OpenCV：对于 mp4 文件，保存电影作品但编解码器错误

Question

I am making a video editor in Python with OpenCV, and testing different combinations of codecs and file formats. For this example, I'm focusing on avi and mp4 formats. I have tried multiple different four-character codecs ( http://www.fourcc.org/codecs.php ) and they all saved an actual movie I can play.

The confusing things is when I reload them and extract the codec from the movie, it turns out that only the movie saved as avi seems to have been saved using the original codec. Below I have a chart showing the codec used for encoding (created using cv2.VideoWriter_fourcc() ), and then the corresponding codec extracted from the video after loading it later (extracted using get(cv2.CAP_PROP_FOURCC) ). I also am putting the size in MB in parentheses.

For avi (encoded -> extracted):

MJPG -> MJPG (3.9)
FFV1 -> FFV1 (12.3)
DIVX -> DIVX (0.7)

For mp4:

MJPG -> mp4v (3.9)
FFV1 -> avc1 (26.1)
DIVX -> mp4v (0.7)

So you can see that the encoding does not match for the mp4 file types, even though the file sizes roughly match up. The videos all look pretty much the same for avi versus mp4.

Is this metadata from the loaded movies not trustworthy for mp4, is the encoding not working for mp4 so it is picking a default, or something else?

Answer 1

Most probably you try to use combination of codec (how the video content is compressed) and container (the actual file format used to transport the compressed video (and usually audio, subtitiles,...) that are not compatible.