Summary of 3GPP Technical Document S4-260131
Document Information
- Type: Change Request (CR 0001 rev 3)
- Specification: TS 26.966 v19.0.0
- Category: B (addition of feature)
- Release: Rel-20
- Work Item: FS_AVFOPS_MED
- Source: Xiaomi Communications
- Meeting: SA4#135, India, 9-13 Feb 2026
Main Purpose
This CR proposes adding a new scenario to TS 26.966 addressing video with changeable background functionality, progressing objective 1 on identifying relevant new representation formats not yet documented in TS 26.265.
Technical Contributions
New Scenario #5: Video with Changeable Background (Clause 5.6)
Overview (5.6.1)
The CR introduces a scenario addressing the growing use case of mobile video editing where users:
- Record and edit videos directly on their devices
- Upload source video to cloud services for editing, or
- Use local applications to generate final video
The key technical requirement is video compositing where:
- One video is overlaid on another visual content
- Alpha blending is performed using an alpha channel
- Pixel-accurate transparency information is required
- Alpha channel is typically carried in lossless manner
The CR includes four illustrative figures showing:
1. Original video frame
2. Associated alpha plane
3. Video frame with alpha plane applied
4. Alpha blended video frame with background image
Review of Previous Work (5.6.2)
The CR notes that coded representation of alpha auxiliary channels as part of video bitstream has not been addressed in 3GPP specifications until now.
Review of Related Work (5.6.3)
MV-HEVC Support (5.6.3.1)
- MV-HEVC video coding standard enables encoding of alpha channel as auxiliary layer
- Auxiliary layer identified as alpha auxiliary layer
- Contains alpha channel information SEI message
- Reference to Clause 6.9 Solution #4.1: MV-HEVC with auxiliary depth/alpha channels
ISO/IEC 23091-2:2025 CICP Video
- ISO/IEC JTC 1/SC 29/WG 5 specification defines codec-agnostic code points
- New colour primaries field code point (value 129) indicates decoded picture represents alpha channel
- Properties:
- Colour Primaries: Alpha channel
- HasChromaticityCoordinates: 0
- One colour component representing alpha channel
- Can be carried in VUI parameters in AVC and HEVC bitstreams
- Enables alpha channel indication without relying on auxiliary layers design
MIAF and HEIF Support (5.6.3.2)
- Multi-Image Application Format (MIAF) builds on HEIF for storing multiple images
- Both specify same alpha plane format
- Alpha planes defined as "image specifying transparency information of master image"
- Identified using auxiliary image item type:
urn:mpeg:mpegB:cicp:systems:auxiliary:alpha
Alpha Plane Interpretation Rules:
- Minimum sample value: Transparency (co-located pixel is transparent)
- Maximum sample value: Opacity (co-located pixel is opaque)
- Alpha value: Normalized between 0.0 and 1.0
- Sample values divided by maximum value (e.g., 255 for 8-bit) provides multiplier for master image intensity
- Requirement: Encoded alpha planes must use full sample range (0-255 for 8-bit)
SMPTE RP 157:2012 (5.6.3.3)
Defines signal properties for key/alpha/matte signals (terms used interchangeably):
Properties:
- Black level: Complete transparency
- White level: Complete opacity
- Transfer function: Out of scope, but assumed linear; black/white levels conform to image format specifications
- Alpha value: Normalized 0.0 (fully transparent) to 1.0 (fully opaque)
- Sample mapping: Co-located with corresponding fill luminance or RGB samples (zero pixel offset)
- Timing: Timed coincident with associated fill video signal
ISO/IEC 23008-2 HEVC / ITU-T H.265 (5.6.3.4)
- MV-HEVC enables alpha channel encoding as auxiliary layer
- MPEG Systems WG03 developing Amendment 2 of ISO/IEC 14496-12 8th edition
- Specifies storage of alpha maps in auxiliary video track linked to main video track
- Covers: media handler, track referencing, metadata box for alpha map interpretation
ISO/IEC 14496-12 ISOBMFF (5.6.3.5)
- Amendment 2 in development for alpha map storage
- Auxiliary video track linked to main video track
- Addresses media handler, track referencing, and metadata box aspects
SMPTE ST 268-1:2014 DPX Format (5.6.3.6)
- Digital Picture Exchange Format v2.0 for exchanging digital moving pictures
- Uses ".dpx" file extension
- Stores single raster image with up to eight image elements
- Supports alpha signal carriage
- Alpha element uses same pixel format/data type as other elements (8-bit, 10-bit, 12-bit, 16-bit, or 32-bit floating point)
- Allows both pre-multiplied and non-pre-multiplied alpha
Code Points Supporting Alpha:
- Value 4: Alpha (matte)
- Value 51: R, G, B, Alpha (A)
- Value 52: A, B, G, R
- Value 101: CB, Y, A, CR, Y, A (4:2:2:4)
- Value 103: CB, Y, CR, A (4:4:4:4)
Functional Requirements (5.6.4)
The CR establishes functional analysis framework based on:
Hardware Impact Assessment
Two possibilities:
1. Existing hardware support: Reference to example hardware products
2. No existing hardware support: Discussion/description with justifications on expected hardware implementation impact, or reference to existing demos
Codec Capabilities
- Capability to losslessly encode alpha information
Impact
- Affected Clauses: 5.6 (new)
- Other Specifications: None affected
- Test Specifications: None affected
- O&M Specifications: None affected
Revision History