[FFmpeg-devel] [PATCH v8] avformat/rcwtdec: add RCWT Closed Captions demuxer

[Marth64 <marth64@proxyid.net>]

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>
---
 Changelog                |   2 +-
 doc/demuxers.texi        |  30 ++++++++++
 libavformat/Makefile     |   1 +
 libavformat/allformats.c |   1 +
 libavformat/rcwtdec.c    | 125 +++++++++++++++++++++++++++++++++++++++
 5 files changed, 158 insertions(+), 1 deletion(-)
 create mode 100644 libavformat/rcwtdec.c

diff --git a/Changelog b/Changelog
index e3ca52430c..0ae05c6cce 100644
--- a/Changelog
+++ b/Changelog
@@ -19,7 +19,7 @@ version <next>:
 - lavu/eval: introduce randomi() function in expressions
 - VVC decoder
 - fsync filter
-- Raw Captions with Time (RCWT) closed caption muxer
+- RCWT (Raw Captions with Time) Closed Captions muxer and demuxer
 - ffmpeg CLI -bsf option may now be used for input as well as output
 - ffmpeg CLI options may now be used as -/opt <path>, which is equivalent
   to -opt <contents of file <path>>
diff --git a/doc/demuxers.texi b/doc/demuxers.texi
index b70f3a38d7..b4c4daacd9 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 Closed Captions to ASS, using the CC decoder (cc_dec):
+@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 SCC format, using the SCC muxer (scc):
+@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 formats
+that can be stored in RCWT.
+@end itemize
+
 @section sbg
 
 SBaGen script demuxer.
diff --git a/libavformat/Makefile b/libavformat/Makefile
index 94a949f555..a6de720d8c 100644
--- a/libavformat/Makefile
+++ b/libavformat/Makefile
@@ -493,6 +493,7 @@ OBJS-$(CONFIG_QOA_DEMUXER)               += qoadec.o
 OBJS-$(CONFIG_R3D_DEMUXER)               += r3d.o
 OBJS-$(CONFIG_RAWVIDEO_DEMUXER)          += rawvideodec.o
 OBJS-$(CONFIG_RAWVIDEO_MUXER)            += rawenc.o
+OBJS-$(CONFIG_RCWT_DEMUXER)              += rcwtdec.o subtitles.o
 OBJS-$(CONFIG_RCWT_MUXER)                += rcwtenc.o subtitles.o
 OBJS-$(CONFIG_REALTEXT_DEMUXER)          += realtextdec.o subtitles.o
 OBJS-$(CONFIG_REDSPARK_DEMUXER)          += redspark.o
diff --git a/libavformat/allformats.c b/libavformat/allformats.c
index e15d0fa6d7..3140018f8d 100644
--- a/libavformat/allformats.c
+++ b/libavformat/allformats.c
@@ -391,6 +391,7 @@ extern const FFInputFormat  ff_qoa_demuxer;
 extern const FFInputFormat  ff_r3d_demuxer;
 extern const FFInputFormat  ff_rawvideo_demuxer;
 extern const FFOutputFormat ff_rawvideo_muxer;
+extern const FFInputFormat  ff_rcwt_demuxer;
 extern const FFOutputFormat ff_rcwt_muxer;
 extern const FFInputFormat  ff_realtext_demuxer;
 extern const FFInputFormat  ff_redspark_demuxer;
diff --git a/libavformat/rcwtdec.c b/libavformat/rcwtdec.c
new file mode 100644
index 0000000000..ca5bdc1940
--- /dev/null
+++ b/libavformat/rcwtdec.c
@@ -0,0 +1,125 @@
+/*
+ * RCWT (Raw Captions With Time) demuxer
+ *
+ * This file is part of FFmpeg.
+ *
+ * FFmpeg is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * FFmpeg is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with FFmpeg; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+/*
+ * 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.
+ *
+ * This demuxer implements the specification as of March 2024, which has
+ * been stable and unchanged since April 2014.
+ *
+ * A free specification of RCWT can be found here:
+ * @url{https://github.com/CCExtractor/ccextractor/blob/master/docs/BINARY_FILE_FORMAT.TXT}
+ */
+
+#include "avformat.h"
+#include "demux.h"
+#include "internal.h"
+#include "subtitles.h"
+#include "libavutil/intreadwrite.h"
+
+#define RCWT_HEADER_SIZE                    11
+
+typedef struct RCWTContext {
+    FFDemuxSubtitlesQueue q;
+} RCWTContext;
+
+static int rcwt_read_header(AVFormatContext *avf)
+{
+    RCWTContext *rcwt = avf->priv_data;
+
+    AVStream      *st;
+    uint8_t       header[RCWT_HEADER_SIZE];
+    int           nb_bytes;
+
+    /* read header */
+    nb_bytes = ffio_read_size(avf->pb, header, RCWT_HEADER_SIZE);
+    if (nb_bytes < 0)
+        return nb_bytes;
+    if (nb_bytes != RCWT_HEADER_SIZE)
+        return AVERROR_INVALIDDATA;
+
+    if (AV_RB16(header + 6) != 0x0001) {
+        av_log(avf, AV_LOG_ERROR, "RCWT format version is not compatible "
+                                  "(only version 0.001 is known)\n");
+        return AVERROR_INVALIDDATA;
+    }
+
+    av_log(avf, AV_LOG_DEBUG, "RCWT writer application: %02X version: %02x\n",
+                              header[3], header[5]);
+
+    /* setup stream */
+    st = avformat_new_stream(avf, NULL);
+    if (!st)
+        return AVERROR(ENOMEM);
+
+    st->codecpar->codec_type = AVMEDIA_TYPE_SUBTITLE;
+    st->codecpar->codec_id   = AV_CODEC_ID_EIA_608;
+
+    avpriv_set_pts_info(st, 64, 1, 1000);
+
+    /* demux */
+    while (!avio_feof(avf->pb)) {
+        AVPacket      *sub;
+        int           ret;
+        int64_t       cluster_pos       = avio_tell(avf->pb);
+        int64_t       cluster_pts       = avio_rl64(avf->pb);
+        int           cluster_nb_blocks = avio_rl16(avf->pb);
+
+        if (cluster_nb_blocks == 0)
+            continue;
+
+        sub = ff_subtitles_queue_insert(&rcwt->q, NULL, 0, 0);
+        if (!sub)
+            return AVERROR(ENOMEM);
+
+        if ((ret = av_get_packet(avf->pb, sub, cluster_nb_blocks * 3)) < 0)
+            return ret;
+
+        sub->pos = cluster_pos;
+        sub->pts = cluster_pts;
+    }
+
+    ff_subtitles_queue_finalize(avf, &rcwt->q);
+
+    return 0;
+}
+
+static int rcwt_probe(const AVProbeData *p)
+{
+    return p->buf_size > RCWT_HEADER_SIZE   &&
+           AV_RB16(p->buf) == 0xCCCC        &&
+           AV_RB8(p->buf + 2) == 0xED       &&
+           AV_RB16(p->buf + 6) == 0x0001    ? 50 : 0;
+}
+
+const FFInputFormat ff_rcwt_demuxer = {
+    .p.name         = "rcwt",
+    .p.long_name    = NULL_IF_CONFIG_SMALL("RCWT (Raw Captions With Time)"),
+    .p.extensions   = "bin",
+    .p.flags        = AVFMT_TS_DISCONT,
+    .priv_data_size = sizeof(RCWTContext),
+    .flags_internal = FF_FMT_INIT_CLEANUP,
+    .read_probe     = rcwt_probe,
+    .read_header    = rcwt_read_header,
+    .read_packet    = ff_subtitles_read_packet,
+    .read_seek2     = ff_subtitles_read_seek,
+    .read_close     = ff_subtitles_read_close
+};
-- 
2.34.1

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