Add dataset record liveview

.gitignore

@@ -1,6 +1,7 @@
 # ============================
 # Python
 # ============================
+**/logs/**
 __pycache__/
 *.py[cod]
 *$py.class

README.md

@@ -181,6 +181,17 @@ uv run record --config_path config/gello/xarm7_gello_record_config.yaml -a
 
 Controls: `Space` start the episode, `→` save it, `←` discard and re-record it, `Esc` stop recording. The arm resets to its initial point between episodes.
 
+The xArm7 GELLO example also enables a lightweight browser camera preview at
+`http://127.0.0.1:8765/` (or `http://<recorder-ip>:8765/` from another machine).
+It reuses frames already captured by the recorder: JPEG encoding and HTTP
+streaming run on background threads, the preview is capped at 8 FPS, and stale
+preview frames are dropped rather than delaying dataset collection. Disable it
+or tune it in the top-level `web_preview` section of the YAML configuration.
+When `robot.enable_logs` is enabled for diagnostics, one
+`logs/gello_record_sync_*.csv` file is written per episode. The
+`preview_clients`, `record_period_ms`, `frame_overrun_ms`, `action_age_ms`, and
+`preview_publish_ms` columns can be used to measure preview timing impact.
+
 > During collection the **relative position between the robot arm and the camera must not change**, and the camera setup at inference time must match the one used during collection. If the arm or camera moves, previously collected data becomes invalid.
 
 > If the dataset root already exists and `-r` is not given, the script asks whether to overwrite it, resume, or cancel.

config/gello/xarm7_gello_record_config.yaml

@@ -51,6 +51,17 @@ teleop:
   gripper_open_deg: 198.28125
   gripper_close_deg: 155.75
 
+# Lossy browser preview of the frames already captured for the dataset.
+# JPEG encoding and network I/O run in the background; preview frames are
+# dropped instead of delaying the 30 Hz recorder or 60 Hz GELLO control loop.
+web_preview:
+  enabled: true
+  host: "0.0.0.0"
+  port: 8765
+  fps: 8
+  width: 480
+  jpeg_quality: 65
+
 dataset:
   # Dataset path relative to the directory where the command is started.
   root: "datasets/xarm7_gello_joint_safe_datas"

src/lerobot_robot_ufactory/scripts/uf_lerobot_record.py

@@ -7,7 +7,7 @@ import argparse
 import logging
 import shutil
 import threading
-from dataclasses import dataclass
+from dataclasses import dataclass, field
 from pathlib import Path
 import lerobot_robot_ufactory # patch
 from lerobot.scripts.lerobot_record import *
@@ -16,13 +16,17 @@ from lerobot_robot_ufactory.teleoperators.uf_mock_teleop import UFMockTeleop
 from lerobot_robot_ufactory.teleoperators.base_teleop import UFBaseTeleop
 from lerobot_robot_ufactory.utils.realtime_teleop import RealtimeTeleopController
 from lerobot_robot_ufactory.utils.utils import init_keyboard_listener
+from lerobot_robot_ufactory.utils.web_preview import RecordingWebPreview, WebPreviewConfig
 
 
 @dataclass
 class UFRecordConfig(LeRobotRecordConfig):
     """RecordConfig variant that permits UFACTORY manual-mode recording."""
 
+    web_preview: WebPreviewConfig = field(default_factory=WebPreviewConfig)
+
     def __post_init__(self):
+        self.web_preview.validate()
         manual_mode = getattr(self.robot, "manual_mode", False)
         if manual_mode:
             if self.teleop is not None or self.policy is not None:
@@ -276,19 +280,24 @@ def _disconnect_recording_resources(robot, teleop, listener):
 
 
 class _RecordingCleanup:
-    def __init__(self, robot, teleop, listener, async_episode_saver):
+    def __init__(self, robot, teleop, listener, async_episode_saver, web_preview=None):
         self.robot = robot
         self.teleop = teleop
         self.listener = listener
         self.async_episode_saver = async_episode_saver
+        self.web_preview = web_preview
 
     def __enter__(self):
         return self
 
     def __exit__(self, exc_type, exc_value, traceback):
+        try:
             try:
                 if self.async_episode_saver is not None:
                     self.async_episode_saver.close()
+            finally:
+                if self.web_preview is not None:
+                    self.web_preview.stop()
         finally:
             _disconnect_recording_resources(self.robot, self.teleop, self.listener)
         return False
@@ -321,6 +330,7 @@ def record_loop(
     manual_mode: bool = False,
     manual_gripper_keys: dict[str, bool] | None = None,
     manual_gripper_speed: float = 0.5,
+    web_preview: RecordingWebPreview | None = None,
 ):
     if dataset is not None and dataset.fps != fps:
         raise ValueError(f"The dataset fps should be equal to requested fps ({dataset.fps} != {fps}).")
@@ -402,7 +412,16 @@ def record_loop(
                     "observation_end_s",
                     "state_to_observation_end_ms",
                     "camera_timings",
+                    "preview_publish_ms",
+                    "preview_clients",
+                    "preview_source_generation",
+                    "preview_encoded_frames",
+                    "preview_last_encode_ms",
+                    "preview_max_encode_ms",
+                    "record_period_ms",
                     "frame_loop_ms",
+                    "frame_budget_ms",
+                    "frame_overrun_ms",
                 ],
             )
             sync_log_writer.writeheader()
@@ -410,8 +429,15 @@ def record_loop(
 
     timestamp = 0
     start_episode_t = time.perf_counter()
+    previous_loop_start_t = None
     while timestamp < control_time_s:
         start_loop_t = time.perf_counter()
+        record_period_ms = (
+            0.0
+            if previous_loop_start_t is None
+            else (start_loop_t - previous_loop_start_t) * 1000
+        )
+        previous_loop_start_t = start_loop_t
 
         if events["exit_early"]:
             events["exit_early"] = False
@@ -435,6 +461,13 @@ def record_loop(
 
         # Applies a pipeline to the raw robot observation, default is IdentityProcessor
         obs_processed = robot_observation_processor(obs)
+        preview_publish_ms = 0.0
+        if web_preview is not None:
+            # This only replaces references in a latest-frame slot. All image
+            # processing and network I/O remain on preview background threads.
+            before_preview_publish_t = time.perf_counter()
+            web_preview.publish(obs_processed)
+            preview_publish_ms = (time.perf_counter() - before_preview_publish_t) * 1000
 
         if policy is not None or dataset is not None:
             observation_frame = build_dataset_frame(dataset.features, obs_processed, prefix=OBS_STR)
@@ -542,6 +575,9 @@ def record_loop(
                 robot, "_last_realtime_observation_end_monotonic_s", observation_monotonic_s
             )
             camera_timings = getattr(robot, "_last_realtime_camera_timings", {})
+            preview_stats = web_preview.timing_stats() if web_preview is not None else {}
+            frame_loop_ms = (time.perf_counter() - start_loop_t) * 1000
+            frame_budget_ms = 1000 / fps
             sync_log_writer.writerow(
                 {
                     "frame": sync_frame_index,
@@ -551,7 +587,18 @@ def record_loop(
                     "observation_end_s": f"{observation_end_s:.9f}",
                     "state_to_observation_end_ms": f"{(observation_end_s - observation_monotonic_s) * 1000:.3f}",
                     "camera_timings": repr(camera_timings),
-                    "frame_loop_ms": f"{(time.perf_counter() - start_loop_t) * 1000:.3f}",
+                    "preview_publish_ms": f"{preview_publish_ms:.6f}",
+                    "preview_clients": preview_stats.get("preview_clients", 0),
+                    "preview_source_generation": preview_stats.get(
+                        "preview_source_generation", 0
+                    ),
+                    "preview_encoded_frames": preview_stats.get("preview_encoded_frames", 0),
+                    "preview_last_encode_ms": f'{preview_stats.get("preview_last_encode_ms", 0.0):.3f}',
+                    "preview_max_encode_ms": f'{preview_stats.get("preview_max_encode_ms", 0.0):.3f}',
+                    "record_period_ms": f"{record_period_ms:.3f}",
+                    "frame_loop_ms": f"{frame_loop_ms:.3f}",
+                    "frame_budget_ms": f"{frame_budget_ms:.3f}",
+                    "frame_overrun_ms": f"{max(0.0, frame_loop_ms - frame_budget_ms):.3f}",
                 }
             )
             sync_frame_index += 1
@@ -759,12 +806,23 @@ def record(cfg: UFRecordConfig, async_save: bool = False) -> LeRobotDataset:
             },
         )
 
+    web_preview = None
     try:
         robot.connect()
         if teleop is not None:
             teleop.connect()
+        if cfg.web_preview.enabled:
+            web_preview = RecordingWebPreview(cfg.web_preview)
+            web_preview.start()
+            print(f"Camera web preview: {web_preview.url}")
+            if cfg.web_preview.host == "0.0.0.0":
+                print(
+                    f"From another machine: http://<recorder-ip>:{cfg.web_preview.port}/"
+                )
     except BaseException:
         try:
+            if web_preview is not None:
+                web_preview.stop()
             _disconnect_recording_resources(robot, teleop, None)
         except BaseException:
             logging.exception("Failed to clean up after recording device connection failure")
@@ -829,7 +887,9 @@ def record(cfg: UFRecordConfig, async_save: bool = False) -> LeRobotDataset:
     if async_episode_saver is not None:
         print('Async episode saving is enabled.')
 
-    with _RecordingCleanup(robot, teleop, listener, async_episode_saver), VideoEncodingManager(dataset):
+    with _RecordingCleanup(
+        robot, teleop, listener, async_episode_saver, web_preview
+    ), VideoEncodingManager(dataset):
         recorded_episodes = 0
         while recorded_episodes < cfg.dataset.num_episodes and not events["stop_recording"]:
             time.sleep(0.01)
@@ -875,6 +935,7 @@ def record(cfg: UFRecordConfig, async_save: bool = False) -> LeRobotDataset:
                     manual_mode=manual_mode,
                     manual_gripper_keys=manual_gripper_keys,
                     manual_gripper_speed=getattr(cfg.robot, "manual_gripper_speed", 0.5),
+                    web_preview=web_preview,
                 )
             else:
                 continue

src/lerobot_robot_ufactory/utils/web_preview.py

@@ -0,0 +1,332 @@
+"""Best-effort web preview for frames already captured by a recording loop.
+
+The recorder only publishes references to its latest frames.  Resizing, JPEG
+encoding, and network writes happen on background threads, and preview frames
+are deliberately dropped whenever those threads cannot keep up.
+"""
+
+from __future__ import annotations
+
+import http.server
+import json
+import logging
+import threading
+import time
+from dataclasses import dataclass
+from typing import Any
+from urllib.parse import parse_qs, urlsplit
+
+import cv2
+import numpy as np
+
+
+logger = logging.getLogger(__name__)
+
+
+@dataclass
+class WebPreviewConfig:
+    """Configuration for the optional recording-time camera preview."""
+
+    enabled: bool = False
+    host: str = "0.0.0.0"
+    port: int = 8765
+    fps: float = 8.0
+    width: int = 480
+    jpeg_quality: int = 65
+
+    def validate(self) -> None:
+        if not self.host:
+            raise ValueError("web_preview.host must not be empty")
+        if not 1 <= self.port <= 65535:
+            raise ValueError("web_preview.port must be between 1 and 65535")
+        if self.fps <= 0:
+            raise ValueError("web_preview.fps must be positive")
+        if self.width <= 0:
+            raise ValueError("web_preview.width must be positive")
+        if not 1 <= self.jpeg_quality <= 100:
+            raise ValueError("web_preview.jpeg_quality must be between 1 and 100")
+
+
+class _PreviewFeed:
+    def __init__(self, name: str) -> None:
+        self.name = name
+        self.condition = threading.Condition()
+        self.jpeg: bytes | None = None
+        self.frame_id = 0
+        self.clients = 0
+        self.stopped = False
+
+    def add_client(self) -> None:
+        with self.condition:
+            self.clients += 1
+
+    def remove_client(self) -> None:
+        with self.condition:
+            self.clients = max(0, self.clients - 1)
+
+    def publish_jpeg(self, jpeg: bytes) -> None:
+        with self.condition:
+            self.jpeg = jpeg
+            self.frame_id += 1
+            self.condition.notify_all()
+
+    def wait_for_jpeg(self, previous_id: int) -> tuple[bytes | None, int, bool]:
+        with self.condition:
+            self.condition.wait_for(
+                lambda: self.frame_id > previous_id or self.stopped,
+                timeout=1.0,
+            )
+            return self.jpeg, self.frame_id, self.stopped
+
+    def stop(self) -> None:
+        with self.condition:
+            self.stopped = True
+            self.condition.notify_all()
+
+
+class RecordingWebPreview:
+    """Serve a lossy, asynchronous preview of recording observations."""
+
+    def __init__(self, config: WebPreviewConfig) -> None:
+        config.validate()
+        self.config = config
+        self._condition = threading.Condition()
+        self._latest_frames: dict[str, np.ndarray] = {}
+        self._source_generation = 0
+        self._encoded_frames = 0
+        self._last_encode_ms = 0.0
+        self._max_encode_ms = 0.0
+        self._stop_event = threading.Event()
+        self._feeds: dict[str, _PreviewFeed] = {}
+        self._encoder_thread: threading.Thread | None = None
+        self._server: http.server.ThreadingHTTPServer | None = None
+        self._server_thread: threading.Thread | None = None
+
+    @property
+    def url(self) -> str:
+        host = "127.0.0.1" if self.config.host == "0.0.0.0" else self.config.host
+        return f"http://{host}:{self.config.port}/"
+
+    def start(self) -> None:
+        if self._server is not None:
+            return
+        handler = type("RecordingPreviewHandler", (_PreviewHandler,), {"preview": self})
+        self._server = http.server.ThreadingHTTPServer(
+            (self.config.host, self.config.port), handler
+        )
+        self._server.daemon_threads = True
+        self._encoder_thread = threading.Thread(
+            target=self._encode_loop,
+            name="uf-recording-preview-encoder",
+            daemon=True,
+        )
+        self._server_thread = threading.Thread(
+            target=self._server.serve_forever,
+            name="uf-recording-preview-http",
+            daemon=True,
+        )
+        self._encoder_thread.start()
+        self._server_thread.start()
+
+    def publish(self, observation: dict[str, Any]) -> None:
+        """Non-blockingly replace the latest previewable image references."""
+        frames = {
+            key: value
+            for key, value in observation.items()
+            if isinstance(value, np.ndarray) and value.ndim == 3 and value.shape[2] in (3, 4)
+        }
+        if not frames:
+            return
+        with self._condition:
+            self._latest_frames = frames
+            for name in frames:
+                self._feeds.setdefault(name, _PreviewFeed(name))
+            self._source_generation += 1
+            self._condition.notify()
+
+    def camera_names(self) -> list[str]:
+        with self._condition:
+            return sorted(self._feeds)
+
+    def feed(self, name: str | None) -> _PreviewFeed | None:
+        if name is None:
+            return None
+        with self._condition:
+            return self._feeds.get(name)
+
+    def timing_stats(self) -> dict[str, float | int]:
+        """Return a cheap snapshot for recording synchronization diagnostics."""
+        with self._condition:
+            return {
+                "preview_clients": sum(feed.clients for feed in self._feeds.values()),
+                "preview_source_generation": self._source_generation,
+                "preview_encoded_frames": self._encoded_frames,
+                "preview_last_encode_ms": self._last_encode_ms,
+                "preview_max_encode_ms": self._max_encode_ms,
+            }
+
+    def _has_clients(self) -> bool:
+        return any(feed.clients > 0 for feed in self._feeds.values())
+
+    def _encode_loop(self) -> None:
+        last_generation = 0
+        next_encode_at = 0.0
+        period = 1.0 / self.config.fps
+        while not self._stop_event.is_set():
+            with self._condition:
+                self._condition.wait_for(
+                    lambda: self._stop_event.is_set()
+                    or (
+                        self._source_generation > last_generation
+                        and self._has_clients()
+                    ),
+                    timeout=1.0,
+                )
+                if self._stop_event.is_set():
+                    return
+                generation = self._source_generation
+                frames = dict(self._latest_frames)
+
+            delay = next_encode_at - time.monotonic()
+            if delay > 0 and self._stop_event.wait(delay):
+                return
+            next_encode_at = time.monotonic() + period
+            last_generation = generation
+
+            for name, frame in frames.items():
+                feed = self.feed(name)
+                if feed is None or feed.clients == 0:
+                    continue
+                try:
+                    encode_started = time.perf_counter()
+                    jpeg = self._encode_frame(frame)
+                    encode_ms = (time.perf_counter() - encode_started) * 1000
+                except Exception:
+                    logger.exception("Failed to encode web preview frame for %s", name)
+                    continue
+                feed.publish_jpeg(jpeg)
+                with self._condition:
+                    self._encoded_frames += 1
+                    self._last_encode_ms = encode_ms
+                    self._max_encode_ms = max(self._max_encode_ms, encode_ms)
+
+    def _encode_frame(self, frame: np.ndarray) -> bytes:
+        height, width = frame.shape[:2]
+        if width != self.config.width:
+            target_height = max(1, round(height * self.config.width / width))
+            frame = cv2.resize(
+                frame,
+                (self.config.width, target_height),
+                interpolation=cv2.INTER_AREA,
+            )
+        if frame.shape[2] == 4:
+            frame = cv2.cvtColor(frame, cv2.COLOR_RGBA2BGR)
+        else:
+            frame = cv2.cvtColor(frame, cv2.COLOR_RGB2BGR)
+        ok, buffer = cv2.imencode(
+            ".jpg", frame, [cv2.IMWRITE_JPEG_QUALITY, self.config.jpeg_quality]
+        )
+        if not ok:
+            raise RuntimeError("JPEG encoding failed")
+        return buffer.tobytes()
+
+    def stop(self) -> None:
+        if self._stop_event.is_set():
+            return
+        self._stop_event.set()
+        with self._condition:
+            self._condition.notify_all()
+        for feed in list(self._feeds.values()):
+            feed.stop()
+        if self._server is not None:
+            self._server.shutdown()
+            self._server.server_close()
+        if self._server_thread is not None:
+            self._server_thread.join(timeout=2.0)
+        if self._encoder_thread is not None:
+            self._encoder_thread.join(timeout=2.0)
+
+
+_WEB_PAGE = """<!doctype html>
+<html lang="zh-CN"><head><meta charset="utf-8">
+<meta name="viewport" content="width=device-width,initial-scale=1">
+<title>LeRobot 录制预览</title>
+<style>
+:root{color-scheme:dark;font-family:system-ui,sans-serif}body{margin:0;background:#101214;color:#eee}
+header{padding:14px 18px;background:#191c20;border-bottom:1px solid #333}h1{margin:0;font-size:20px}
+#status{margin-top:7px;color:#aeb5bd;font-size:13px}.grid{display:grid;grid-template-columns:repeat(auto-fit,minmax(320px,1fr));gap:12px;padding:14px}
+.tile{overflow:hidden;background:#191c20;border:1px solid #333;border-radius:7px}.tile h2{margin:0;padding:9px 11px;font-size:14px}.tile img{display:block;width:100%;background:#08090a}
+</style></head><body><header><h1>LeRobot 录制相机预览</h1><div id="status">正在等待相机帧…</div></header>
+<main id="grid" class="grid"></main><script>
+let signature='';async function refresh(){try{const r=await fetch('/api/status',{cache:'no-store'});const p=await r.json();
+document.getElementById('status').textContent=p.cameras.length?`${p.cameras.length} 路相机 · 预览 ${p.fps} FPS · 采集优先，预览允许丢帧`:'正在等待相机帧…';
+const next=p.cameras.join('|');if(next!==signature){signature=next;const grid=document.getElementById('grid');grid.replaceChildren();
+for(const name of p.cameras){const tile=document.createElement('section');tile.className='tile';const h=document.createElement('h2');h.textContent=name;
+const img=document.createElement('img');img.alt=name;img.src='/stream?camera='+encodeURIComponent(name);tile.append(h,img);grid.appendChild(tile);}}}catch(e){document.getElementById('status').textContent='预览服务不可用：'+e}}
+refresh();setInterval(refresh,2000);</script></body></html>""".encode()
+
+
+class _PreviewHandler(http.server.BaseHTTPRequestHandler):
+    preview: RecordingWebPreview
+
+    def do_GET(self) -> None:
+        parsed = urlsplit(self.path)
+        if parsed.path == "/":
+            self._send(200, "text/html; charset=utf-8", _WEB_PAGE)
+        elif parsed.path == "/api/status":
+            body = json.dumps(
+                {"ok": True, "cameras": self.preview.camera_names(), "fps": self.preview.config.fps}
+            ).encode()
+            self._send(200, "application/json; charset=utf-8", body)
+        elif parsed.path == "/stream":
+            self._stream(parse_qs(parsed.query).get("camera", [None])[0])
+        elif parsed.path == "/favicon.ico":
+            self._send(204, "text/plain", b"")
+        else:
+            self.send_error(404)
+
+    def _send(self, status: int, content_type: str, body: bytes) -> None:
+        try:
+            self.send_response(status)
+            self.send_header("Content-Type", content_type)
+            self.send_header("Content-Length", str(len(body)))
+            self.end_headers()
+            self.wfile.write(body)
+        except (BrokenPipeError, ConnectionResetError):
+            pass
+
+    def _stream(self, name: str | None) -> None:
+        feed = self.preview.feed(name)
+        if feed is None:
+            self._send(404, "text/plain; charset=utf-8", b"Unknown camera\n")
+            return
+        feed.add_client()
+        with self.preview._condition:
+            self.preview._condition.notify()
+        try:
+            self.send_response(200)
+            self.send_header("Cache-Control", "no-store")
+            self.send_header("Content-Type", "multipart/x-mixed-replace; boundary=frame")
+            self.end_headers()
+            frame_id = 0
+            while True:
+                jpeg, next_id, stopped = feed.wait_for_jpeg(frame_id)
+                if stopped:
+                    return
+                if jpeg is None or next_id == frame_id:
+                    continue
+                frame_id = next_id
+                self.wfile.write(
+                    b"--frame\r\nContent-Type: image/jpeg\r\n"
+                    + f"Content-Length: {len(jpeg)}\r\n\r\n".encode()
+                    + jpeg
+                    + b"\r\n"
+                )
+                self.wfile.flush()
+        except (BrokenPipeError, ConnectionResetError):
+            pass
+        finally:
+            feed.remove_client()
+
+    def log_message(self, format: str, *args: Any) -> None:
+        pass

tests/test_web_preview.py

@@ -0,0 +1,122 @@
+import socket
+import threading
+import time
+from pathlib import Path
+from urllib.request import urlopen
+
+import numpy as np
+import pytest
+import yaml
+
+from lerobot_robot_ufactory.utils.web_preview import (
+    RecordingWebPreview,
+    WebPreviewConfig,
+    _WEB_PAGE,
+)
+
+
+def _preview(**overrides):
+    values = {
+        "enabled": True,
+        "host": "127.0.0.1",
+        "port": 8765,
+        "fps": 30,
+        "width": 32,
+        "jpeg_quality": 60,
+    }
+    values.update(overrides)
+    return RecordingWebPreview(WebPreviewConfig(**values))
+
+
+def _start_encoder(preview):
+    preview._encoder_thread = threading.Thread(target=preview._encode_loop, daemon=True)
+    preview._encoder_thread.start()
+
+
+def test_config_rejects_invalid_resource_limits():
+    with pytest.raises(ValueError, match="fps"):
+        WebPreviewConfig(fps=0).validate()
+    with pytest.raises(ValueError, match="jpeg_quality"):
+        WebPreviewConfig(jpeg_quality=101).validate()
+
+
+def test_embedded_page_has_parseable_camera_signature_expression():
+    page = _WEB_PAGE.decode()
+
+    assert "const next=p.cameras.join('|')" in page
+    assert "join('\n')" not in page
+
+
+def test_gello_record_config_enables_low_rate_web_preview():
+    config_path = Path("config/gello/xarm7_gello_record_config.yaml")
+    config = yaml.safe_load(config_path.read_text())
+
+    assert config["web_preview"]["enabled"] is True
+    assert config["web_preview"]["fps"] == 8
+    assert config["web_preview"]["width"] == 480
+
+
+def test_publish_keeps_only_latest_frame_references():
+    preview = _preview()
+    first = np.zeros((24, 32, 3), dtype=np.uint8)
+    second = np.ones((24, 32, 3), dtype=np.uint8)
+
+    preview.publish({"J1.pos": 0.0, "camera": first})
+    preview.publish({"camera": second})
+
+    assert preview.camera_names() == ["camera"]
+    assert preview._latest_frames["camera"] is second
+    assert preview._source_generation == 2
+
+
+def test_encoder_is_idle_without_a_browser_client(monkeypatch):
+    preview = _preview()
+    calls = []
+    monkeypatch.setattr(preview, "_encode_frame", lambda frame: calls.append(frame) or b"jpeg")
+    _start_encoder(preview)
+    try:
+        preview.publish({"camera": np.zeros((24, 32, 3), dtype=np.uint8)})
+        time.sleep(0.08)
+        assert calls == []
+    finally:
+        preview.stop()
+
+
+def test_client_receives_background_encoded_latest_frame():
+    preview = _preview()
+    _start_encoder(preview)
+    try:
+        frame = np.zeros((24, 32, 3), dtype=np.uint8)
+        frame[:, :, 0] = 255
+        preview.publish({"camera": frame})
+        feed = preview.feed("camera")
+        feed.add_client()
+        with preview._condition:
+            preview._condition.notify()
+        jpeg, frame_id, stopped = feed.wait_for_jpeg(0)
+        feed.remove_client()
+
+        assert jpeg is not None and jpeg.startswith(b"\xff\xd8")
+        assert frame_id == 1
+        assert not stopped
+    finally:
+        preview.stop()
+
+
+def test_status_endpoint_lists_published_cameras():
+    try:
+        with socket.socket() as sock:
+            sock.bind(("127.0.0.1", 0))
+            port = sock.getsockname()[1]
+    except PermissionError:
+        pytest.skip("local sockets are disabled by the test sandbox")
+    preview = _preview(port=port)
+    preview.start()
+    try:
+        preview.publish({"camera2": np.zeros((24, 32, 3), dtype=np.uint8)})
+        with urlopen(preview.url + "api/status", timeout=2) as response:
+            body = response.read()
+        assert response.status == 200
+        assert b'"camera2"' in body
+    finally:
+        preview.stop()