Frame timestamps when transcoding must be set to None

[MiguelFRomeroR]

I was able to replicate this issue with the following example:

from fractions import Fraction
import av
import logging
logging.basicConfig()

input_container = av.open('input.mp4')
video_stream = input_container.streams.video[0]

output_container = av.open('output.mp4', mode='w')
stream = output_container.add_stream('libx264', rate=Fraction(30000,1001))
stream.width = 512
stream.height = 512
stream.pix_fmt = 'yuv420p'

for frame in input_container.decode(video=0):
	packet = stream.encode(frame)
	output_container.mux(packet)

packet = stream.encode()
output_container.mux(packet)

output_container.close()

The fps of the file 'output.mp4' is 0.06 fps, and therefore the duration of the video is really high. I tried lots of things (changing the rate, time_base, bit_rate, max_bit_rate, ... of the OutputContainer and the CodecContext), until I found out that to solve it, you simply have to set frame.pts and frame.time_base to None, to allow the encoder to generate it's own timestamps.

from fractions import Fraction
import av
import logging
logging.basicConfig()

input_container = av.open('input.mp4')
video_stream = input_container.streams.video[0]

output_container = av.open('output.mp4', mode='w')
stream = output_container.add_stream('libx264', rate=Fraction(30000,1001))
stream.width = 512
stream.height = 512
stream.pix_fmt = 'yuv420p'

for frame in input_container.decode(video=0):
	frame.pts = None
	frame.time_base = None
	packet = stream.encode(frame)
	output_container.mux(packet)

packet = stream.encode()
output_container.mux(packet)

output_container.close()

This error is very subtle but I wanted to remark it, because someone else may have a related issue.

I'm using the latest (31-01-2018) ffmpeg version N-89907-g293f24b, the PyAV version built from source under Ubuntu 16.04.

[mikeboers]

This should land in the documentation, as PyAV/FFmpeg are working as they need to.

[jlaine]

Could this be due to the fact the frames had a bogus timebase until recently?

[jlaine]

I ran the following, which worked fine using PyAV 0.5.1:

import av
import logging
logging.basicConfig()

input_container = av.open('input.mp4')
video_stream = input_container.streams.video[0]

output_container = av.open('output.mp4', mode='w')
stream = output_container.add_stream('libx264')
stream.width = 512
stream.height = 512
stream.pix_fmt = 'yuv420p'
stream.time_base = video_stream.time_base

for frame in input_container.decode(video=0):
    packet = stream.encode(frame)
    output_container.mux(packet)

output_container.close()

As you can see:

• I didn't pass the rate parameter when opening the container, but copied time_base from the input to the output stream
• I didn't touch the frames' pts or time_base
• I didn't use the final stream.encode(), as this results in a packet with no PTS causing ffmpeg to bomb

The output file has the exact same framerate as my input.

[mikeboers]

@jlaine It might have been the wrong timebase on the frames.

My comment from Mar. 21 was because I thought the timebase didn't match on the two streams (which it wouldn't), and so if you don't remap the pts from one to the other it will be wonky. But that is what the great time overhaul of v0.4.0 was supposed to do at all.

This could very likely be fixed, but I'm leaving the issue here as a signal to document that time needs your attention all the time.

[jlaine]

To the best of my knowledge this is fixed