ffmpeg将可变帧率.webm转换为恒定帧率视频

我有一个以16fps录制游戏的.webm文件。但是，在尝试使用OpenCV处理视频时，似乎视频是以可变帧率录制的，因此当我尝试使用OpenCV通过获取第16帧来每秒获取一帧时，由于该视频，该视频将无法工作流将过早结束。

因此，我正在尝试将声称帧速率为16 fps的可变帧.webm视频转换为具有恒定帧的视频，以便可以每秒提取一帧。我已经从https://ffmpeg.zeranoe.com/forum/viewtopic.php?t=5518尝试了以下ffmpeg命令：

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

但是，将发生以下错误：

[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.

这是我试图用来每秒处理一帧的代码：

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

由于视频的FPS并不恒定，因此代码会在接近视频结尾之前完成。如何将可变FPS视频转换为恒定FPS视频？