[FFmpeg-devel] [PATCH v2] libavformat: add RCWT closed caption muxer

[Marth64 <marth64@proxyid.net>]

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

Thank you for the good feedback and review. Most feedback is addressed.

>  nit: no need to shift
I left this alone only since I see it being done this way in lavf/ccfifo
and other documentation. I assumed it could be doing the shift for a reason,
but I can study further why if you think it shouldn't shift.

> I don't remember if new elements addition entails a minor library bump (probably it should)
I reviewed APIchangelog and didn't see similar type of bumps for adding to allformats.
The publically exposed codec ID has been in ffmpeg for a long time.
But I am happy to patch in a version bump with your confirmation.

Much appreciated,
Marth64

---
 Changelog                |   1 +
 doc/muxers.texi          |  40 ++++++++++
 libavformat/Makefile     |   1 +
 libavformat/allformats.c |   1 +
 libavformat/rcwtenc.c    | 166 +++++++++++++++++++++++++++++++++++++++
 tests/fate/subtitles.mak |   3 +
 tests/ref/fate/sub-rcwt  |   1 +
 7 files changed, 213 insertions(+)
 create mode 100644 libavformat/rcwtenc.c
 create mode 100644 tests/ref/fate/sub-rcwt

diff --git a/Changelog b/Changelog
index 5b2899d05b..3d60f688ca 100644
--- a/Changelog
+++ b/Changelog
@@ -18,6 +18,7 @@ version <next>:
 - lavu/eval: introduce randomi() function in expressions
 - VVC decoder
 - fsync filter
+- Raw Captions with Time (RCWT) closed caption demuxer
 
 version 6.1:
 - libaribcaption decoder
diff --git a/doc/muxers.texi b/doc/muxers.texi
index 7b705b6a9e..0bdeaeeaf3 100644
--- a/doc/muxers.texi
+++ b/doc/muxers.texi
@@ -2232,6 +2232,46 @@ Extensions: thd
 
 SMPTE 421M / VC-1 video.
 
+@anchor{rcwt}
+@section rcwt
+
+Raw Captions With Time (RCWT) is a format native to ccextractor, a commonly
+used open source tool for processing 608/708 closed caption (CC) sources.
+It can be used to archive the original, raw CC bitstream and to produce
+a source file for later CC processing or conversion. As a result,
+it also allows for interopability with ccextractor for processing CC data
+extracted via ffmpeg. The format is simple to parse and can be used
+to retain all lines and variants of CC.
+
+This muxer implements the specification as of 2024-01-05, which has
+been stable and unchanged for 10 years as of this writing.
+
+This muxer will have some nuances from the way that ccextractor muxes RCWT.
+No compatibility issues when processing the output with ccextractor
+have been observed as a result of this so far, but mileage may vary
+and outputs will not be a bit-exact match.
+
+Specifically, the differences are:
+@enumerate
+@item
+This muxer will identify as "FF" as the writing program identifier, so
+as to be honest about the output's origin.
+@item
+ffmpeg's MPEG-1/2, H264, HEVC, etc. decoders extract closed captioning
+data differently than ccextractor from embedded SEI/user data.
+For example, DVD captioning bytes will be translated to ATSC A53 format.
+This allows ffmpeg to handle 608/708 in a consistant way downstream.
+This is a lossless conversion and the meaningful data is retained.
+@item
+This muxer will not alter the extracted data except to remove invalid
+packets in between valid CC blocks. On the other hand, ccextractor
+will by default remove mid-stream padding, and add padding at the end
+of the stream (in order to convey the end time of the source video).
+@end enumerate
+
+A free specification of RCWT can be found here:
+@url{https://github.com/CCExtractor/ccextractor/blob/master/docs/BINARY_FILE_FORMAT.TXT}
+
 @anchor{segment}
 @section segment, stream_segment, ssegment
 
diff --git a/libavformat/Makefile b/libavformat/Makefile
index 581e378d95..dcc99eeac4 100644
--- a/libavformat/Makefile
+++ b/libavformat/Makefile
@@ -490,6 +490,7 @@ 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-$(CONFIG_RCWT_MUXER)                += rcwtenc.o subtitles.o
 OBJS-$(CONFIG_REALTEXT_DEMUXER)          += realtextdec.o subtitles.o
 OBJS-$(CONFIG_REDSPARK_DEMUXER)          += redspark.o
 OBJS-$(CONFIG_RKA_DEMUXER)               += rka.o apetag.o img2.o
diff --git a/libavformat/allformats.c b/libavformat/allformats.c
index ce6be5f04d..b04b43cab3 100644
--- a/libavformat/allformats.c
+++ b/libavformat/allformats.c
@@ -389,6 +389,7 @@ extern const AVInputFormat  ff_qoa_demuxer;
 extern const AVInputFormat  ff_r3d_demuxer;
 extern const AVInputFormat  ff_rawvideo_demuxer;
 extern const FFOutputFormat ff_rawvideo_muxer;
+extern const FFOutputFormat ff_rcwt_muxer;
 extern const AVInputFormat  ff_realtext_demuxer;
 extern const AVInputFormat  ff_redspark_demuxer;
 extern const AVInputFormat  ff_rka_demuxer;
diff --git a/libavformat/rcwtenc.c b/libavformat/rcwtenc.c
new file mode 100644
index 0000000000..17382548aa
--- /dev/null
+++ b/libavformat/rcwtenc.c
@@ -0,0 +1,166 @@
+/*
+ * Raw Captions With Time (RCWT) muxer
+ * Author: Marth64 <marth64@proxyid.net>
+ *
+ * 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
+ */
+
+#include "avformat.h"
+#include "internal.h"
+#include "mux.h"
+#include "libavutil/log.h"
+#include "libavutil/intreadwrite.h"
+
+#define RCWT_CLUSTER_MAX_BLOCKS             65535
+#define RCWT_BLOCK_SIZE                     3
+
+typedef struct RCWTContext {
+    int cluster_nb_blocks;
+    int cluster_pos;
+    int64_t cluster_pts;
+    uint8_t cluster_buf[RCWT_CLUSTER_MAX_BLOCKS * RCWT_BLOCK_SIZE];
+} RCWTContext;
+
+static void rcwt_init_cluster(AVFormatContext *avf)
+{
+    RCWTContext *rcwt = avf->priv_data;
+
+    rcwt->cluster_nb_blocks = 0;
+    rcwt->cluster_pos = 0;
+    rcwt->cluster_pts = AV_NOPTS_VALUE;
+    memset(rcwt->cluster_buf, 0, sizeof(rcwt->cluster_buf));
+}
+
+static void rcwt_flush_cluster(AVFormatContext *avf)
+{
+    RCWTContext *rcwt = avf->priv_data;
+
+    if (rcwt->cluster_nb_blocks > 0) {
+        avio_wl64(avf->pb, rcwt->cluster_pts);
+        avio_wl16(avf->pb, rcwt->cluster_nb_blocks);
+        avio_write(avf->pb, rcwt->cluster_buf, (rcwt->cluster_nb_blocks * RCWT_BLOCK_SIZE));
+    }
+
+    rcwt_init_cluster(avf);
+}
+
+static int rcwt_write_header(AVFormatContext *avf)
+{
+    if (avf->nb_streams != 1 || avf->streams[0]->codecpar->codec_id != AV_CODEC_ID_EIA_608) {
+        av_log(avf, AV_LOG_ERROR,
+                "RCWT supports only one CC (608/708) stream, more than one stream was "
+                "provided or its codec type was not CC (608/708)\n");
+        return AVERROR(EINVAL);
+    }
+
+    avpriv_set_pts_info(avf->streams[0], 64, 1, 1000);
+
+    /* magic number */
+    avio_wb16(avf->pb, 0xCCCC);
+    avio_w8(avf->pb, 0xED);
+
+    /* program version (identify as ffmpeg) */
+    avio_wb16(avf->pb, 0xFF00);
+    avio_w8(avf->pb, 0x60);
+
+    /* format version, only version 0.001 supported for now */
+    avio_wb16(avf->pb, 0x0001);
+
+    /* reserved */
+    avio_wb16(avf->pb, 0x000);
+    avio_w8(avf->pb, 0x00);
+
+    rcwt_init_cluster(avf);
+
+    return 0;
+}
+
+static int rcwt_write_packet(AVFormatContext *avf, AVPacket *pkt)
+{
+    RCWTContext *rcwt = avf->priv_data;
+
+    int in_block = 0;
+    int nb_block_bytes = 0;
+
+    if (pkt->size == 0)
+        return 0;
+
+    /* new PTS, new cluster */
+    if (pkt->pts != rcwt->cluster_pts) {
+        rcwt_flush_cluster(avf);
+        rcwt->cluster_pts = pkt->pts;
+    }
+
+    if (pkt->pts == AV_NOPTS_VALUE) {
+        av_log(avf, AV_LOG_WARNING, "Ignoring CC packet with no PTS\n");
+        return 0;
+    }
+
+    for (int i = 0; i < pkt->size; i++) {
+        uint8_t cc_valid;
+        uint8_t cc_type;
+
+        if (rcwt->cluster_nb_blocks == RCWT_CLUSTER_MAX_BLOCKS) {
+            av_log(avf, AV_LOG_WARNING, "Starting new cluster due to size\n");
+            rcwt_flush_cluster(avf);
+        }
+
+        cc_valid = (pkt->data[i] & 0x04) >> 2;
+        cc_type = pkt->data[i] & 0x03;
+
+        if (!in_block && !(cc_valid || cc_type == 3))
+            continue;
+
+        memcpy(&rcwt->cluster_buf[rcwt->cluster_pos], &pkt->data[i], 1);
+        rcwt->cluster_pos++;
+
+        if (!in_block) {
+            in_block = 1;
+            nb_block_bytes = 1;
+            continue;
+        }
+
+        nb_block_bytes++;
+
+        if (nb_block_bytes == RCWT_BLOCK_SIZE) {
+            in_block = 0;
+            nb_block_bytes = 0;
+            rcwt->cluster_nb_blocks++;
+        }
+    }
+
+    return 0;
+}
+
+static int rcwt_write_trailer(AVFormatContext *avf)
+{
+    rcwt_flush_cluster(avf);
+
+    return 0;
+}
+
+const FFOutputFormat ff_rcwt_muxer = {
+    .p.name             = "rcwt",
+    .p.long_name        = NULL_IF_CONFIG_SMALL("Raw Captions With Time"),
+    .p.extensions       = "bin",
+    .p.flags            = AVFMT_GLOBALHEADER | AVFMT_VARIABLE_FPS | AVFMT_TS_NONSTRICT,
+    .p.subtitle_codec   = AV_CODEC_ID_EIA_608,
+    .priv_data_size     = sizeof(RCWTContext),
+    .write_header       = rcwt_write_header,
+    .write_packet       = rcwt_write_packet,
+    .write_trailer      = rcwt_write_trailer
+};
diff --git a/tests/fate/subtitles.mak b/tests/fate/subtitles.mak
index 59595b9cc1..d7edd31e85 100644
--- a/tests/fate/subtitles.mak
+++ b/tests/fate/subtitles.mak
@@ -118,6 +118,9 @@ fate-sub-scc: CMD = fmtstdout ass -ss 57 -i $(TARGET_SAMPLES)/sub/witch.scc
 FATE_SUBTITLES-$(call DEMMUX, SCC, SCC) += fate-sub-scc-remux
 fate-sub-scc-remux: CMD = fmtstdout scc -i $(TARGET_SAMPLES)/sub/witch.scc -ss 4:00 -map 0 -c copy
 
+FATE_SUBTITLES-$(call DEMMUX, SCC, RCWT) += fate-sub-rcwt
+fate-sub-rcwt: CMD = md5 -i $(TARGET_SAMPLES)/sub/witch.scc -map 0 -c copy -f rcwt
+
 FATE_SUBTITLES-$(call ALLYES, MPEGTS_DEMUXER DVBSUB_DECODER DVBSUB_ENCODER) += fate-sub-dvb
 fate-sub-dvb: CMD = framecrc -i $(TARGET_SAMPLES)/sub/dvbsubtest_filter.ts -map s:0 -c dvbsub
 
diff --git a/tests/ref/fate/sub-rcwt b/tests/ref/fate/sub-rcwt
new file mode 100644
index 0000000000..722cbe1c5b
--- /dev/null
+++ b/tests/ref/fate/sub-rcwt
@@ -0,0 +1 @@
+d86f179094a5752d68aa97d82cf887b0
-- 
2.34.1

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