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>

Changelog

@@ -2,6 +2,7 @@ Entries are sorted chronologically from oldest to youngest within each release,
 releases are sorted from youngest to oldest.
 
 version <next>:
+- Raw Captions with Time (RCWT) closed caption demuxer
 
 
 version 7.0:

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.

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

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;

libavformat/rcwtdec.c

@@ -0,0 +1,122 @@
+/*
+ * 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           ret;
+
+    /* read header */
+    ret = ffio_read_size(avf->pb, header, RCWT_HEADER_SIZE);
+    if (ret < 0)
+        return ret;
+
+    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;
+        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);
+
+        ret = av_get_packet(avf->pb, sub, cluster_nb_blocks * 3);
+        if (ret < 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.flags        = AVFMT_TS_DISCONT,
+    .priv_data_size = sizeof(RCWTContext),
+    .flags_internal = FF_INFMT_FLAG_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
+};