# 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
- S4-252016
- S4aV250069