Why does opencv Videocapture function read video frames with wrong pixel values?

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!

lenik lenik · Accepted Answer · 2019-09-27T03:17:14

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.

Why does opencv Videocapture function read video frames with wrong pixel values?

1 Answers