Codecs, transport, displays, input and current limits.

Wire protocol

• Protocol version: 3.
• Control: bounded UTF-8 JSON messages with a type discriminator.
• Legacy single-monitor video: 10-byte big-endian header plus Annex-B access unit.
• Region video: 26-byte big-endian header carrying monitor ID, topology generation and stream epoch.
• Audio: 8-byte binary header plus PCM or Opus payload.
• Clipboard: 20-byte binary chunk header; chunks are at most 1 MiB.
• Hard USB lab: normalized control/interrupt URBs on the reliable authenticated QUIC stream.

The shared transport crate contains reviewed stream/datagram primitives for later carrier work. Current Direct products do not send video as QUIC datagrams.

Arcen does not operate an identity provider, certificate authority, session broker or online licence service for the Direct path.

Capture and encode by host

Color and stream structure

• Chroma and bit depth are separate: 4:4:4 means no chroma subsampling; it does not imply 10-bit or 12-bit samples.
• Current Arcen contract: HEVC Range Extensions with YUV 4:4:4 and 8-bit samples. The media planner explicitly rejects other bit depths today.
• Not currently implemented: HEVC Main 4:4:4 10, Main 4:4:4 12, Main10, HDR transfer functions or HDR metadata.
• Why 4:4:4: preserving chroma at every pixel improves colored text, thin lines and desktop UI compared with 4:2:0. It does not increase tonal precision beyond 8-bit.
• Compatibility modes: H.264 4:2:0.
• Software conversion: BGRA to NV12/I420 uses BT.709 limited-range conversion in shared safe Rust.
• Decode: macOS VideoToolbox decodes Annex-B H.264 and H.265.
• Latency configuration: NVENC uses the ultra-low-latency tuning path with no lookahead/B-frame path.
• Keyframes: explicit IDR requests are used for recovery, reconnect and topology changes.
• Bitrate planning: one per-monitor budget derived from pixel rate, clamped to 500-50,000 kbps. There is no live adaptive-bitrate feedback loop yet.

Multi-monitor is not one stitched video canvas. Deck sends the local display topology. Pier applies a host topology, assigns each monitor a nonzero session monitor ID and stream epoch, and runs independent capture, encode, queue, decode and presentation state per monitor.

How work is divided

1. Topology: Deck reports logical placement, physical/backing size, scale, rotation and display identity.
2. Apply: Pier binds exact host outputs and publishes the applied host-pixel rectangles.
3. Plan: every monitor receives its own codec, chroma, encoded size, fps and bitrate budget.
4. Measure: Pier opens the complete proposed encoder set and measures p50/p95 encode latency, queue age, delivered fps and fairness.
5. Commit: the full roster is accepted, reassigned or rejected atomically. A partial display subset is not silently served.
6. Route: every region frame carries monitor ID, topology generation and stream epoch before it reaches the matching decoder.

Damage and idle cadence

arcen-keel divides each region into a 16 x 16 activity grid. The grid identifies changed content; it does not divide the display into separate network resolutions. Static regions may skip conversion, encode and emission between bounded refresh deadlines. Startup frames, requested IDRs, recovery, keyframe deadlines and input/focus wake events always force service.

Typed pen contract

• Normalized position, pressure, tilt, rotation, tip/eraser, proximity, touching and two barrel-button bits.
• One globally ordered input sequence shared with keyboard and pointer events.
• Only superseded hover/motion samples may coalesce; contact, tool, proximity and button edges are preserved.
• Linux pressure maps to 0..=8191; tilt maps to whole degrees -90..=90; rotation is not advertised.
• Windows pressure maps to the Win32 PT_PEN ceiling 0..=1024; tilt and rotation are supported through Windows Ink.
• Windows Wintab-only applications are not covered by the synthetic PT_PEN path.
• Tablet multi-touch is not transported. One active tablet/tool is supported per Deck session.
• ExpressKeys and touch rings use client-side Wacom mappings to ordinary keyboard or wheel events.

Physical Wacom USB lab evidence

Revision eac2494 captures one exact Intuos5 Touch L device (056a:0317) on macOS, forwards control and interrupt URBs over the authenticated QUIC session, and enumerates native Pen, Pad and Finger devices on Linux through usb-vhci. The current lab build must run with administrator authorization. The corrected pending-interrupt behavior still requires one physical pen-report retest. This is not a production support claim or a general USB forwarding feature.

The Windows virtual microphone driver source exists but remains default-off pending protected driver signing, HLK/HVCI evidence and release approval. Arcen does not change the Windows default recording endpoint.

• Clipboard protocol version 1 must be negotiated by both peers.
• Pier controls direction, allowed content and maximum encoded size.
• Default policy is both directions, UTF-8 text and PNG, maximum 8 MiB.
• Operator configuration accepts 1-20 MiB; 20 MiB is the protocol ceiling.
• Wire chunks are contiguous, at most 1 MiB, sequence-numbered and expire after five seconds without progress.
• Text truncates only at a UTF-8 boundary. Images reject rather than truncate.
• No files, HTML, RTF or delayed rendering are transported.
• Clipboard payloads are not logged or persisted by Arcen.

Known technical limits

• Current streams are 8-bit SDR; no 10-bit or HDR pipeline is claimed.
• H.264 4:4:4 is not used on the macOS VideoToolbox path; Arcen's 4:4:4 mode is paired with HEVC.
• Linux software H.264 is limited to 1920 x 1200 at 30 fps.
• Linux typed-pen rotation is not injected.
• Windows typed pen uses Windows Ink, not Wintab.
• Multi-monitor pointer lock is disabled because region-scoped relative input is not defined.
• Display topology is fixed for an attachment; display changes require reconnect.
• There is no current live adaptive-bitrate controller.
• Native USB bridging is not general USB forwarding and excludes storage, cameras, audio devices, hubs, smart cards and network adapters.