add ffmpeg backend for linux compatibility

test_ffmpeg.py

@@ -0,0 +1,46 @@
+import av
+import cv2
+import time
+
+DEVICE = "/dev/video4"
+TARGET_FPS = 30
+WIDTH = 1280
+HEIGHT = 720
+
+# Open v4l2 device with same style options as ffmpeg CLI
+container = av.open(
+    DEVICE,
+    format="v4l2",
+    options={
+        "input_format": "mjpeg",
+        "video_size": f"{WIDTH}x{HEIGHT}",
+        "framerate": str(TARGET_FPS),
+    },
+)
+
+stream = container.streams.video[0]
+frame_count = 0
+t_start = time.perf_counter()
+
+for frame in container.decode(stream):
+    t0 = time.perf_counter()
+    img = frame.to_ndarray(format="bgr24")  # OpenCV-compatible
+    t1 = time.perf_counter()
+
+    cv2.imshow("pyav_cam", img)
+    key = cv2.waitKey(1) & 0xFF
+    t2 = time.perf_counter()
+
+    frame_count += 1
+    if frame_count % 30 == 0:
+        elapsed = t2 - t_start
+        print(
+            f"Frame {frame_count}: convert={1000*(t1-t0):.1f}ms, "
+            f"imshow+waitKey={1000*(t2-t1):.1f}ms, avg_fps={frame_count/elapsed:.1f}"
+        )
+
+    if key == ord("q"):
+        break
+
+container.close()
+cv2.destroyAllWindows()