Re: [FFmpeg-devel] Advice on patch: Decklink flushes most buffered frames at end of video instead of displaying them

From: Devin Heitmueller <devin.heitmueller@ltnglobal.com>
To: FFmpeg development discussions and patches <ffmpeg-devel@ffmpeg.org>
Cc: Matthew Rademaker <matthew@watchgood.com>
Subject: Re: [FFmpeg-devel] Advice on patch: Decklink flushes most buffered frames at end of video instead of displaying them
Date: Wed, 5 Mar 2025 06:50:42 -0500
Message-ID: <CAHGibzEtQbPZsbuUWAMMTy8c=3TMkRbDgiVqsAWUjdpdwACNkA@mail.gmail.com> (raw)
In-Reply-To: <CAGBDY-suNJVT1F7cga+8uW7xfTepdp1GfnG_KRPkfCWLqWAogA@mail.gmail.com>

On Tue, Mar 4, 2025 at 10:47 PM Matthew Rademaker via ffmpeg-devel
<ffmpeg-devel@ffmpeg.org> wrote:
>
> When playing out video to a Decklink SDI 4K card I've found that all/most
> of the buffered frames get flushed instead of being displayed, essentially
> shortening the video by the amount of pre-roll. I found a way to fix this
> by adding a counter of outstanding frames, then delaying the call to
> StopScheduledPlayback until all frames are returned.
>
> I would submit this as a patch, but this is my first attempt at any ffmpeg
> development so I wanted to see if this was the best way to fix this issue.
> Are the outstanding frames already counted some other way? Is blocking via
> sleep ideal? The StopScheduledPlayback has a pts parameter, which I assume
> means that it should only stop playback at that time - should this not
> prevent the issue I am seeing? Why would it flush the frames?

Thanks for reaching out.

This is a longstanding issue that has been reported before, and I just
never got around to submitting a fix for (since for my particular use
case, I actually want it to exit immediately, although I realize this
isn't the common use case).  The correct fix is simply to change the
StopScheduledPlayback to not exit immediately (for which there is a
specific argument, IIRC).  You shouldn't need to do any sort of
counters/timers to achieve the desired behavior.

If you still can't get it to work, let me know and I'll take a look
and submit the appropriate patch.

Regards,

Devin

-- 
Devin Heitmueller, Senior Software Engineer
LTN Global Communications
o: +1 (301) 363-1001
w: https://ltnglobal.com  e: devin.heitmueller@ltnglobal.com
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