Dedicated LPR fixes (#17325)

* docs fps recommendation

* add detection_threshold to example

* send uncoverted yuv frame

* copy and convert

* ensure thumbnail is encoded as webp

* keep as jpeg

docs/docs/configuration/license_plate_recognition.md

@@ -133,8 +133,8 @@ Users with a dedicated LPR camera can run Frigate's LPR by specifying a camera t
 # LPR global configuration
 lpr:
   enabled: True
-  min_area: 2000
   min_plate_length: 4
+  detection_threshold: 0.7 # change if necessary
 
 # Dedicated LPR camera configuration
 cameras:
@@ -146,7 +146,7 @@ cameras:
     ffmpeg: ...
     detect:
       enabled: False # optional, disable Frigate's standard object detection pipeline
-      fps: 5
+      fps: 5 # keep this at 5, higher values are unnecessary for dedicated LPR mode and could overwhelm the detector
       width: 1920
       height: 1080
     motion:

frigate/data_processing/common/license_plate/mixin.py

@@ -1279,10 +1279,9 @@ class LicensePlateProcessingMixin:
             # save the best snapshot
             logger.debug(f"Writing snapshot for {id}, {top_plate}, {current_time}")
             frame_bgr = cv2.cvtColor(frame, cv2.COLOR_YUV2BGR_I420)
-            _, buffer = cv2.imencode(".jpg", frame_bgr)
             self.sub_label_publisher.publish(
                 EventMetadataTypeEnum.save_lpr_snapshot,
-                (base64.b64encode(buffer).decode("ASCII"), id, camera),
+                (base64.b64encode(frame_bgr).decode("ASCII"), id, camera),
             )
 
         self.detected_license_plates[id] = {