Preview push 30fps : thread de fond + diffusion WS sans polling

Avant : frontend poll WS toutes les 100ms -> gphoto2 bloquant -> lag
Apres : thread dedie capture en continu a 30fps, pusher asyncio
        diffuse a 20fps a tous les clients, frontend dit juste
        preview_start / preview_stop

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/main.py

@@ -2,6 +2,8 @@ import asyncio
 import base64
 import json
 import logging
+import threading
+import time
 from pathlib import Path
 from contextlib import asynccontextmanager
 
@@ -37,7 +39,49 @@ clients_ws: list[WebSocket] = []
 
 capture_en_cours = False
 _dslr_erreurs = 0
-_dernier_envoi_erreur_preview = 0.0  # timestamp du dernier camera_erreur envoye depuis le preview
+
+# --- Preview push : thread de fond qui capture en continu ---
+_derniere_frame_preview: bytes | None = None   # derniere frame JPEG capturee
+_preview_actif = False                          # True quand au moins un client veut le preview
+_preview_lock = threading.Lock()
+_loop_principal: asyncio.AbstractEventLoop | None = None  # loop asyncio principal
+
+
+def _thread_preview():
+    """Thread de fond : capture les frames DSLR en continu et les stocke."""
+    global _derniere_frame_preview, _preview_actif
+    while True:
+        if not _preview_actif or capture_en_cours:
+            time.sleep(0.05)
+            continue
+        try:
+            donnees = camera.preview()
+            with _preview_lock:
+                _derniere_frame_preview = donnees
+        except Exception:
+            with _preview_lock:
+                _derniere_frame_preview = None
+        time.sleep(0.033)  # ~30 fps max
+
+
+async def _pusher_preview():
+    """Tache asyncio : pousse la derniere frame a tous les clients WS abonnes."""
+    global _preview_actif
+    while True:
+        await asyncio.sleep(0.05)  # 20 fps max envoye aux clients
+        if not clients_ws:
+            _preview_actif = False
+            continue
+        with _preview_lock:
+            donnees = _derniere_frame_preview
+        if donnees is None:
+            # Signaler l'erreur si le DSLR est sense etre connecte
+            if camera.mode == "gphoto2" and _preview_actif:
+                await diffuser_ws({"type": "camera_erreur", "message": "DSLR ne repond pas au preview"})
+                _preview_actif = False
+            continue
+        b64 = base64.b64encode(donnees).decode("ascii")
+        await diffuser_ws({"type": "preview", "image": f"data:image/jpeg;base64,{b64}"})
 
 async def surveiller_dslr():
     """Verifie periodiquement si un DSLR est branche et bascule dessus."""
@@ -78,11 +122,18 @@ async def surveiller_dslr():
 @asynccontextmanager
 async def lifespan(app: FastAPI):
     """Demarrage et arret de l'application."""
+    global _loop_principal
     log.info("Demarrage du photobooth")
+    _loop_principal = asyncio.get_event_loop()
     camera.connecter()
-    task = asyncio.create_task(surveiller_dslr())
+    # Thread de capture preview (bloquant, tourne en parallele)
+    t = threading.Thread(target=_thread_preview, daemon=True)
+    t.start()
+    task_dslr = asyncio.create_task(surveiller_dslr())
+    task_push = asyncio.create_task(_pusher_preview())
     yield
-    task.cancel()
+    task_dslr.cancel()
+    task_push.cancel()
     log.info("Arret du photobooth")
     camera.deconnecter()
 
@@ -632,19 +683,11 @@ async def traiter_message_ws(msg: dict, ws: WebSocket):
 
     if type_msg == "ping":
         await ws.send_json({"type": "pong"})
-    elif type_msg == "preview":
-        import time as _time
-        global _dernier_envoi_erreur_preview
-        donnees = camera.preview()
-        if donnees:
-            b64 = base64.b64encode(donnees).decode("ascii")
-            await ws.send_json({"type": "preview", "image": f"data:image/jpeg;base64,{b64}"})
-        # Si le DSLR est cense etre connecte mais le preview echoue, signaler l'erreur
-        if camera.mode == "gphoto2" and not camera.preview_dslr_ok:
-            now = _time.monotonic()
-            if now - _dernier_envoi_erreur_preview > 5.0:
-                _dernier_envoi_erreur_preview = now
-                await ws.send_json({"type": "camera_erreur", "message": "Le DSLR ne repond pas au preview"})
+    elif type_msg == "preview_start":
+        global _preview_actif
+        _preview_actif = True
+    elif type_msg == "preview_stop":
+        _preview_actif = False
 
 
 async def diffuser_ws(message: dict):