Apple Stage-2 Presenter (handoff)

A pickup-ready plan for the stage-2 Apple presenter. The current stage-1 presenter feeds compressed HEVC straight into AVSampleBufferDisplayLayer, which hardware-decodes and presents internally with no per-frame callback — so we can't stamp decode or present, and we can't hand-pace. Stage-2 takes explicit control: decode with VTDecompressionSession, present decoded frames through a CAMetalLayer driven by a display link. Two wins: ~0.5 refresh off the present tail (the biggest client latency term at 60 Hz) and true decode→present / glass-to-glass numbers.

All of this is macOS/iOS/tvOS-only — build + validate on a Mac (swift build && swift test, then live against a Linux host). The host + connector side is already done: PunktfunkConnection.clockOffsetNs (the connect-time skew offset, host minus client) is what makes the present timestamp cross-machine valid. See Status and roadmap §12.

Where it plugs into the existing code

Keep stage-1 behind a UserDefaults flag (e.g. punktfunk.presenter = "stage1" | "stage2") so a regression can fall back — AVSampleBufferDisplayLayer is the known-good path.

Decode: VTDecompressionSession

1. Create the session from the IDR's CMVideoFormatDescription (AnnexB.formatDescription(fromIDR:)):

   VTDecompressionSessionCreate(
     allocator: nil,
     formatDescription: fmt,
     decoderSpecification: nil,           // hardware by default; no need to force
     imageBufferAttributes: [
       kCVPixelBufferMetalCompatibilityKey: true,
       kCVPixelBufferPixelFormatTypeKey:
         kCVPixelFormatType_420YpCbCr8BiPlanarVideoRange, // 8-bit SDR; 10-bit (…10BiPlanar) for HDR later
     ],
     outputCallback: <C-callback>,
     decompressionSessionOut: &session)

2. Per AU: build the same CMSampleBuffer as stage-1 (AnnexB.sampleBuffer(au:format:), PTS = au.ptsNs @ 1e9 timescale) and submit:

   VTDecompressionSessionDecodeFrame(session, sampleBuffer,
     flags: ._EnableAsynchronousDecompression,
     frameRefcon: <pts or a boxed context>, infoFlagsOut: nil)

3. The output callback delivers (status, infoFlags, imageBuffer: CVImageBuffer?, presentationTimeStamp, …). presentationTimeStamp is au.ptsNs (the host capture clock). Stamp decode-completion here (CLOCK_REALTIME ns), retain the CVPixelBuffer, and push {pts, pixelBuffer, decodedNs} into a small NSLock-guarded ring (the "ready" queue) the display link drains.
4. IDR / mode change: when AnnexB.formatDescription yields a new desc, check VTDecompressionSessionCanAcceptFormatDescription; if not, finish-and-recreate the session (same trigger stage-1 uses to refresh format). On decoder error (kVTVideoDecoderBadDataErr, etc.) drop to the next IDR — there's no out-of-band extradata; recovery keyframes re-carry the parameter sets.

Present: CAMetalLayer + display link

• CAMetalLayer (device = system default, pixelFormat = .bgra8Unorm, framebufferOnly = true, drawableSize = stream WxH). The view: macOS NSView/iOS UIView whose layerClass/backing layer is the CAMetalLayer (mirror StreamView/StreamViewIOS).
• Display link drives present: macOS CVDisplayLink (or CADisplayLink on macOS 14+), iOS/tvOS CADisplayLink. Each callback carries the target present timestamp (CVTimeStamp / targetTimestamp).
• Each vsync: pop the newest ready frame (drop older undisplayed ones — low-latency default; no smoothing buffer to start), render a fullscreen quad sampling the biplanar YUV (luma + chroma planes via CVMetalTextureCache) with a BT.709 YUV→RGB fragment shader, then commandBuffer.present(drawable) (or present(drawable, atTime:)). Stamp present time for the frame just shown (use the display link's target timestamp converted to CLOCK_REALTIME).
• Colorspace: BT.709 8-bit for now (matches the host's SDR). HDR (BT.2020/PQ, 10-bit …10BiPlanar + EDR CAMetalLayer.wantsExtendedDynamicRangeContent) is a later tie-in with the HDR roadmap (§10).

Cheaper intermediate (2a) if the Metal path is too big in one step

Decode with VTDecompressionSession (gets the decode-completion timestamp = capture→decoded), then wrap the decoded CVPixelBuffer in a CMSampleBuffer and enqueue it into the existing AVSampleBufferDisplayLayer (it accepts uncompressed pixel buffers too). This yields the decode term without a Metal renderer — but not true present (the layer still presents internally). Ship 2a first if useful; 2b (CAMetalLayer + display link) is required for the on-glass present stamp.

Measurement (the whole point)

Extend LatencyMeter (or add per-stage meters) so each frame records three instants, all CLOCK_REALTIME ns, all shifted by connection.clockOffsetNs to the host clock:

• capture→decoded = decodedNs + offset − pts_ns (VideoToolbox decode latency, cross-machine)
• decode→present = presentedNs − decodedNs (the present tail stage-2 shortens)
• capture→present = presentedNs + offset − pts_ns — the glass-to-glass number (modulo the host render→capture term, still unmeasured; see roadmap §12)

Surface capture→present p50/p95 in the HUD (extend the existing model.latency* line in ContentView). skewCorrected stays false when clockOffsetNs == 0 (old host) — then the numbers are same-host-only, as today.

Validation

• swift test: add a decode-output test (decode a known IDR built like VideoToolboxRoundTripTests → assert a CVPixelBuffer of the right dimensions + the decode callback fires). Present is display-bound — validate it live via the HUD number.
• Live: connect to a Linux host (m3-host --source virtual on the GNOME box; see Ubuntu — GNOME), confirm capture→present is a few ms over capture→client and that decode→present shrank vs. an AVSampleBufferDisplayLayer baseline.
• Compare against the headless reference number: punktfunk-client-rs reports skew-corrected capture→reassembled (~1.3 ms p50 GNOME box → dev box); capture→present should be that + decode + present.

Gotchas

• VT decode is async; the output callback runs on a VT-managed thread — don't block it, just stamp
  • enqueue. Retain the CVPixelBuffer until presented (the ring owns it).
• VTDecompressionSessionDecodeFrame wants the same CMSampleBuffer shape stage-1 builds (AVCC length-prefixed NALs, in-band parameter sets in the format desc, never as extradata).
• CAMetalLayer.drawableSize must track mode changes (the host can Reconfigure mid-stream — watch PunktfunkConnection.mode/the new-IDR dimensions).
• Don't add a jitter/smoothing buffer for the first cut — present newest-ready for lowest latency; a pacing policy can come later if frames look uneven.
• Keep clients/apple/README.md's "Stage 2" item + Status updated when this lands.