为什么 opencv Videocapture function 会读取像素值错误的视频帧？

Question

I am capturing videos from a camera and saving them using the opencv VideoWriter function. I save the captured videos as uncompressed avi files. When I finish recording the video, I have another script that is supposed to read the video frame by frame, process the pixel values. However, when I try to read the frames of the saved video, the pixel values are off a bit.

For example, comparing the first frames of the video being written, and the video being read (supposed to be 100% identical), I notice that the pixel values are off by a small number (RGB values off by a small number, usually less than 5).

I have already made sure that I am using the exact same video codex when writing the video, and when reading the video (Check code below)

def write_video():
    out = cv2.VideoWriter("encrypted.avi" ,cv2.VideoWriter_fourcc(*'XVID'),30, (640,480))
    foreach frame:
        out.write(frame)

def read_video():
    cap = cv2.VideoCapture("encrypted.avi")
    cap.set(cv2.CAP_PROP_FOURCC,cv2.VideoWriter_fourcc(*'XVID'))
    while(cap.isOpened()):
        ret, frame = cap.read()

For my application, the frames being written and read should match 100%. I have included an image highlighting the difference between the first frame in the video being written, and the video being read. Any help is much appreciated!

Answer 1

These are the compression artifacts, since you are using lossy compression. If you would like your frames match down to the last bit, write them as a sequence of.PNG files -- these are losslessly compressed and preserve everything. Beware that.PNG will take much more of your HDD space than compressed video.