Stage-2 was a one-line "next" in the README. Add a full, actionable spec (docs-site apple-stage2-presenter.md) a Mac agent can execute: VTDecompressionSession decode (with decode-completion stamping) -> CAMetalLayer + display-link present, the exact integration points against the existing StreamPump/StreamView/AnnexB/LatencyMeter, the three-stage measurement wiring (capture->decoded / decode->present / capture->present = glass-to-glass, using the already-wired PunktfunkConnection.clockOffsetNs), a cheaper decode-only intermediate, validation, and gotchas. Link it from the Apple README's Stage 2 item. (meta.json nav entry left in the working tree to land with the CI docs WIP.) Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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---
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title: "Apple Stage-2 Presenter (handoff)"
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description: "Implementation plan for the explicit VTDecompressionSession → CAMetalLayer presenter — hand-paced present + true decode→present (glass-to-glass) measurement. Written so a Mac agent can pick it up."
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---
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A pickup-ready plan for the **stage-2 Apple presenter**. The current **stage-1** presenter feeds
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compressed HEVC straight into `AVSampleBufferDisplayLayer`, which hardware-decodes **and presents
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internally with no per-frame callback** — so we can't stamp decode or present, and we can't hand-pace.
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Stage-2 takes explicit control: decode with `VTDecompressionSession`, present decoded frames through a
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`CAMetalLayer` driven by a display link. Two wins: **~0.5 refresh off the present tail** (the biggest
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client latency term at 60 Hz) and **true decode→present / glass-to-glass** numbers.
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All of this is **macOS/iOS/tvOS-only** — build + validate on a Mac (`swift build && swift test`, then
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live against a Linux host). The host + connector side is already done: `PunktfunkConnection.clockOffsetNs`
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(the connect-time skew offset, host minus client) is what makes the present timestamp cross-machine
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valid. See [Status](/docs/status) and roadmap §12.
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## Where it plugs into the existing code
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| Existing (stage-1) | Stage-2 change |
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|---|---|
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| `StreamPump` pulls AUs → `AnnexB.sampleBuffer` → `layer.enqueue` (compressed) | A `Stage2Pump` (or a mode flag on `StreamPump`) feeds AUs to `VTDecompressionSessionDecodeFrame` instead |
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| `StreamView`/`StreamViewIOS` host an `AVSampleBufferDisplayLayer` | Host a `CAMetalLayer` (+ a display link); keep the input-capture + HUD overlay unchanged |
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| `AnnexB.formatDescription(fromIDR:)` builds the format desc, refreshed on every IDR | **Reused** — it's the `VTDecompressionSession`'s format description; recreate the session when it changes |
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| `LatencyMeter` records capture→client-receipt at `onFrame` | Extend to record **decode-completion** and **present** stages (below) |
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Keep stage-1 behind a `UserDefaults` flag (e.g. `punktfunk.presenter = "stage1" | "stage2"`) so a
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regression can fall back — `AVSampleBufferDisplayLayer` is the known-good path.
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## Decode: VTDecompressionSession
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1. Create the session from the IDR's `CMVideoFormatDescription`
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(`AnnexB.formatDescription(fromIDR:)`):
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```
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VTDecompressionSessionCreate(
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allocator: nil,
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formatDescription: fmt,
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decoderSpecification: nil, // hardware by default; no need to force
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imageBufferAttributes: [
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kCVPixelBufferMetalCompatibilityKey: true,
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kCVPixelBufferPixelFormatTypeKey:
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kCVPixelFormatType_420YpCbCr8BiPlanarVideoRange, // 8-bit SDR; 10-bit (…10BiPlanar) for HDR later
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],
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outputCallback: <C-callback>,
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decompressionSessionOut: &session)
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```
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2. Per AU: build the same `CMSampleBuffer` as stage-1 (`AnnexB.sampleBuffer(au:format:)`, PTS =
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`au.ptsNs` @ 1e9 timescale) and submit:
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```
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VTDecompressionSessionDecodeFrame(session, sampleBuffer,
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flags: ._EnableAsynchronousDecompression,
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frameRefcon: <pts or a boxed context>, infoFlagsOut: nil)
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```
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3. The **output callback** delivers `(status, infoFlags, imageBuffer: CVImageBuffer?, presentationTimeStamp, …)`.
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`presentationTimeStamp` is `au.ptsNs` (the host capture clock). **Stamp decode-completion here**
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(`CLOCK_REALTIME` ns), retain the `CVPixelBuffer`, and push `{pts, pixelBuffer, decodedNs}` into a
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small NSLock-guarded ring (the "ready" queue) the display link drains.
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4. **IDR / mode change**: when `AnnexB.formatDescription` yields a new desc, check
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`VTDecompressionSessionCanAcceptFormatDescription`; if not, finish-and-recreate the session (same
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trigger stage-1 uses to refresh `format`). On decoder error (`kVTVideoDecoderBadDataErr`, etc.) drop
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to the next IDR — there's no out-of-band extradata; recovery keyframes re-carry the parameter sets.
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## Present: CAMetalLayer + display link
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- `CAMetalLayer` (device = system default, `pixelFormat = .bgra8Unorm`, `framebufferOnly = true`,
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`drawableSize` = stream WxH). The view: macOS `NSView`/iOS `UIView` whose `layerClass`/backing layer
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is the `CAMetalLayer` (mirror `StreamView`/`StreamViewIOS`).
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- **Display link** drives present: macOS `CVDisplayLink` (or `CADisplayLink` on macOS 14+),
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iOS/tvOS `CADisplayLink`. Each callback carries the **target present timestamp** (`CVTimeStamp` /
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`targetTimestamp`).
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- Each vsync: pop the **newest** ready frame (drop older undisplayed ones — low-latency default; no
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smoothing buffer to start), render a fullscreen quad sampling the **biplanar YUV** (luma +
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chroma planes via `CVMetalTextureCache`) with a BT.709 YUV→RGB fragment shader, then
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`commandBuffer.present(drawable)` (or `present(drawable, atTime:)`). **Stamp present time** for the
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frame just shown (use the display link's target timestamp converted to `CLOCK_REALTIME`).
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- Colorspace: BT.709 8-bit for now (matches the host's SDR). HDR (BT.2020/PQ, 10-bit `…10BiPlanar` +
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EDR `CAMetalLayer.wantsExtendedDynamicRangeContent`) is a later tie-in with the HDR roadmap (§10).
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### Cheaper intermediate (2a) if the Metal path is too big in one step
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Decode with `VTDecompressionSession` (gets the **decode-completion timestamp** = capture→decoded),
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then wrap the decoded `CVPixelBuffer` in a `CMSampleBuffer` and `enqueue` it into the existing
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`AVSampleBufferDisplayLayer` (it accepts uncompressed pixel buffers too). This yields the decode term
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**without** a Metal renderer — but **not** true present (the layer still presents internally). Ship 2a
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first if useful; 2b (CAMetalLayer + display link) is required for the on-glass present stamp.
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## Measurement (the whole point)
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Extend `LatencyMeter` (or add per-stage meters) so each frame records three instants, all
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`CLOCK_REALTIME` ns, all shifted by `connection.clockOffsetNs` to the host clock:
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- **capture→decoded** = `decodedNs + offset − pts_ns` (VideoToolbox decode latency, cross-machine)
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- **decode→present** = `presentedNs − decodedNs` (the present tail stage-2 shortens)
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- **capture→present** = `presentedNs + offset − pts_ns` — **the glass-to-glass number** (modulo the
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host render→capture term, still unmeasured; see roadmap §12)
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Surface `capture→present` p50/p95 in the HUD (extend the existing `model.latency*` line in
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`ContentView`). `skewCorrected` stays false when `clockOffsetNs == 0` (old host) — then the numbers are
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same-host-only, as today.
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## Validation
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- `swift test`: add a decode-output test (decode a known IDR built like
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`VideoToolboxRoundTripTests` → assert a `CVPixelBuffer` of the right dimensions + the
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decode callback fires). Present is display-bound — validate it **live** via the HUD number.
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- Live: connect to a Linux host (`m3-host --source virtual` on the GNOME box; see
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[GNOME Box Setup](/docs/gnome-box)), confirm `capture→present` is a few ms over `capture→client`
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and that `decode→present` shrank vs. an `AVSampleBufferDisplayLayer` baseline.
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- Compare against the headless reference number: `punktfunk-client-rs` reports skew-corrected
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capture→reassembled (~1.3 ms p50 GNOME box → dev box); capture→present should be that **+ decode +
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present**.
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## Gotchas
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- VT decode is **async**; the output callback runs on a VT-managed thread — don't block it, just stamp
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+ enqueue. Retain the `CVPixelBuffer` until presented (the ring owns it).
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- `VTDecompressionSessionDecodeFrame` wants the **same** `CMSampleBuffer` shape stage-1 builds (AVCC
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length-prefixed NALs, in-band parameter sets in the format desc, never as extradata).
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- `CAMetalLayer.drawableSize` must track mode changes (the host can `Reconfigure` mid-stream — watch
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`PunktfunkConnection.mode`/the new-IDR dimensions).
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- Don't add a jitter/smoothing buffer for the first cut — present newest-ready for lowest latency; a
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pacing policy can come later if frames look uneven.
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- Keep `clients/apple/README.md`'s "Stage 2" item + [Status](/docs/status) updated when this lands.
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