FFmpeg Quirks in LPMS #275
Conversation
QUIRKS.md
Outdated
|
|
||
| FPS filter expects monotonic increase in input frame's PTS. We cannot rely on the input to be monotonic thus: | ||
| i. Set a dummy PTS before the frame is sent into filtergraph, that we manually increase monotonically. | ||
| ii. OR use SETPTS filter in the filtergraph before FPS filter, which would do the same thing. |
There was a problem hiding this comment.
We should remove this from the documentation, as we did not take the approach of SETPTS filter due to reasons mentioned here #201 (comment)
QUIRKS.md
Outdated
|
|
||
| ### Solution | ||
| To solve the flushing problem while still reusing the session, we introduced so called sentinel-packets. Sentinel packets are dummy frames with -1 timestamps. We insert these packets at the end of each segment to make sure that the packets that are sent to the buffer earlier always get popped out. We wait until we receive the sentinel packets pack and if we receive sentinel packet we know that this is the end of the segment. | ||
|
|
There was a problem hiding this comment.
We should probably add links to the code snippet that implements this.
We can also note the fact that we give up flushing after sending SENTINEL_MAX packets, which is a pre-processor constant defined as 5 for now.
QUIRKS.md
Outdated
| Instead of erroring on such segments, let's accept them and send back audio-only tracks. The audio output should be exactly the same as the audio input. | ||
|
|
||
| ### Our Solution | ||
| We bypass the usual transcoding process for audio-only segments and just return them back. To do this, we check if video frame is present, and if not present just copy the frames to the output as it is. The bypassing is done by forcing the copy transcoder https://github.com/livepeer/lpms/commit/8bc28e3f702049a17c24ab2041857a47d8af51bf for such segments. |
There was a problem hiding this comment.
While the by-pass check code is still implemented in LPMS we no longer use this function on the transcoder directly, or set the 'copy' encoder.
Lines 110 to 117 in 46f7030
We instead moved the calling logic on the Broadcaster side in livepeer/go-livepeer#1933 and the document should reflect that.
|
Amazing write-up, @jailuthra @oscar-davids! Hijacking this branch to add a small section on reusing transcoding session with HW codecs. I believe much of the information on this is already in the document. |
This document outlines how LPMS tweaks FFmpeg transcoding pipeline to address some specific problems.