On 12/19/23 1:36 AM, Peter Krefting wrote:
Hi!
The video and audio comes out just fine, but the subtitle packets all
have a length of 30 bytes, and the data appears to be garbage (well,
could be raw data I've not explored that).
Are you getting bitmap subtitles? If AVSubtitle::format == 0, you have
bitmap subtitles.
The format is not immediately obvious, have a look at these posts for
some help on how to use them:
https://stackoverflow.com/a/54143538
https://stackoverflow.com/a/60560764
The former has a code snippet that lets you convert an AVSubtitle into
an AVFrame that can be input into an encoder to make an image or video
stream for overlay.
Thanks for the assist!
The issue was a bit more basic. I was not using avcodec_decode_subtitle2
and avcodec_encode_subtitle. Easy to overlook if you don't know you need
it. My documentation is only the ffmpeg tool code itself, which has all
kind of conditional code to handle exceptions with no explanation of
what it is trying to fix.
In light of this new information and deciphering what ffmpeg is doing, I
can decode the subtitle packets coming out of the lavfi filter and back
out. I'm still not 100% on the AVSubtitle structure and what it means.
1) AVSubtitle::start_display_time and AVSubtitle::end_display_time are
always in milliseconds?
2) AVSubtitle::start_display_time and AVSubtitle::end_display_time are
offsets from AVSubtitle::pts?
Would be great if AVSubtitle had time_base and opaque, but even better
if it was an AVFrame as referenced in comments in ffmpeg.
The biggest questions are around this subtitle heartbeat in ffmpeg. What
issue is it trying to solve? I'm guessing I'll eventually run into the
issue myself and then it will all be made clear, but I'd like to learn
something the easy way once in awhile :-) Maybe to cause HLS
segmentation to output an empty subtitle file to keep it numerically in
sync with the video file name count?
Why does ffmpeg (conditionally) decompose the AVSubtitle into smaller
AVSubtitles before feeding it to avcodec_encode_subtitle?
The next issue is I often get "[Closed caption Decoder @ 0x7fffa40a4f40]
Data Ignored since exceeding screen width" errors. I see in
libavcodec/ccaption_dec.c where the error is generated, but it is not
some configurable value for width but instead a fixed value of 32
characters. What would cause this? No demux errors reported, and the
receiver reports no uncorrected errors in the data stream. Even ffmpeg
will report this, so not specific to something I am doing.
Deron
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