diff --git a/tracklab/utils/cv2.py b/tracklab/utils/cv2.py
index 0c198d90..144f7485 100644
--- a/tracklab/utils/cv2.py
+++ b/tracklab/utils/cv2.py
@@ -34,14 +34,10 @@
 class VideoReader:
     def __init__(self):
         self.filename = None
-        self.cap = None  # cv2.VideoCapture(filename)
 
     def set_filename(self, filename):
+        # TODO: remove this function, and set self.filename directly
         self.filename = filename
-        if self.cap is not None:
-            self.cap.release()
-        self.cap = cv2.VideoCapture(self.filename)
-        assert self.cap.isOpened(), "Error opening video stream or file"
 
     def __getitem__(self, idx):
         assert self.filename is not None, "You should first set the filename"
diff --git a/tracklab/wrappers/dataset/external_video.py b/tracklab/wrappers/dataset/external_video.py
index c82a6749..d3083bdc 100644
--- a/tracklab/wrappers/dataset/external_video.py
+++ b/tracklab/wrappers/dataset/external_video.py
@@ -73,7 +73,7 @@ def __init__(self, dataset_path: str, video_path: str, *args, **kwargs):
                 image_metadata.extend(
                     [
                         {
-                            "id": j+1000*i,
+                            "id": j+100_000*i, # TODO: read the number of frames to get the right factor
                             "name": f"{video_name}_{j}",
                             "frame": j,
                             "nframes": nframes,