second.

  • Type: number
  • Required: No, default: 25
  • Minimum: ` >= 0`

MPEG_accessor_timed.extensions

JSON object with extension-specific objects.

  • Type: object
  • Required: No
  • Type of each property: Extension

MPEG_accessor_timed.extras

Application-specific data.

  • Type: any
  • Required: No

Known Implementations

Resources

  • ISO/IEC FDIS 23090-14, Information technology — Coded representation of immersive media — Part 14: Scene Description
  • ISO/IEC WD 23090-24, Information technology — Coded representation of immersive media — Part 24: Conformance and Reference Software for Scene Description for MPEG Media

License

Copyright ISO/IEC 2022

The use of the “MPEG scene description extensions” is subject to the license as accessible here: https://standards.iso.org/ and is subject to the IPR policy as accessible here: https://www.iso.org/iso-standards-and-patents.html.