Add optional idle heartbeat for Birdseye (#20453)

* Add optional idle heartbeat for Birdseye (periodic frame emission when idle)

birdseye: add optional idle heartbeat and FFmpeg tuning envs (default off)

This adds an optional configuration field `birdseye.idle_heartbeat_fps` to
enable a lightweight idle heartbeat mechanism in Birdseye. When set to a value
greater than 0, Birdseye periodically re-sends the last composed frame during
idle periods (no motion or active updates).

This helps downstream consumers such as go2rtc, Alexa, or Scrypted to attach
faster and maintain a low-latency RTSP stream when the system is idle.

Key details:
- Config-based (`birdseye.idle_heartbeat_fps`), default `0` (disabled).
- Uses existing Birdseye rendering pipeline; minimal performance impact.
- Does not alter behavior when unset.

Documentation: added tip section in docs/configuration/restream.md.

* Update docs/docs/configuration/restream.md

Co-authored-by: Nicolas Mowen <nickmowen213@gmail.com>

* Update docs/docs/configuration/reference.md

Co-authored-by: Nicolas Mowen <nickmowen213@gmail.com>

* Refactors Birdseye idle frame broadcasting

Simplifies the idle frame broadcasting logic by removing the dedicated thread.

The idle frame is now resent directly within the main loop,
improving efficiency and reducing complexity.  Also, limits the idle
heartbeat FPS to a maximum of 10 since the framebuffer is limited to 10 anyway

* ruff fix

---------

Co-authored-by: Nicolas Mowen <nickmowen213@gmail.com>
Co-authored-by: Francesco Durighetto <francesco.durighetto@subbyx.com>
Co-authored-by: duri <duri@homelabubuntu.durihome.unifi>

frigate/config/camera/birdseye.py

@@ -55,6 +55,12 @@ class BirdseyeConfig(FrigateBaseModel):
     layout: BirdseyeLayoutConfig = Field(
         default_factory=BirdseyeLayoutConfig, title="Birdseye Layout Config"
     )
+    idle_heartbeat_fps: float = Field(
+        default=0.0,
+        ge=0.0,
+        le=10.0,
+        title="Idle heartbeat FPS (0 disables, max 10)",
+    )
 
 
 # uses BaseModel because some global attributes are not available at the camera level

frigate/output/birdseye.py

@@ -9,6 +9,7 @@ import os
 import queue
 import subprocess as sp
 import threading
+import time
 import traceback
 from typing import Any, Optional
 
@@ -791,6 +792,10 @@ class Birdseye:
         self.frame_manager = SharedMemoryFrameManager()
         self.stop_event = stop_event
         self.requestor = InterProcessRequestor()
+        self.idle_fps: float = self.config.birdseye.idle_heartbeat_fps
+        self._idle_interval: Optional[float] = (
+            (1.0 / self.idle_fps) if self.idle_fps > 0 else None
+        )
 
         if config.birdseye.restream:
             self.birdseye_buffer = self.frame_manager.create(
@@ -848,6 +853,15 @@ class Birdseye:
             if frame_layout_changed:
                 coordinates = self.birdseye_manager.get_camera_coordinates()
                 self.requestor.send_data(UPDATE_BIRDSEYE_LAYOUT, coordinates)
+        if self._idle_interval:
+            now = time.monotonic()
+            is_idle = len(self.birdseye_manager.camera_layout) == 0
+            if (
+                is_idle
+                and (now - self.birdseye_manager.last_output_time)
+                >= self._idle_interval
+            ):
+                self.__send_new_frame()
 
     def stop(self) -> None:
         self.converter.join()