PRD: Video Quality Controller + PriorityMode

Status: proposed Resolves: Road-to-video Phase V5 (video adaptive controller, audio-priority gate, ScreenShare slide-mode). Depends on: PRD #3 (BWE), PRD #5 (video v1).

Problem

Audio and video share a finite bandwidth budget. The FaceTime model — audio absolute priority, video elastic on top — is right for the default voice/video call, but it's wrong for screen-share / presentation where a frozen slide deck is worse than slightly degraded audio.

We need: a single VideoQualityController consuming BWE, with a policy gate driven by a user/product-selectable PriorityMode.

Goals

PriorityMode enum carried on QualityProfile.
Per-mode allocation gates: AudioFirst, VideoFirst, ScreenShare, Balanced.
Mid-call SetPriorityMode signal for runtime override.
ScreenShare slide-fallback: when bandwidth drops below SD video floor, encoder switches to single-I-frame-every-N-seconds mode (no wire format change).
Sensible defaults per call type (voice/video call → AudioFirst; presentation app → ScreenShare).

Non-goals

Multi-stream priority (e.g., one HD + one screen-share in the same session — separate work).
Custom user-defined modes; only the four enum variants.

Design

`PriorityMode`

#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
pub enum PriorityMode {
    AudioFirst,    // default for voice/video calls
    VideoFirst,    // user override
    ScreenShare,   // video + slide fallback; audio = intelligible speech only
    Balanced,      // proportional split
}

Carried on QualityProfile:

pub struct QualityProfile {
    ...
    pub priority_mode: PriorityMode,    // default AudioFirst
    pub video_bitrate_kbps: Option<u32>,
    pub video_resolution: Option<(u16, u16)>,
    pub video_fps: Option<u8>,
}

Mid-call change:

SignalMessage::SetPriorityMode {
    version: u8,
    mode: PriorityMode,
}

Allocation gates

let bwe = bandwidth_estimator.target_send_bps();

match priority_mode {
    AudioFirst => {
        audio_budget = max(24_kbps, audio_tier_min);  // audio floor first
        video_budget = bwe.saturating_sub(audio_budget);
        // video → 0 before audio degrades below floor
    }
    VideoFirst => {
        video_budget = max(video_floor, target_video_bps);
        audio_budget = bwe.saturating_sub(video_budget);
        // audio degrades to Opus 16k floor first
    }
    ScreenShare => {
        // Audio gets just enough for intelligible speech.
        audio_budget = 16_kbps;
        video_budget = bwe.saturating_sub(audio_budget);
        if video_budget < SD_VIDEO_FLOOR {
            encoder.set_mode(EncoderMode::SlideFallback);
        }
    }
    Balanced => {
        audio_budget = (bwe as f64 * 0.15) as u64;
        video_budget = bwe - audio_budget;
    }
}

`VideoQualityController`

pub struct VideoQualityController {
    bwe: Arc<BandwidthEstimator>,
    mode: AtomicU8,    // PriorityMode
    encoder: Arc<dyn VideoEncoder>,
    loss_pct: AtomicU8,
    rtt_ms: AtomicU32,
    encoder_queue_ms: AtomicU32,
}

impl VideoQualityController {
    pub fn tick(&self) {
        let budget = self.allocate();
        let target = self.derive_target(budget);  // (bitrate, fps, resolution, layer)
        self.encoder.set_target(target);
    }
}

derive_target maps (budget, loss, rtt, queue) to encoder parameters via a step table. Smoothed; no jumps larger than 2× per second.

ScreenShare slide-fallback

Pure encoder policy:

Normal video: continuous frames, target fps (5–15 for screen content).
When video_budget < SD_VIDEO_FLOOR (e.g., 150 kbps): switch to slide mode.
Slide mode: emit one high-quality I-frame every 2–5 s. No P-frames. Encoder prefers H.265 or AV1 (text legibility).
Wire format: KeyFrame=1 on every packet, FrameEnd=1 on last packet of slide. No new fields.

Receiver doesn't know slide mode is on — just sees keyframes arriving slowly.

Defaults

Product flow	Default mode
Voice call	AudioFirst (no video)
Video call	AudioFirst
Screen share	ScreenShare
User toggle in settings	VideoFirst or Balanced

Implementation outline

PriorityMode enum + serde + QualityProfile field (T5.1).
SetPriorityMode signal variant (T5.1).
VideoQualityController::new + tick (T5.2).
Per-mode allocation gates (T5.2).
EncoderMode::SlideFallback in wzp-video (T5.3).
Integration: CallEngine honors SetPriorityMode within 1 s.
UI plumbing for runtime toggle (out of scope here; tracked by platform team).

Acceptance criteria

100 kbps shaped link, AudioFirst: audio holds Opus 24 k, video drops to 0.
100 kbps shaped link, ScreenShare: audio holds Opus 16 k, video in slide mode emits 1 I-frame / 3 s.
100 kbps shaped link, VideoFirst: audio drops to Opus 16 k, video holds floor.
5 Mbps link, AudioFirst: video reaches HD within 10 s.
SetPriorityMode mid-call applied within 1 s.

Risks

Mode flapping under unstable BWE. Mitigation: 10 s dwell time before allowing mode-driven encoder reconfiguration.
Slide mode mistaken for poor connection by users. Mitigation: UI indicator distinguishing "slide mode active" from "poor connection".
AudioFirst floor too aggressive for low-bandwidth music calls. Mitigation: when audio profile is Opus 64k music, floor raised to 48 k.

Effort

~6 engineer-days (Wave 5 tasks T5.1–T5.3).

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