It seems the image is revealed by a 1px area sliding from the top to the bottom of the video. Let's merge these 1px areasfrom all frames into a single image.
1) Install OpenCV and tqdm: `pip3 install opencv-python tqdm`2) Run the exploit:```% cat subliminal.pyimport sysimport cv2print('CV2 versin: ', cv2.__version__)import numpy as npfrom tqdm import tqdm
def extractImages(pathIn, pathOut):    count = 0    vidcap = cv2.VideoCapture(pathIn)     # get video properties    width = round(vidcap.get(cv2.CAP_PROP_FRAME_WIDTH))    height = round(vidcap.get(cv2.CAP_PROP_FRAME_HEIGHT))    fps = round(vidcap.get(cv2.CAP_PROP_FPS))    frame_count = round(vidcap.get(cv2.CAP_PROP_FRAME_COUNT))
    print("width={}, height={}, fps={}, frame_count={}".format(width, height, fps, frame_count))
    result = np.zeros((height, width, 3), dtype=np.uint8)
    for i in tqdm(range(frame_count)):        # get frame        vidcap.set(cv2.CAP_PROP_POS_FRAMES, i)        success, image = vidcap.read()        assert success
        for j in range(width):            for k in range(3):                result[i][j][k] = image[i][j][k]
    cv2.imwrite(pathOut, result)
if __name__ == '__main__':    input_video = sys.argv[1]    output_name = sys.argv[2]    extractImages(input_video, output_name)
% python3 subliminal.py subliminal_challenge.avi result.jpgCV2 versin:  4.5.1width=1280, height=720, fps=60, frame_count=720100%|█████████████████████████████████| 720/720 [00:16<00:00, 42.59it/s]```3) Capture the flag in the `result.jpg`:![](https://i.imgur.com/TCKbgQO.jpg)The flag is `Hero{Fr4gM3nt3D_1m4Ge}`