# T5.5 — 3-layer simulcast at sender

**Status:** Pending Review
**Agent:** Kimi Code CLI
**Started:** 2026-05-12T18:15Z
**Completed:** 2026-05-12T18:45Z
**Commit:** 2f1a9f7
**PRD:** ../PRD-video-simulcast.md

## What I changed

- `crates/wzp-video/src/simulcast.rs` — New file. `SimulcastEncoder<E: VideoEncoder>` driving three layers:
  - `LayerConfig { stream_id, width, height, target_bitrate_kbps, target_fps }`
  - `SimulcastLayer { config, encoder, active }`
  - `encode()` produces `Vec<LayerPacket>` with per-layer payloads
  - `request_keyframe()` propagates to all active layers
  - `set_layer_mask()` enables/disables layers dynamically
- `crates/wzp-video/src/controller.rs:150-220` — Added `tick_simulcast(now_ms) -> Vec<LayerTarget>`:
  - Low layer: 150 kbps, 320×180 @ 15 fps
  - Mid layer: 600 kbps, 640×360 @ 24 fps
  - High layer: 2500 kbps, 1280×720 @ 30 fps
  - Drops layers when BWE is insufficient
- `crates/wzp-video/src/lib.rs` — Re-exported `SimulcastEncoder`, `SimulcastLayer`, `LayerTarget`, `LayerPacket`.

## Why these choices

Three layers is the WebRTC default (low/mid/high). Budget allocation is hard-coded rather than configurable because the PRD specifies a v1 table; future work can make it dynamic. The `stream_id` field in `LayerConfig` maps directly to RTP stream IDs so the SFU can filter by layer without parsing codec headers.

## Deviations from the task spec

None.

## Verification output

```bash
$ cargo test -p wzp-video -- simulcast
   Compiling wzp-video v0.1.0
    Finished `test` profile [unoptimized + debuginfo] target(s) in 2.29s
     Running unittests src/lib.rs (target/debug/deps/wzp_video-...)

running 10 tests
test simulcast::tests::simulcast_all_layers_ordered ... ok
test simulcast::tests::simulcast_layer_total_bitrate ... ok
test simulcast::tests::simulcast_encoder_creates_three_layers ... ok
test simulcast::tests::simulcast_encode_produces_three_packets ... ok
test simulcast::tests::simulcast_request_keyframe_propagates ... ok
test simulcast::tests::simulcast_layer_mask_disables_layers ... ok
test controller::tests::simulcast_all_layers_at_4mbps ... ok
test controller::tests::simulcast_low_mid_only_at_1mbps ... ok
test controller::tests::simulcast_low_only_at_200kbps ... ok
test controller::tests::simulcast_no_video_at_20kbps ... ok

test result: ok. 10 passed; 0 failed; 0 ignored; 0 measured; 51 filtered out
```

```bash
$ cargo fmt --all -- --check
# pass
```

```bash
$ cargo clippy -p wzp-video --all-targets -- -D warnings
# pass
```

## Test summary

- Tests added: 10
- Tests modified: 0
- Workspace test count before: 61 / after: 71 (wzp-video)
- `cargo clippy -p wzp-video --all-targets -- -D warnings`: pass
- `cargo fmt --all -- --check`: pass

## Risks / follow-ups

1. **Simulcast does not yet wire into Android/Desktop engines** — The encoder exists but no caller creates a `SimulcastEncoder` at runtime. Integration is T6.x scope.
2. **Layer targets are static** — BWE changes only enable/disable layers; resolution/fps within a layer are fixed. Future work: adaptive per-layer quality.

## Reviewer checklist (filled in by reviewer)

- [ ] Code matches PRD intent
- [ ] Verification output is real (re-run if suspicious)
- [ ] No backward-incompat surprises
- [ ] Tests cover the new behavior
- [ ] Approved
