ffmpeg convert variable framerate .webm to constant framerate video

Question

I have a .webm file of a recording of a game at 16fps. However, upon trying to process the video with OpenCV, it seems the video is recorded with a variable framerate, so when I try to use OpenCV to get a frame every second by getting the every 16th frame, it won't work since the video stream will end prematurely.

Therefore, I'm trying to convert a variable-frame .webm video, which claims it has a framerate of 16 fps, to a video with a constant frame, so I can extract one frame for every second. I've tried the following ffmpeg command from https://ffmpeg.zeranoe.com/forum/viewtopic.php?t=5518:

ffmpeg -i input.webm -c:v copy -b:v copy -r 16 output.webm

However, the following error will occur:

[NULL @ 00000272ccbc0c40] [Eval @ 000000bc11bfe2f0] Undefined constant or missing '(' in 'copy'
[NULL @ 00000272ccbc0c40] Unable to parse option value "copy"
[NULL @ 00000272ccbc0c40] Error setting option b to value copy.

Error setting up codec context options.

Here's is the code I'm trying to use to process a frame every second:

video = cv2.VideoCapture(test_mp4_vod_path)
print("Opened ", test_mp4_vod_path)
print("Processing MP4 frame by frame")

# forward over to the frames you want to start reading from.
# manually set this, fps * time in seconds you wanna start from
video.set(1, 0)
success, frame = video.read()
#fps = int(video.get(cv2.CAP_PROP_FPS))  # this will return 0!
fps = 16  # hardcode fps
total_frame_count = int(video.get(cv2.CAP_PROP_FRAME_COUNT))
print("Loading video %d seconds long with FPS %d and total frame count %d " % (total_frame_count/fps, fps, total_frame_count))

count = 1
while video.isOpened():
    success, frame = video.read()
    if not success:
        break

    if count % fps == 0:
        print("%dth frame is %d seconds on video"%(count, count/fps))
    count += 1

The code will finish before it gets near the end of the video, since the video isn't at a constant FPS. How can I convert a variable-FPS video to a constant FPS video?

VC.One VC.One · Accepted Answer · 2019-07-13T14:14:13

For webM options in FFmpeg, read: https://trac.ffmpeg.org/wiki/Encode/VP9.
Don't use a codec copy option if converting frame rates.

Possible solution (the 2M is a testing value, adjust for your video) :

ffmpeg -i input.we -c:v libvpx-vp9 -minrate 2M -maxrate 2M -b:v 2M -pix_fmt yuv420p -r 16 output.webm

ffmpeg convert variable framerate .webm to constant framerate video

2 Answers