Piik/docs/adr/0007-path-isolated-representation-quality.md

ADR-0007: Framework-Owned Media Quality Adaptation

• Status: Accepted
• Date: 2026-08-19
• Last reviewed: 2026-08-28

Context

Screener sends one realtime game-screen source through independent P2P connections and, when needed, one shared LiveKit publication. WebRTC and LiveKit already own congestion control, encoder adaptation, retransmission, simulcast construction, subscriber bandwidth estimation, and layer selection.

Earlier experiments added a Screener-defined lower SFU encoding and treated quality preference as an application policy. Production evidence showed that an always-active lower encoding can compete with the highest encoding on a constrained Host-to-SFU path. It did not justify replacing LiveKit's native adaptation with a single HIGH representation or another application controller.

The product needs standard game-motion intent and user-selected ceilings, but it does not need a second bitrate, resolution, FPS, or layer-control system.

Decision

1. Browser video makes one internal, share-generation-scoped codec decision per sending endpoint. A bounded actual-RTCPeerConnection preflight measures H.264 encoded progress against a deterministic moving probe track at the current share's effective target dimensions and frame rate. The real capture supplies those target settings, but its current motion does not decide encoder capability. A proved sender prefers H.264 and retains VP8 as the native negotiation fallback; an unsupported, inconclusive, slow, or failed preflight uses VP8 only. The Host advanced settings expose one local VP8 | Auto | H264 selector with Auto default: it is editable only before a share, strict H264/VP8 bypass the gate, and share start locks it. Viewer relay senders remain Auto. The selection is not persisted or inferred from UA, GPU, capability advertisement, or powerEfficientEncoder alone; it creates no room state, wire field, or active-edge codec switching.
2. Display video uses the standard contentHint = "motion"; display audio uses contentHint = "music". The hint expresses content intent and does not promise a resolution, frame rate, bitrate, encoder, or hardware path.
3. The three recommended profiles and advanced controls supply capture and sender ceilings plus degradationPreference. Every sender reads back the parameters that the browser actually accepted.
4. Direct and peer paths leave media adaptation to each RTCPeerConnection. After accepting an answer, the Host reapplies the current video profile to the negotiated sender because the browser may replace or rewrite encoding parameters during negotiation.
5. Auto direct and Browser-relay offers with a proved sender order H.264 before VP8 in one standard codec-preference list; endpoints select their first common codec without a parallel media connection. Failed Auto or manual VP8 offers VP8 only; manual H264 offers H.264 mode 1 only. Existing edges are not renegotiated when a later relay gate completes; its result applies to future children.
6. The SFU publisher uses the Host source decision for its one publication, disables backup codec, sets the selected HIGH ceiling and degradation preference, but does not set screenShareSimulcastLayers, mutate lower encodings, or select a subscriber layer. Pinned LiveKit defaults own the selected codec's representation set and source-replacement/republish behavior.
7. LiveKit Dynacast and server send-side BWE remain enabled. AdaptiveStream remains disabled because a Viewer may relay its received track to peer children; local DOM size or visibility cannot represent that downstream demand.
8. WebRTC and LiveKit remain the media-adaptation owners. ADR-0005's deployed native-edge convergence may consume only a persistent categorical limitation from the exact sender and comparative delivery proof from the same Viewer over a real prepared candidate. It does not set bitrate, resolution, FPS, or layer, infer a physical bottleneck, or combine quality measurements into a score.
9. A healthy decoded route remains sticky. Manual media reconnect rebuilds the current exact P2P parent or current SFU subscription; it does not search for a better parent. If current-route recovery genuinely exhausts, ADR-0005's existing controller tries other eligible P2P parents before SFU.

Evidence Boundary

Chrome 151 production measurements established three durable facts:

• direct and browser-relay VP8 can sustain about 60 fps after stock bandwidth estimation warms up; sender ceilings are not startup guarantees;
• forcing an always-active lower SFU encoding can reduce the highest encoding on the same Host-to-SFU congestion budget; and
• pinned LiveKit 2.22.0 plus server 1.13.5 can select lower and higher VP8 screen-share representations per subscriber when server send-side BWE is enabled.

Separate content-hint probes showed that motion changes Chromium's adaptation tradeoff and may spatially downscale to preserve motion. That is the standard behavior requested for games, not a quality floor. Real-game readability, weaker Hosts, heterogeneous devices, and public-network SFU quality still require physical evidence.

The exact Chrome 151/153 comparison in realtime-quality research established why this decision is runtime-evidence driven: the cadence-failing AMD H.264 MFT and the full-cadence NVIDIA H.264 MFT both reported hardware efficiency. Source versus encoded-frame progress separated them. Chrome 153 then sustained the same H.264 path with two senders, Browser relay re-encoding, and one pinned LiveKit HIGH+LOW publication without backup codec.

Consequences

• Screener owns fewer media mechanisms and follows the pinned frameworks' supported control surfaces.
• A capable Browser Host or relay can use H.264 hardware encoding while another sender remains on VP8 without a room-protocol branch; the Host can explicitly override one future share for diagnosis or preference.
• A constrained Viewer may receive a lower LiveKit representation without lowering every subscriber, subject to the publisher and network actually sustaining the framework contract.
• A Host may need to lower its explicit share profile when encoder or uplink capacity is insufficient; Screener does not silently remove constrained Viewer support.
• ADR-0005's native-edge local convergence is deployed and performs bounded real-candidate comparisons without owning media adaptation. Weighted or global topology optimization still requires another route-model decision.

Stop Lines

• No Screener resolution/FPS/bitrate ladder or representation formula.
• No codec wire/state, GPU/MFT allowlist, persistent codec cache, parallel VP8/H.264 route, active-edge codec churn, or LiveKit backup publication. The local pre-share Host selector is the only manual codec surface.
• No manual SFU layer selector, forced single HIGH publication, or per-Viewer encoder.
• No quality score, all-pairs probing, periodic rebalancing, or parent switching outside ADR-0005's native-edge operation.
• No AdaptiveStream while a subscriber can relay the track.
• No quality or hardware claim from configured options alone.

References

• MediaStreamTrack Content Hints
• WebRTC
• WebRTC Statistics
• LiveKit video simulcast and Dynacast
• LiveKit selective subscription
• LiveKit client 2.22.0 encoding construction
• LiveKit server 1.13.5 layer forwarding