# Summary of 3GPP Technical Document S4-260239

## Document Overview

This is a pseudo-CR to TR 26.958 v0.1.1 addressing viewport-adaptive delivery workflows for large-scale 3D Gaussian Splatting (3DGS) scenes in the context of FS_3DGS_MED study. The contribution focuses on enabling delivery of massive 3DGS environments (e.g., city-scale digital twins) to mobile devices with constrained resources.

## Problem Statement

Large-scale 3DGS scenes (as defined in clause 5.4) cannot be fully loaded into mobile device memory due to:
- Bandwidth limitations
- Memory constraints
- Rendering capacity restrictions

Static delivery workflows would result in:
- Excessive latency
- Immediate resource saturation
- Inability to deliver complete scenes

Simple capability negotiation alone is insufficient for these use cases.

## Main Technical Contributions

### Viewport-Adaptive Workflow (Clause 9.2.3)

The document proposes a new clause 9.2.3 introducing a viewport-adaptive workflow that extends existing capability negotiation mechanisms by incorporating continuous spatial feedback.

#### Core Mechanism

- **Dynamic Spatial Context**: UE continuously transmits 6DoF pose and Field of View (FoV) to server
- **Metadata Format**: Adheres to formats defined in TR 26.928 (XR services)
- **Rendering Budget Management**: Server optimizes 3DGS stream relative to user's perspective while staying within negotiated rendering budget

### Spatial Optimization Strategies (Clause 9.2.3.2)

Two approaches are defined:

#### Tiled Environments with LOD
- Environment partitioned into spatial tiles
- Multiple levels of detail (LOD) per tile
- Server selects appropriate LOD based on:
  - Proximity to user
  - Visibility within frustum
- **LOD Distribution**:
  - High-density tiles (e.g., LOD 4) for viewport center
  - Lower-density tiles (e.g., LOD 1-3) for peripheral/distant areas
- Concentrates point budget where user is looking

#### Unstructured Scenes
- Real-time frustum culling, pruning, and merging
- High point density in center of FoV
- Aggressive simplification in peripheral zones
- Dynamic primitive removal/merging for non-visible areas

### Server-Centric Decision Workflow (Clause 9.2.3.3)

**Two-Phase Approach**:

#### Static Initialization Phase
1. **Hardware Capabilities Assessment**: UE evaluates resources via system APIs or OpenXR
2. **Capability Reporting**: UE transmits comprehensive capability report to server
3. **Server-Side Capability Decision**: Server defines global rendering budget (max point count, SH degree) for session

#### Dynamic Delivery Phase
4. **Viewpoint and FoV Determination**: UE calculates current 6DoF pose and camera frustum
5. **Viewpoint and FoV Information**: UE sends spatial metadata to server
6. **Content Adaptation Based on FoV**: Server selects visible spatial tiles and adapts content (pruning, merging, LOD selection, quantization) to fit budget and user's view
7. **Optimized 3DGS Data**: Server streams adapted content payload (N points) to UE
8. **Local Adaptation**: UE performs final on-device adjustments if necessary
9. **3DGS Rendering**: UE renders the scene

**Key Characteristic**: Server maintains control over rendering budget throughout session based on initial capability assessment.

### Client-Centric Decision Workflow (Clause 9.2.3.4)

**UE-Driven Approach**:

#### Initialization Phase
1. **Hardware Assessment Analysis**: UE performs internal audit of hardware capabilities
2. **Decision of Best Representation Format**: UE selects optimal configuration (max point count, SH degree)
3. **3DGS Format Request**: UE requests content from server, specifying desired format parameters (point budget, SH degrees, quantization)

#### Delivery Phase
4. **Viewpoint and FoV Determination**: UE calculates current spatial position and FoV
5. **Viewpoint and FoV Information**: UE sends spatial metadata to server
6. **Content Adaptation Based on FoV**: Server filters scene spatially (frustum culling/tile selection) and adapts data to match format requested in step 3
7. **Optimized 3DGS Data**: Server delivers visible content conforming to requested parameters
8. **Local Adaptation**: UE applies final local refinements for runtime stability
9. **3DGS Rendering**: UE renders received content

**Key Characteristic**: UE explicitly requests specific representation format during initialization; server's role restricted to spatial operations while adhering to UE-imposed format constraints.

## Alignment with Existing Specifications

- Builds upon capability negotiation described in clause 9.2.2
- Aligns with viewport-dependent streaming principles from TR 26.928 (XR services)
- Addresses use case defined in clause 5.4 (Large 3DGS scenes)

## Proposal

The document proposes to agree the changes introducing clause 9.2.3 and its subclauses (9.2.3.1-9.2.3.4) to TR 26.958, including two workflow diagrams (Figures 5 and 6) and one illustration of tile/LOD selection (Figure 4).