T5.3: EncoderMode::SlideFallback — SD-floor detection + VideoEncoder::set_mode() trait hook

crates/wzp-video/src/controller.rs

@@ -108,6 +108,10 @@ const BALANCED_AUDIO_RATIO: f64 = 0.15;
 /// Maximum bitrate change ratio per second (2x up or down).
 const MAX_CHANGE_RATIO_PER_SEC: f64 = 2.0;
 
+/// SD video floor (kbps).  When ScreenShare video budget drops below this,
+/// the controller recommends [`EncoderMode::SlideFallback`].
+const SD_VIDEO_FLOOR_KBPS: u32 = 150;
+
 /// Video quality controller.
 ///
 /// Consumes a [`BandwidthEstimator`] and a [`PriorityMode`] and produces
@@ -157,6 +161,23 @@ impl VideoQualityController {
         }
     }
 
+    /// Recommend the encoder operating mode based on priority + budget.
+    ///
+    /// Returns [`EncoderMode::SlideFallback`] when the current mode is
+    /// [`PriorityMode::ScreenShare`] and the video budget is below the
+    /// SD floor (150 kbps).  Otherwise returns [`EncoderMode::Normal`].
+    pub fn encoder_mode(&self) -> crate::EncoderMode {
+        if self.mode() != PriorityMode::ScreenShare {
+            return crate::EncoderMode::Normal;
+        }
+        let (_audio, video) = self.allocate();
+        if video < SD_VIDEO_FLOOR_KBPS {
+            crate::EncoderMode::SlideFallback
+        } else {
+            crate::EncoderMode::Normal
+        }
+    }
+
     /// Compute audio and video budgets from the current BWE and priority mode.
     ///
     /// Returns `(audio_budget_kbps, video_budget_kbps)`.
@@ -381,4 +402,34 @@ mod tests {
         ctrl.set_mode(PriorityMode::ScreenShare);
         assert_eq!(ctrl.mode(), PriorityMode::ScreenShare);
     }
+
+    #[test]
+    fn screenshare_above_floor_is_normal() {
+        // 1 Mbps → ~900 kbps after 90% factor. Video budget ~884 kbps > 150.
+        let bwe = dummy_bwe(1_000_000);
+        let ctrl = VideoQualityController::new(bwe);
+        ctrl.set_mode(PriorityMode::ScreenShare);
+        assert_eq!(ctrl.encoder_mode(), crate::EncoderMode::Normal);
+    }
+
+    #[test]
+    fn screenshare_below_floor_is_slide_fallback() {
+        // 100 kbps → ~90 kbps after 90% factor. Video budget ~74 kbps < 150.
+        let bwe = dummy_bwe(100_000);
+        let ctrl = VideoQualityController::new(bwe);
+        ctrl.set_mode(PriorityMode::ScreenShare);
+        assert_eq!(ctrl.encoder_mode(), crate::EncoderMode::SlideFallback);
+    }
+
+    #[test]
+    fn non_screenshare_never_slide_fallback() {
+        let bwe = dummy_bwe(50_000);
+        let ctrl = VideoQualityController::new(bwe);
+        ctrl.set_mode(PriorityMode::AudioFirst);
+        assert_eq!(ctrl.encoder_mode(), crate::EncoderMode::Normal);
+        ctrl.set_mode(PriorityMode::VideoFirst);
+        assert_eq!(ctrl.encoder_mode(), crate::EncoderMode::Normal);
+        ctrl.set_mode(PriorityMode::Balanced);
+        assert_eq!(ctrl.encoder_mode(), crate::EncoderMode::Normal);
+    }
 }

crates/wzp-video/src/encoder.rs

@@ -44,6 +44,11 @@ pub trait VideoEncoder: Send {
     /// Default implementation is a no-op; platform encoders override to
     /// reconfigure the underlying session.
     fn set_target(&mut self, _target: &crate::VideoTarget) {}
+
+    /// Switch the encoder operating mode (normal vs slide fallback).
+    ///
+    /// Default implementation is a no-op.
+    fn set_mode(&mut self, _mode: crate::EncoderMode) {}
 }
 
 /// Raw video frame input for encoding.

crates/wzp-video/src/encoder_mode.rs

@@ -0,0 +1,15 @@
+//! Encoder operating mode — normal continuous video or slide fallback.
+//!
+//! See `docs/PRD/PRD-video-quality-priority.md` (ScreenShare slide-fallback).
+
+/// Operating mode for the video encoder.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Default)]
+pub enum EncoderMode {
+    /// Normal continuous-frame encoding at the target fps.
+    #[default]
+    Normal,
+    /// Slide fallback: emit one high-quality I-frame every 2–5 s,
+    /// no P-frames.  Used when bandwidth is below the SD video floor
+    /// during a ScreenShare session.
+    SlideFallback,
+}

crates/wzp-video/src/lib.rs

@@ -8,6 +8,7 @@ pub mod controller;
 pub mod decoder;
 pub mod depacketizer;
 pub mod encoder;
+pub mod encoder_mode;
 pub mod framer;
 pub mod mediacodec;
 pub mod nack;
@@ -17,6 +18,7 @@ pub use controller::{VideoQualityController, VideoTarget};
 pub use decoder::VideoDecoder;
 pub use depacketizer::H264Depacketizer;
 pub use encoder::{VideoEncoder, VideoError, VideoFrame};
+pub use encoder_mode::EncoderMode;
 pub use framer::{FramedPacket, H264Framer};
 pub use nack::{CachedPacket, NackAction, NackReceiver, NackSender};
 pub use videotoolbox::{VideoToolboxDecoder, VideoToolboxEncoder};