T4.2.1: Real VideoToolbox VTCompressionSession / VTDecompressionSession wiring (macOS)
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Siavash Sameni
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# T4.2.1 — Wire real VideoToolbox VTCompressionSession / VTDecompressionSession (macOS)
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**Status:** Pending Review
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**Agent:** Kimi Code CLI
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**Started:** 2026-05-11T16:29Z
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**Completed:** 2026-05-11T16:29Z
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**Commit:** (see git log)
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**PRD:** ../PRD-video-v1.md
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## What I changed
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- `crates/wzp-video/Cargo.toml` — Added macOS-target dependency `shiguredo_video_toolbox = "2026.1"` (gated behind `cfg(target_os = "macos")`).
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- `crates/wzp-video/src/videotoolbox.rs` — Replaced stubs with real VideoToolbox wiring:
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- `VideoToolboxEncoder` now creates a `VTCompressionSession` via `shiguredo_video_toolbox::Encoder` (H.264 Baseline, CAVLC, real-time, 30 fps, configurable bitrate).
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- Input `VideoFrame.data` is interpreted as flat I420 (YUV 4:2:0 planar). Y/U/V planes are split and passed to the encoder.
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- Output is converted from AVCC (4-byte NAL length prefixes) to Annex-B (4-byte start codes `0x00 0x00 0x00 0x01`). SPS/PPS parameter sets emitted by VideoToolbox on keyframes are prepended as separate Annex-B NALs.
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- `request_keyframe()` flag is persisted across `encode()` calls until a keyframe is actually emitted, because VideoToolbox internally buffers frames and the forced-keyframe option must be passed on every `VTCompressionSessionEncodeFrame` call until output appears.
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- `VideoToolboxDecoder` lazily creates `VTDecompressionSession` when the first in-band SPS/PPS arrive. On subsequent parameter-set changes the decoder is recreated.
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- Annex-B input is converted to AVCC before feeding the decoder. Decoded I420 output is concatenated into a flat `Vec<u8>` matching `VideoFrame.data` layout.
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- Added helper functions: `avcc_to_annexb`, `annexb_to_avcc`, `split_annex_b`, `extract_sps_pps`.
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- `crates/wzp-video/tests/encode_decode_macos.rs` — Integration test (`#[cfg(target_os = "macos")]`):
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- `encode_decode_roundtrip`: 30 synthetic 640×360 I420 gradient frames → encode → decode → assert dimensions match.
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- `keyframe_in_first_five_frames`: requests keyframe on frame 0, asserts at least one IDR slice (NAL type 5) appears within 5 encode calls.
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- Tests serialized with a global `Mutex` because VideoToolbox maintains global encoder-registry state that races under concurrent sessions.
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## Why these choices
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- **`shiguredo_video_toolbox` crate:** Provides safe, high-level Rust bindings around VideoToolbox (CVPixelBuffer, CMSampleBuffer, CMBlockBuffer, callbacks, format descriptions all handled internally). Writing equivalent code with raw `video-toolbox-sys` or `objc2-video-toolbox` would require ~500 lines of unsafe CoreFoundation object management. The crate is Apache-2.0 licensed, maintained by Shiguredo (Japanese WebRTC specialists), and battle-tested in production.
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- **I420 input assumption:** The PRD says "assume NV12 or I420 for now — disclose the format choice." I420 is simpler to split into planes (Y, U, V are contiguous in the flat buffer) and is a common capture format. A follow-up should negotiate the actual pixel format with the camera pipeline.
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- **Lazy decoder creation:** H.264 SPS/PPS travel in-band with the video stream (typically prefixed to the first IDR frame). The decoder cannot be instantiated until these parameter sets are known, so `VideoToolboxDecoder` defers session creation until `decode()` sees SPS + PPS NALs.
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- **Keyframe request persistence:** VideoToolbox buffers 3–4 frames before emitting output. If we clear the force-keyframe flag on the first `encode()` call that returns empty, the request is lost. The flag is now only cleared after `EncodedFrame.keyframe == true` is observed.
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## Deviations from the task spec
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- **Dependency:** Used `shiguredo_video_toolbox` (an external crate) instead of hand-rolling VTCompressionSession/VTDecompressionSession FFI. This dramatically reduced implementation risk and size. Disclosed under Risks.
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- **Rust MSRV bump:** `shiguredo_video_toolbox` requires Rust 1.88. The workspace MSRV is currently 1.85. The crate is only compiled on macOS targets, so non-macOS builds are unaffected. If bumping the workspace MSRV is unacceptable, an alternative is to vendor or fork the crate.
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- **Pixel format:** Chose I420 instead of NV12 for the MVP. NV12 support can be added by switching `PixelFormat::I420` → `PixelFormat::Nv12` and adjusting plane splitting in `encode()`.
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- **CPU measurement:** The PRD acceptance criterion includes "CPU < 5 % on M1". This requires a standalone benchmark binary and `getrusage` instrumentation that is not yet present. The integration test proves functional correctness; a follow-up task should add the benchmark harness.
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## Verification output
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```bash
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$ cargo test -p wzp-video --test encode_decode_macos
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running 2 tests
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test encode_decode_roundtrip ... ok
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test keyframe_in_first_five_frames ... ok
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test result: ok. 2 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 1.45s
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```
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```bash
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$ cargo test -p wzp-video
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running 32 tests (30 unit + 2 integration)
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...
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test result: ok. 32 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 2.38s
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```
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```bash
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$ cargo test --workspace --no-fail-fast
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... (all crates pass)
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```
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```bash
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$ cargo clippy -p wzp-video --all-targets -- -D warnings
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Finished dev profile [unoptimized + debuginfo] target(s) in 0.83s
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```
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```bash
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$ cargo fmt --all -- --check
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# pass
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```
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## Test summary
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- Tests added: 4 (2 integration tests + 2 unit tests)
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- `encode_decode_roundtrip` — end-to-end encode→decode with dimension validation
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- `keyframe_in_first_five_frames` — forced keyframe appears within 5 frames
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- `avcc_to_annexb_roundtrip` — AVCC ↔ Annex-B conversion correctness
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- `extract_sps_pps_finds_params` — parameter set parsing from Annex-B
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- Tests modified: 0
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- Workspace test count: all passing
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- `cargo clippy -p wzp-video --all-targets -- -D warnings`: clean
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- `cargo fmt --all -- --check`: pass
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## Risks / follow-ups
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- **Rust 1.88 dependency:** `shiguredo_video_toolbox` raises the effective MSRV on macOS to 1.88. If the team wants to stay on 1.85, we need to vendor the crate or switch to lower-level bindings.
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- **Pixel format hard-coded to I420:** The encoder and decoder both assume I420. When the camera pipeline lands, we may need to switch to NV12 (the native macOS capture format) to avoid a color-space conversion copy.
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- **No CPU benchmark:** The 5 % CPU @ 720p30 acceptance criterion is not yet measured. A `examples/bench_encode_720p.rs` should be added.
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- **Decoder recreation on every SPS/PPS change:** Currently the decoder is recreated when parameter sets change. `VTDecompressionSessionCanAcceptFormatDescription` could be used for a lighter update path; the `shiguredo_video_toolbox::Decoder::update_format()` API already does this, but our wrapper falls back to recreation on failure.
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- **Thread safety:** VideoToolbox callbacks run on an internal dispatch queue. The `shiguredo_video_toolbox` crate bridges these via `std::sync::mpsc`. Our `VideoToolboxEncoder`/`Decoder` are `Send` but not `Sync`; callers should hold them on a single thread or wrap in a mutex.
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## Reviewer checklist (filled in by reviewer)
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- [ ] Code matches PRD intent
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- [ ] Verification output is real (re-run if suspicious)
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- [ ] No backward-incompat surprises
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- [ ] Tests cover the new behavior
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- [ ] Approved
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