playback issues

sam-tool-tauri/src-tauri/src/decoder.rs

@@ -309,6 +309,20 @@ impl Drop for DecoderSession {
     }
 }
 
+/// Pack a frame for the JS wire: 8-byte little-endian i64 frame index
+/// followed by the raw JPEG. The backend is authoritative on which frame
+/// actually decoded — JS reads the prefix to bind paints to the real frame
+/// number rather than the optimistic request, which matters when codec
+/// keyframe-snap or seek-ahead returns something other than the requested
+/// index. (Today our ffmpeg path is exact-seek so they always match, but
+/// the prefix makes the system robust to that ever changing.)
+pub fn pack_frame(idx: u64, jpeg: &[u8]) -> Vec<u8> {
+    let mut out = Vec::with_capacity(8 + jpeg.len());
+    out.extend_from_slice(&(idx as i64).to_le_bytes());
+    out.extend_from_slice(jpeg);
+    out
+}
+
 /// Pick a VAAPI render node. `SAM_TOOL_VAAPI_DEVICE` overrides; otherwise we
 /// probe `/dev/dri/renderD128..199` for the first that exists. Multi-GPU
 /// systems can end up with the integrated card on D128 and a discrete card

sam-tool-tauri/src-tauri/src/lib.rs

@@ -6,6 +6,7 @@ mod extract;
 mod sam;
 mod settings;
 mod state;
+mod stream;
 mod user;
 mod video;
 
@@ -62,12 +63,15 @@ fn current_video(state: State<'_, AppState>) -> Result<Option<VideoInfo>, String
 
 #[tauri::command]
 fn decode_frame(state: State<'_, AppState>, frame_idx: u64) -> Result<Response, String> {
-    let bytes = {
+    let jpeg = {
         let mut guard = state.decoder.lock().map_err(|e| format!("lock: {e}"))?;
         let decoder = guard.as_mut().ok_or("no video loaded")?;
         decoder.decode_at(frame_idx)?
     };
-    Ok(Response::new(bytes))
+    // Wire format: 8-byte LE frame number + JPEG bytes. JS unpacks the
+    // prefix in the worker so paints bind to the backend-authoritative
+    // index, not the optimistic request.
+    Ok(Response::new(decoder::pack_frame(frame_idx, &jpeg)))
 }
 
 #[tauri::command]
@@ -309,6 +313,10 @@ pub fn run() {
             sam_health_check,
             sam_list_models,
             sam_segment_bbox,
+            stream::start_stream,
+            stream::set_stream_state,
+            stream::stop_stream,
+            stream::stream_current_frame,
         ])
         .run(tauri::generate_context!())
         .expect("error while running tauri application");

sam-tool-tauri/src-tauri/src/state.rs

@@ -1,14 +1,22 @@
 use crate::decoder::DecoderSession;
-use std::sync::Mutex;
+use crate::stream::StreamControl;
+use std::sync::{Arc, Mutex};
 
 pub struct AppState {
-    pub decoder: Mutex<Option<DecoderSession>>,
+    /// Wrapped in Arc so the stream producer thread can hold its own
+    /// handle and reach back into the decoder without going through Tauri.
+    pub decoder: Arc<Mutex<Option<DecoderSession>>>,
+    /// Atomic control struct read by the stream producer each tick. JS
+    /// mutates via tauri commands; the producer polls it for play / pause /
+    /// speed / target / stride.
+    pub stream_control: Arc<StreamControl>,
 }
 
 impl AppState {
     pub fn new() -> Self {
         Self {
-            decoder: Mutex::new(None),
+            decoder: Arc::new(Mutex::new(None)),
+            stream_control: Arc::new(StreamControl::new()),
         }
     }
 }

sam-tool-tauri/src-tauri/src/stream.rs

@@ -0,0 +1,208 @@
+//! Forward playback transport. A Rust producer thread decodes via the
+//! existing DecoderSession and pushes packed frames to the JS side via a
+//! Tauri Channel — eliminates the per-frame IPC round-trip that pure
+//! `invoke()` playback paid for. Backward / step / scrub / pause stay on
+//! the invoke-pull path (see lib.rs::decode_frame).
+
+use crate::decoder::{pack_frame, DecoderSession};
+use crate::state::AppState;
+use std::sync::atomic::{AtomicBool, AtomicU64, Ordering};
+use std::sync::{Arc, Mutex};
+use std::thread;
+use std::time::{Duration, Instant};
+use tauri::ipc::{Channel, InvokeResponseBody};
+use tauri::State;
+
+pub struct StreamControl {
+    /// Bumped on each `start_stream`. Producer threads check this and exit
+    /// when it changes — guarantees only one producer at a time.
+    pub session_id: AtomicU64,
+    /// True while the producer should keep emitting. JS clears via
+    /// `set_stream_state` / `stop_stream` to pause.
+    pub playing: AtomicBool,
+    /// Speed multiplier × 100 (atomic u64; floats aren't atomic).
+    pub speed_x100: AtomicU64,
+    /// Next frame to emit. JS may overwrite this for in-stream seeks.
+    pub current_frame: AtomicU64,
+    /// Advance step (1 = every frame, N = every Nth — frame_skip).
+    pub stride: AtomicU64,
+    /// Stop when current_frame >= total_frames.
+    pub total_frames: AtomicU64,
+}
+
+impl StreamControl {
+    pub fn new() -> Self {
+        Self {
+            session_id: AtomicU64::new(0),
+            playing: AtomicBool::new(false),
+            speed_x100: AtomicU64::new(100),
+            current_frame: AtomicU64::new(0),
+            stride: AtomicU64::new(1),
+            total_frames: AtomicU64::new(0),
+        }
+    }
+}
+
+#[tauri::command]
+pub fn start_stream(
+    state: State<'_, AppState>,
+    channel: Channel<InvokeResponseBody>,
+    from_idx: u64,
+    speed: f64,
+    stride: u64,
+) -> Result<u64, String> {
+    let (total, fps) = {
+        let guard = state.decoder.lock().map_err(|e| format!("lock: {e}"))?;
+        let info = guard.as_ref().ok_or("no video loaded")?.info();
+        (info.total_frames, info.fps)
+    };
+    if from_idx >= total {
+        return Err("from_idx beyond total_frames".into());
+    }
+
+    let control = state.stream_control.clone();
+    // fetch_add returns the previous value; +1 = our new id. Any existing
+    // producer with the prior id will exit on its next iteration.
+    let sid = control.session_id.fetch_add(1, Ordering::SeqCst) + 1;
+    control
+        .speed_x100
+        .store((speed.max(0.01) * 100.0) as u64, Ordering::SeqCst);
+    control.current_frame.store(from_idx, Ordering::SeqCst);
+    control.stride.store(stride.max(1), Ordering::SeqCst);
+    control.total_frames.store(total, Ordering::SeqCst);
+    // playing must be the LAST flag we set — once it's true the producer
+    // can start consuming the other fields.
+    control.playing.store(true, Ordering::SeqCst);
+
+    let decoder = state.decoder.clone();
+    thread::spawn(move || {
+        producer_loop(decoder, control, channel, sid, fps);
+    });
+
+    Ok(sid)
+}
+
+#[tauri::command]
+pub fn set_stream_state(
+    state: State<'_, AppState>,
+    playing: Option<bool>,
+    speed: Option<f64>,
+    target_frame: Option<u64>,
+    stride: Option<u64>,
+) -> Result<(), String> {
+    let c = &state.stream_control;
+    if let Some(s) = speed {
+        c.speed_x100
+            .store((s.max(0.01) * 100.0) as u64, Ordering::SeqCst);
+    }
+    if let Some(t) = target_frame {
+        c.current_frame.store(t, Ordering::SeqCst);
+    }
+    if let Some(s) = stride {
+        c.stride.store(s.max(1), Ordering::SeqCst);
+    }
+    if let Some(p) = playing {
+        c.playing.store(p, Ordering::SeqCst);
+    }
+    Ok(())
+}
+
+#[tauri::command]
+pub fn stop_stream(state: State<'_, AppState>) -> Result<(), String> {
+    let c = &state.stream_control;
+    c.playing.store(false, Ordering::SeqCst);
+    // Bump session_id too so a producer that misses the playing flag still
+    // exits on its session check.
+    c.session_id.fetch_add(1, Ordering::SeqCst);
+    Ok(())
+}
+
+#[tauri::command]
+pub fn stream_current_frame(state: State<'_, AppState>) -> Result<u64, String> {
+    Ok(state.stream_control.current_frame.load(Ordering::Relaxed))
+}
+
+fn producer_loop(
+    decoder: Arc<Mutex<Option<DecoderSession>>>,
+    control: Arc<StreamControl>,
+    channel: Channel<InvokeResponseBody>,
+    my_sid: u64,
+    fps: f64,
+) {
+    let mut next_tick = Instant::now();
+    loop {
+        if control.session_id.load(Ordering::Relaxed) != my_sid {
+            return;
+        }
+        if !control.playing.load(Ordering::Relaxed) {
+            return;
+        }
+
+        let speed = (control.speed_x100.load(Ordering::Relaxed) as f64 / 100.0).max(0.01);
+        let stride = control.stride.load(Ordering::Relaxed).max(1);
+        let total = control.total_frames.load(Ordering::Relaxed);
+        let frame = control.current_frame.load(Ordering::Relaxed);
+
+        if total == 0 || frame >= total {
+            control.playing.store(false, Ordering::SeqCst);
+            return;
+        }
+
+        // Decode this frame via the existing DecoderSession (preserves
+        // hwaccel fallback + stderr capture + frame cache).
+        let jpeg = {
+            let mut guard = match decoder.lock() {
+                Ok(g) => g,
+                Err(_) => return,
+            };
+            match guard.as_mut() {
+                Some(d) => d.decode_at(frame),
+                None => Err("no video loaded".into()),
+            }
+        };
+
+        let send_ok = match jpeg {
+            Ok(bytes) => {
+                let packed = pack_frame(frame, &bytes);
+                channel.send(InvokeResponseBody::Raw(packed)).is_ok()
+            }
+            Err(_) => {
+                // Surface decode errors as an empty packed frame so JS knows
+                // to stop. With just an 8-byte prefix and no JPEG it's
+                // unambiguous on the receive side.
+                let packed = pack_frame(frame, &[]);
+                let _ = channel.send(InvokeResponseBody::Raw(packed));
+                false
+            }
+        };
+        if !send_ok {
+            // Channel closed (frontend tore down) — stop cleanly.
+            control.playing.store(false, Ordering::SeqCst);
+            return;
+        }
+
+        // Advance the cursor. JS may have already moved it (an in-stream
+        // seek) — in that case the next iteration picks up the new value.
+        let next_frame = frame.saturating_add(stride);
+        // Only advance if no one else moved it (compare_exchange would be
+        // strictly correct, but a plain store is fine: a race here just
+        // means we emit the seeked-to frame on the next iteration).
+        let _ = control.current_frame.compare_exchange(
+            frame,
+            next_frame,
+            Ordering::SeqCst,
+            Ordering::Relaxed,
+        );
+
+        // Pace: stride frames per (fps × speed) per second.
+        let frame_interval = Duration::from_secs_f64(stride as f64 / (fps * speed));
+        next_tick += frame_interval;
+        let now = Instant::now();
+        if next_tick > now {
+            thread::sleep(next_tick - now);
+        } else {
+            // Behind real-time — reset baseline instead of accumulating debt.
+            next_tick = now;
+        }
+    }
+}

sam-tool-tauri/src/ipc.ts

@@ -1,4 +1,4 @@
-import { invoke } from "@tauri-apps/api/core";
+import { invoke, Channel } from "@tauri-apps/api/core";
 import type {
   CocoCategory,
   CocoShape,
@@ -70,6 +70,25 @@ export const api = {
   readFolderSource: (folder: string) =>
     invoke<string | null>("read_folder_source", { folder }),
 
+  // Forward playback uses a Rust producer thread that pushes packed-frame
+  // ArrayBuffers via this channel. Pass `new Channel<ArrayBuffer>()` from the
+  // caller and attach `.onmessage` before invoking — Rust starts sending
+  // immediately.
+  startStream: (
+    channel: Channel<ArrayBuffer>,
+    fromIdx: number,
+    speed: number,
+    stride: number
+  ) => invoke<number>("start_stream", { channel, fromIdx, speed, stride }),
+  setStreamState: (state: {
+    playing?: boolean;
+    speed?: number;
+    targetFrame?: number;
+    stride?: number;
+  }) => invoke<void>("set_stream_state", state),
+  stopStream: () => invoke<void>("stop_stream"),
+  streamCurrentFrame: () => invoke<number>("stream_current_frame"),
+
   getSettings: () => invoke<Settings>("get_settings"),
   saveSettings: (settings: Settings) =>
     invoke<void>("save_settings", { settings }),

sam-tool-tauri/src/modes/ExtractMode.tsx

@@ -1,10 +1,12 @@
 import { useCallback, useEffect, useMemo, useRef, useState } from "react";
 import { open as openDialog } from "@tauri-apps/plugin-dialog";
 import { listen } from "@tauri-apps/api/event";
+import { Channel } from "@tauri-apps/api/core";
 import { api } from "../ipc";
 import type { CocoShape, ImportedCoco, Settings, VideoInfo } from "../types";
 import { FrameCanvas } from "../components/extract/FrameCanvas";
 import { drawOverlay } from "../components/extract/overlay";
+import { sharedFrameDecoderPool } from "../services/frameDecoderPool";
 
 const SPEEDS = [0.25, 0.5, 1, 1.5, 2, 3, 4];
 const FRAME_BITMAP_CACHE_LIMIT = 24;
@@ -186,34 +188,44 @@ export function ExtractMode({ username, settings }: Props) {
   );
 
   // Returns true if we drew, false if we were preempted by a newer request,
-  // and throws if bitmap creation failed — pump uses this to decide whether
-  // to mark the frame rendered or surface the error to the playback loop.
+  // and throws if worker decode failed. The packed buffer carries an 8-byte
+  // LE frame-number prefix; the worker is authoritative on which frame
+  // actually decoded, so we use that for caching + drawing rather than the
+  // optimistic `atFrameIdx` (`atFrameIdx` only stays useful as a fallback
+  // for the preempt check).
   const renderFrame = useCallback(
     async (bytes: ArrayBuffer, atFrameIdx: number): Promise<boolean> => {
-      // Diagnostic timing log: set `window.SAM_FRAME_DEBUG = true` in devtools
-      // on a slow machine to see per-frame bitmap and draw timings — confirms
-      // where the bottleneck lives (decode vs createImageBitmap vs drawImage).
       const debug = (window as unknown as { SAM_FRAME_DEBUG?: boolean })
         .SAM_FRAME_DEBUG === true;
       const t0 = debug ? performance.now() : 0;
-      const blob = new Blob([bytes], { type: "image/jpeg" });
+      let frameNumber: number;
       let bitmap: ImageBitmap;
       try {
-        bitmap = await createImageBitmap(blob);
+        const result = await sharedFrameDecoderPool.decode(bytes);
+        frameNumber = result.frameNumber;
+        bitmap = result.bitmap;
       } catch (e) {
         throw new Error(
-          `createImageBitmap failed: ${e instanceof Error ? e.message : String(e)}`
+          `worker decode failed: ${e instanceof Error ? e.message : String(e)}`
         );
       }
       const t1 = debug ? performance.now() : 0;
-      rememberBitmap(atFrameIdx, bitmap);
-      if (requestedIdx.current !== atFrameIdx) return false;
-      drawBitmap(bitmap, atFrameIdx);
+      rememberBitmap(frameNumber, bitmap);
+      // Preempt check: if a newer request landed during decode, drop this
+      // paint. Match against either the prefix frame number (authoritative)
+      // or the original request — keyframe snap can make them disagree.
+      if (
+        requestedIdx.current !== frameNumber &&
+        requestedIdx.current !== atFrameIdx
+      ) {
+        return false;
+      }
+      drawBitmap(bitmap, frameNumber);
       if (debug) {
         const t2 = performance.now();
         // eslint-disable-next-line no-console
         console.debug(
-          `[SAM] f${atFrameIdx} bytes=${bytes.byteLength} bitmap=${(t1 - t0).toFixed(0)}ms draw=${(t2 - t1).toFixed(0)}ms`
+          `[SAM] f${frameNumber} bytes=${bytes.byteLength} decode=${(t1 - t0).toFixed(0)}ms draw=${(t2 - t1).toFixed(0)}ms`
         );
       }
       return true;
@@ -329,109 +341,126 @@ export function ExtractMode({ username, settings }: Props) {
     return () => ro.disconnect();
   }, [video, drawBitmap, getCachedBitmap, pump]);
 
-  // --- playback loop ---------------------------------------------------
-  // Render-paced: advance ONLY after the previous frame actually drew. On a
-  // fast box this hits target fps; on a slow box it plays smoothly at the
-  // achievable decode/render rate instead of dropping 9-of-10 frames at the
-  // canvas while the wall-clock UI counter races ahead.
+  // --- playback (Channel-push transport) -----------------------------------
+  // Forward playback runs through a Rust producer thread that pushes packed
+  // frames over a Tauri Channel — eliminates the per-frame IPC round-trip
+  // that was the main stutter source. Backward / step / scrub / pause all
+  // stay on the invoke-pull path (pump() above). Three guards live here:
+  //  * `paused` — set true on review-mode stop so any in-flight messages
+  //    queued behind us don't paint;
+  //  * `lastPainted` — direction-aware out-of-order drop in case the worker
+  //    pool returns N+1 before N;
+  //  * cleanup stops the stream and re-fetches the last painted frame via
+  //    invoke so the canvas exactly matches `frameIdx` on pause.
   useEffect(() => {
     if (!playing || !video) return;
     let cancelled = false;
-    let nextDueAt = performance.now();
-
-    const waitForFrame = (target: number) =>
-      new Promise<void>((resolve) => {
-        const check = () => {
-          if (
-            cancelled ||
-            renderedIdx.current === target ||
-            decodeErrorRef.current !== null
-          ) {
-            resolve();
+    let paused = false;
+    let lastPainted = frameIdxRef.current;
+    const skip = Math.max(1, frameSkipRef.current | 0);
+    const startFrom = Math.min(
+      video.total_frames - 1,
+      frameIdxRef.current + skip
+    );
+    if (startFrom <= frameIdxRef.current) {
+      setPlaying(false);
       return;
     }
-          requestAnimationFrame(check);
+
+    const channel = new Channel<ArrayBuffer>();
+    channel.onmessage = async (buffer: ArrayBuffer) => {
+      if (cancelled || paused) return;
+      // Empty-frame sentinel from the producer = decode failed.
+      if (buffer.byteLength <= 8) {
+        setPlaybackError("decode failed during playback");
+        setPlaying(false);
+        return;
+      }
+      let frameNumber: number;
+      let bitmap: ImageBitmap;
+      try {
+        const result = await sharedFrameDecoderPool.decode(buffer);
+        frameNumber = result.frameNumber;
+        bitmap = result.bitmap;
+      } catch (e) {
+        if (cancelled) return;
+        setPlaybackError(e instanceof Error ? e.message : String(e));
+        setPlaying(false);
+        return;
+      }
+      if (cancelled || paused) {
+        bitmap.close();
+        return;
+      }
+      // Direction-aware out-of-order drop: forward play is monotonic, so
+      // any frame number < the last painted came from a parallel worker
+      // and is stale. Cheap defence against worker-pool races.
+      if (frameNumber < lastPainted) {
+        bitmap.close();
+        return;
+      }
+      lastPainted = frameNumber;
+      rememberBitmap(frameNumber, bitmap);
+      drawBitmap(bitmap, frameNumber);
+      renderedIdx.current = frameNumber;
+      requestedIdx.current = frameNumber;
+      frameIdxRef.current = frameNumber;
+      setFrameIdx(frameNumber);
+      decodeErrorRef.current = null;
+
+      // Review-mode stops AT the annotated frame, so the canvas already
+      // shows it by the time we ask the producer to halt.
+      if (reviewMode && annotatedFrames.current.has(frameNumber)) {
+        paused = true;
+        try { await api.stopStream(); } catch { /* ignore */ }
+        setPlaying(false);
+      }
     };
-        check();
+
+    api
+      .startStream(channel, startFrom, speed, skip)
+      .catch((e) => {
+        if (cancelled) return;
+        setPlaybackError(`start_stream failed: ${e}`);
+        setPlaying(false);
       });
 
-    const loop = async () => {
-      while (!cancelled && playing) {
-        const skip = Math.max(1, frameSkipRef.current | 0);
-        const current = frameIdxRef.current;
-        let target = Math.min(video.total_frames - 1, current + skip);
-        if (target <= current) {
-          setPlaying(false);
-          return;
-        }
-        // Review-mode scan: with skip > 1 a naive +skip jump can leap past
-        // annotations between current+1 and current+skip. Walk that window
-        // and clamp `target` down to the first annotated frame.
-        if (reviewMode) {
-          for (let i = current + 1; i <= target; i++) {
-            if (annotatedFrames.current.has(i)) {
-              target = i;
-              break;
-            }
-          }
-        }
-        requestedIdx.current = target;
-        // Fast path: await pump directly. When pump owns the queue it
-        // resolves the instant target is drawn (no 16ms rAF latency tax).
-        // If another caller (ResizeObserver / overlay-change effect) was
-        // already running pump, our call short-circuits — fall back to
-        // rAF polling in that case so we still wait correctly.
-        await pump();
-        if (
-          !cancelled &&
-          renderedIdx.current !== target &&
-          decodeErrorRef.current === null
-        ) {
-          await waitForFrame(target);
-        }
-        if (cancelled) return;
-        // Decode failed mid-play: stop and surface so the UI doesn't claim to
-        // be playing while the canvas is frozen on the last good frame.
-        if (decodeErrorRef.current) {
-          setPlaybackError(decodeErrorRef.current);
-          setPlaying(false);
-          return;
-        }
-        const advanced = Math.max(1, target - current);
-        // Real-time pacing regardless of skip: skip controls density,
-        // `speed` controls speed. Without the advanced-frame factor here,
-        // skip=3 would play at 3x wall-clock, which is what the speed slider
-        // is for. Compute after review-mode clamping so annotated stops do not
-        // wait for the full skip window.
-        const interval = (advanced * 1000) / (video.fps * speed);
-        // Keep the fast playback loop's cursor in sync immediately. Waiting
-        // for React's frameIdx effect can make a slow decode path replay the
-        // same target once and add an extra frame interval per rendered frame.
-        frameIdxRef.current = target;
-        setFrameIdx(target);
-        // Pause AFTER rendering so the user actually sees the annotated frame.
-        if (reviewMode && annotatedFrames.current.has(target)) {
-          setPlaying(false);
-          return;
-        }
-
-        const now = performance.now();
-        const due = nextDueAt + interval;
-        if (due > now) {
-          await new Promise((r) => setTimeout(r, due - now));
-          nextDueAt = due;
-        } else {
-          // Behind real-time — don't accumulate debt and then "catch up" by
-          // skipping frames; just reset the clock to now.
-          nextDueAt = now;
-        }
-      }
-    };
-    loop();
     return () => {
       cancelled = true;
+      // Fire-and-forget: stop the producer, then re-fetch the last painted
+      // frame via the authoritative invoke path so the canvas matches
+      // `frameIdx` exactly (the producer may have been mid-flight).
+      const lastFrame = frameIdxRef.current;
+      api
+        .stopStream()
+        .catch(() => { /* ignore */ })
+        .then(() => api.decodeFrame(lastFrame))
+        .then((buf) => sharedFrameDecoderPool.decode(buf))
+        .then(({ frameNumber, bitmap }) => {
+          // Newer request may have started by the time this resolves —
+          // only paint if it still matches.
+          if (requestedIdx.current !== frameNumber) {
+            bitmap.close();
+            return;
+          }
+          rememberBitmap(frameNumber, bitmap);
+          drawBitmap(bitmap, frameNumber);
+          renderedIdx.current = frameNumber;
+        })
+        .catch(() => { /* invoke path will recover on next seek */ });
     };
-  }, [playing, video, speed, reviewMode, pump]);
+  }, [playing, video, speed, reviewMode, drawBitmap, rememberBitmap]);
+
+  // Live-update the producer when speed or frame-skip changes mid-play, so
+  // the user doesn't have to pause + resume to see the new setting.
+  useEffect(() => {
+    if (!playing) return;
+    api.setStreamState({ speed }).catch(() => { /* ignore */ });
+  }, [playing, speed]);
+  useEffect(() => {
+    if (!playing) return;
+    api.setStreamState({ stride: Math.max(1, settings?.frame_skip ?? 1) }).catch(() => { /* ignore */ });
+  }, [playing, settings?.frame_skip]);
 
   // --- navigation helpers ----------------------------------------------
   const seekToFrame = useCallback((idx: number) => {
@@ -443,11 +472,19 @@ export function ExtractMode({ username, settings }: Props) {
     if (cached) {
       drawBitmap(cached, clamped);
       renderedIdx.current = clamped;
-    } else {
+    } else if (!playing) {
+      // Pump only handles seeks while paused. During Channel-push play the
+      // producer is the source of frames; we re-target it instead.
       pump();
     }
+    if (playing) {
+      // Hand the new target to the running producer so it resumes from the
+      // scrub destination instead of where it was. The producer's monotonic
+      // session_id keeps the existing channel valid.
+      api.setStreamState({ targetFrame: clamped }).catch(() => { /* ignore */ });
+    }
     setFrameIdx((prev) => (prev === clamped ? prev : clamped));
-  }, [drawBitmap, getCachedBitmap, pump, video]);
+  }, [drawBitmap, getCachedBitmap, pump, video, playing]);
 
   const stepFrames = useCallback((delta: number) => {
     if (!video) return;

sam-tool-tauri/src/services/frameDecoderPool.ts

@@ -0,0 +1,118 @@
+// Worker pool that takes a packed-frame ArrayBuffer (8-byte LE i64 frame
+// number + JPEG bytes), decodes the JPEG to an ImageBitmap off the main
+// thread, and returns both via transferable postMessage so the main thread
+// only does the final drawImage. Inlined as a Blob URL so we don't need a
+// separate build entry for the worker file.
+
+const WORKER_SOURCE = `
+self.onmessage = async (e) => {
+  const { id, buffer } = e.data;
+  let frameNumber = -1;
+  try {
+    const headerView = new BigInt64Array(buffer, 0, 1);
+    frameNumber = Number(headerView[0]);
+    const jpegView = new Uint8Array(buffer, 8);
+    // Copy out of the transferred buffer so Blob ownership is independent.
+    const jpegCopy = jpegView.slice();
+    const blob = new Blob([jpegCopy], { type: "image/jpeg" });
+    const bitmap = await createImageBitmap(blob);
+    self.postMessage({ id, frameNumber, bitmap }, [bitmap]);
+  } catch (err) {
+    const msg = err && err.message ? err.message : String(err);
+    self.postMessage({ id, frameNumber, error: msg });
+  }
+};
+`;
+
+const workerBlob = new Blob([WORKER_SOURCE], {
+  type: "application/javascript",
+});
+const workerUrl = URL.createObjectURL(workerBlob);
+
+export interface DecodeResult {
+  frameNumber: number;
+  bitmap: ImageBitmap;
+}
+
+interface Pending {
+  resolve: (r: DecodeResult) => void;
+  reject: (e: Error) => void;
+}
+
+interface WorkerMessage {
+  id: number;
+  frameNumber: number;
+  bitmap?: ImageBitmap;
+  error?: string;
+}
+
+export class FrameDecoderPool {
+  private workers: Worker[] = [];
+  private next = 0;
+  private pending = new Map<number, Pending>();
+  private nextId = 1;
+
+  constructor(size: number = 2) {
+    const count = Math.max(1, Math.min(size, 8));
+    for (let i = 0; i < count; i++) {
+      const worker = new Worker(workerUrl);
+      worker.onmessage = (e: MessageEvent<WorkerMessage>) => {
+        const { id, frameNumber, bitmap, error } = e.data;
+        const slot = this.pending.get(id);
+        if (!slot) {
+          // The caller discarded this request — close the bitmap so it
+          // doesn't leak (an ImageBitmap holds GPU memory).
+          bitmap?.close?.();
+          return;
+        }
+        this.pending.delete(id);
+        if (error || !bitmap) {
+          slot.reject(new Error(error ?? "decode failed"));
+        } else {
+          slot.resolve({ frameNumber, bitmap });
+        }
+      };
+      worker.onerror = (e) => {
+        // Surface worker-thread errors to any pending request on this
+        // worker. We don't know which id failed, so reject the oldest.
+        const first = this.pending.entries().next().value;
+        if (first) {
+          const [id, slot] = first;
+          this.pending.delete(id);
+          slot.reject(new Error(`worker error: ${e.message}`));
+        }
+      };
+      this.workers.push(worker);
+    }
+  }
+
+  /**
+   * Decode a packed-frame buffer. The buffer ownership transfers to the
+   * worker — do not read it after this call returns.
+   */
+  decode(buffer: ArrayBuffer): Promise<DecodeResult> {
+    return new Promise((resolve, reject) => {
+      const id = this.nextId++;
+      this.pending.set(id, { resolve, reject });
+      const worker = this.workers[this.next];
+      this.next = (this.next + 1) % this.workers.length;
+      worker.postMessage({ id, buffer }, [buffer]);
+    });
+  }
+
+  destroy() {
+    for (const w of this.workers) w.terminate();
+    this.workers = [];
+    for (const slot of this.pending.values()) {
+      slot.reject(new Error("decoder pool destroyed"));
+    }
+    this.pending.clear();
+  }
+}
+
+/**
+ * Shared pool used by ExtractMode (and later, AnnotateMode). Two workers is
+ * enough for sequential playback decode; raising it helps the seek path
+ * when several scrub requests overlap.
+ */
+export const sharedFrameDecoderPool = new FrameDecoderPool(2);