# Change Request Summary: FS_AVFOPS_MED Solutions and Mapping Updates

## Document Information
- **CR Number**: 0005
- **Specification**: TS 26.966 v19.0.0
- **Category**: B (addition of feature)
- **Release**: Rel-19
- **Work Item**: FS_AVFOPS_MED

## Purpose
This CR updates the possible solutions related to new use cases, specifically adding solutions for **Scenario #5: Video with changeable background**.

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## Main Technical Contributions

### 1. Updated Solution-to-Scenario Mapping (Clause 6.0)

The CR extends Table 6.0-1 to include two new solutions for Scenario #5:

- **Solution #5.1**: Multi-layer HEVC with auxiliary alpha layer
- **Solution #5.2**: Multi-HEVC bitstreams with alpha signalling using CICP

These solutions address video with changeable background use cases.

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### 2. Solution #5.1: Multi-layer HEVC with Auxiliary Alpha Layer (New Clause 6.10)

#### High-level Description

This solution leverages HEVC multi-layer extensions to carry alpha planes as auxiliary channels:

- **Base layer**: Contains the video content
- **Auxiliary layer**: Contains the alpha plane for background changing

#### Technical Approach

**Auxiliary Picture Signalling**:
- Uses `scalability_mask_flag` in the Video Parameter Set (VPS)
- Sets scalability mask index to '3' (reserved for "Auxiliary" scalability dimension)
- `AuxId` value determines auxiliary picture type:
  - `AuxId = 1`: Alpha plane (AUX_ALPHA)
  - `AuxId = 2`: Depth picture (AUX_DEPTH)
- Additional interpretation information carried via SEI messages (Alpha channel information, Depth representation information)

#### Profile Considerations

Two possible approaches identified for further study:
1. Multiview profiles (though only one view is present)
2. Combination of non-Multiview profile for base layer with monochrome profile for auxiliary layer

**Open Issues**:
- Different chroma subsampling between layers
- Different encoding configurations
- Spatial resolution differences
- Bit depth variations

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### 3. Solution #5.2: Multi-HEVC Bitstreams with Alpha Signalling Using CICP (New Clause 6.11)

#### High-level Description

This solution uses **two independent HEVC bitstreams**:
1. **First bitstream**: Video content
2. **Second bitstream**: Alpha plane sequence

#### Technical Approach

**Alpha Plane Signalling**:
- Alpha plane carried as a single-layer HEVC bitstream
- Current HEVC specification lacks explicit signalling for alpha plane sequences
- **Proposed solution**: Use specific code points in VUI information
- Reference to potential CICP extension [x2] for signalling via `colour_primaries` parameter in VUI

**VUI Parameters**:
- Signalling through `colour_description_present_flag` and related parameters
- `colour_primaries`, `transfer_characteristics`, and `matrix_coeffs` fields in VUI

#### Profile Considerations

Since bitstreams are independent:
- **Video content bitstream**: Any HEVC profile
- **Alpha plane bitstream**: 
  - Monochrome profiles (Monochrome, Monochrome 10, Monochrome 12, Monochrome 16)
  - 4:2:0 profiles

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## References Added

- **[x2]**: JVET-AN1005, "Future CICP extensions (Draft 2)", Joint Video Experts Team, 40th Meeting, Geneva, October 2025

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## Items for Further Study

Both solutions (#5.1 and #5.2) have evaluation sections marked as "For further study", indicating:
- Performance evaluation pending
- Profile compatibility analysis needed
- Implementation considerations to be determined