How to seek a specific frame quickly?

[Akascape]

I want to get a particular video frame using pyav. I found that container.seek method only seeks to time(int) offset, so it will not help in finding the required frame.

Perhaps, I used this method to get the required frame:

container.seek(0, whence='time', backward=True) # seek to the starting
frame_num = 30  # the frame I want to seek
time.sleep(0.2) # limit cpu usage
for _ in range(frame_num): 
    frame = next(container.decode(video=0))
return frame

This method works properly but it is too slow while seeking the last frames of any long video file. (with frames>1000)

I tried to solve this issue by modifying the seek method :

frame_num = 30
framerate = container.streams.video[0].average_rate # get the frame rate
sec = int(frame_num/framerate) # timestamp (sec) for that frame_num
container.seek(sec*1000000, whence='time', backward=True)  # seek to that nearest timestamp
sec_frame = sec * framerate # get the key frame number of that timestamp
       
for _ in range(sec_frame, frame_num):
    frame = next(container.decode(video=0))
return frame

This reduces the time while seeking any frame of the video, but it doesn't return the proper frame because container.seek method doesn't seek to the proper timestamp. I found that container.seek is giving the same key frame for consecutive 3 seconds. I don't know if I am missing something...

Is there any other way to get the required frame with pyav?

[Akascape]

So, I somehow managed to fix the lag issue:

frame_num = 1000 # the frame I want
framerate = container.streams.video[0].average_rate # get the frame rate
time_base = container.streams.video[0].time_base # get the time base
sec = int(frame_num/framerate) # timestamp for that frame_num
container.seek(sec*1000000, whence='time', backward=True)  # seek to that nearest timestamp
frame = next(container.decode(video=0)) # get the next available frame
sec_frame = int(frame.pts * time_base * framerate) # get the proper key frame number of that timestamp

for _ in range(sec_frame, frame_num):
    frame = next(container.decode(video=0))
return frame

Now it returns the required frame in less than a second.

[pauljurczak]

Same here: getting the first frame.

[Scr44gr]

@pauljurczak @haoxurt I guess is the version, which version did u use? I test with 11.0.0 and it works

[pauljurczak]

pyav 12.0.5. I'm curious why backward=True? Shouldn't seek search forward to find frame 622 after opening the video file?

[robonrrd]

I am using 13.1.0 and have the same issue with only getting the first frame

[Max-Thuc]

the constant 1000000 is different for each video