Opencv fails to save video when cropped using numpy slicing

Question

I'm using OpenCV (3.4.6) to capture video from a Raspberry Pi HQ camera, attached to a Raspberry Pi 4 running Raspbian 10. Bounding boxes suggested by an object detection algorithm are then drawn onto the captured frames, and the output saved to a video file.

Everything works great, except when I try to crop the video to my region of interest.

Using the following line successfully crops the image (as confirmed by cv2.imshow):

frame = frame[0:720, 280:1000]

But, it also stops the output being saved by out.write - I just get an empty file, and being new to both Python and OpenCV, I'm struggling to work out why.

I've trimmed down the code to the relevant part below (apologies - it's still a bit long - I'm not entirely sure what might be important):

# begin video capture
    try:
        vid = cv2.VideoCapture(int(video_path), cv2.CAP_V4L2)
        
    except:
        vid = cv2.VideoCapture(video_path)

    out = None
    
    # set capture resolution     
    inputcodec = cv2.VideoWriter_fourcc('M', 'J', 'P', 'G') # forces MJPG codec
    vid.set(6, inputcodec) # sets inputcodec for video capture
    vid.set(3, 1280) # sets video capture width
    vid.set(4, 720) # sets video capture height
    
    if FLAGS.output:
        # by default VideoCapture returns float instead of int
        width = int(vid.get(cv2.CAP_PROP_FRAME_WIDTH))
        height = int(vid.get(cv2.CAP_PROP_FRAME_HEIGHT))
        fps = int(vid.get(cv2.CAP_PROP_FPS))
        codec = cv2.VideoWriter_fourcc(*FLAGS.output_format)
        out = cv2.VideoWriter(FLAGS.output, codec, fps, (width, height))

    while True:
        return_value, frame = vid.read()
        frame = frame[0:720, 280:1000] # crop video to square region of interest
        if return_value:
            frame = cv2.cvtColor(frame, cv2.COLOR_BGR2RGB)
            image = Image.fromarray(frame)
        else:
            print('Video has ended or failed, try a different video format!')
            break
    
        frame_size = frame.shape[:2]
        image_data = cv2.resize(frame, (input_size, input_size))
        image_data = image_data / 255.
        image_data = image_data[np.newaxis, ...].astype(np.float32)
        start_time = time.time()

        if FLAGS.framework == 'tflite':
            interpreter.set_tensor(input_details[0]['index'], image_data)
            interpreter.invoke()
            pred = [interpreter.get_tensor(output_details[i]['index']) for i in range(len(output_details))]
            if FLAGS.model == 'yolov3' and FLAGS.tiny == True:
                boxes, pred_conf = filter_boxes(pred[1], pred[0], score_threshold=0.25,
                                                input_shape=tf.constant([input_size, input_size]))
            else:
                boxes, pred_conf = filter_boxes(pred[0], pred[1], score_threshold=0.25,
                                                input_shape=tf.constant([input_size, input_size]))
        else:
            batch_data = tf.constant(image_data)
            pred_bbox = infer(batch_data)
            for key, value in pred_bbox.items():
                boxes = value[:, :, 0:4]
                pred_conf = value[:, :, 4:]

        boxes, scores, classes, valid_detections = tf.image.combined_non_max_suppression(
            boxes=tf.reshape(boxes, (tf.shape(boxes)[0], -1, 1, 4)),
            scores=tf.reshape(
                pred_conf, (tf.shape(pred_conf)[0], -1, tf.shape(pred_conf)[-1])),
            max_output_size_per_class=50,
            max_total_size=50,
            iou_threshold=FLAGS.iou,
            score_threshold=FLAGS.score
        )
        pred_bbox = [boxes.numpy(), scores.numpy(), classes.numpy(), valid_detections.numpy()]
        image = utils.draw_bbox(frame, pred_bbox)
        fps = 1.0 / (time.time() - start_time)
        print("FPS: %.2f" % fps)
        result = np.asarray(image)
        cv2.namedWindow("result", cv2.WINDOW_AUTOSIZE)
        result = cv2.cvtColor(image, cv2.COLOR_RGB2BGR)
        
        if not FLAGS.dont_show:
            cv2.imshow("result", result)
        
        if FLAGS.output:
            out.write(result)
        if cv2.waitKey(1) & 0xFF == ord('q'): break
    cv2.destroyAllWindows()

Any suggestions for how to resolve this would be greatly appreciated.

Opencv fails to save video when cropped using numpy slicing

Answers (1)

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