avformat/rcwtdec: add RCWT Closed Captions demuxer

RCWT (Raw Captions With Time) is a format native to ccextractor,
a commonly used OSS tool for processing 608/708 Closed Captions (CC).
RCWT can be used to archive the original extracted CC bitstream.
The muxer was added in January 2024. In this commit, add the demuxer.

One can now demux RCWT files for rendering in ccaption_dec or interop
with ccextractor (which produces RCWT). Using the muxer/demuxer combo,
the CC bits can be kept for processing or rendering with either tool.
This can be an effective way to backup an original CC stream, including
format extensions like EIA-708 and overall original presentation.

Signed-off-by: Marth64 <marth64@proxyid.net>

1 files changed, 30 insertions, 0 deletions

diff --git a/doc/demuxers.texi b/doc/demuxers.texi
index b70f3a38d7..04293c4813 100644
--- a/doc/demuxers.texi
+++ b/doc/demuxers.texi
@@ -1038,6 +1038,36 @@ the command:
 ffplay -f rawvideo -pixel_format rgb24 -video_size 320x240 -framerate 10 input.raw
 @end example
 
+@anchor{rcwtdec}
+@section rcwt
+
+RCWT (Raw Captions With Time) is a format native to ccextractor, a commonly
+used open source tool for processing 608/708 Closed Captions (CC) sources.
+For more information on the format, see @ref{rcwtenc,,,ffmpeg-formats}.
+
+This demuxer implements the specification as of March 2024, which has
+been stable and unchanged since April 2014.
+
+@subsection Examples
+
+@itemize
+@item
+Render CC to ASS using the built-in decoder:
+@example
+ffmpeg -i CC.rcwt.bin CC.ass
+@end example
+Note that if your output appears to be empty, you may have to manually
+set the decoder's @option{data_field} option to pick the desired CC substream.
+
+@item
+Convert an RCWT backup to Scenarist (SCC) format:
+@example
+ffmpeg -i CC.rcwt.bin -c:s copy CC.scc
+@end example
+Note that the SCC format does not support all of the possible CC extensions
+that can be stored in RCWT (such as EIA-708).
+@end itemize
+
 @section sbg
 
 SBaGen script demuxer.