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

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

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 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.

A free specification of RCWT can be found here:
https://github.com/CCExtractor/ccextractor/blob/master/docs/BINARY_FILE_FORMAT.TXT
This muxer implements the specification as of 01/05/2024, 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:
(1)  This muxer will identify as "FF" as the writing program identifier, so
as to be honest about the output's origin.

(2)  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.

(3)  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).

---
 libavformat/Makefile     |   1 +
 libavformat/allformats.c |   1 +
 libavformat/rcwtenc.c    | 203 +++++++++++++++++++++++++++++++++++++++
 tests/fate/subtitles.mak |   3 +
 tests/ref/fate/sub-rcwt  |   1 +
 5 files changed, 209 insertions(+)
 create mode 100644 libavformat/rcwtenc.c
 create mode 100644 tests/ref/fate/sub-rcwt

On date Friday 2024-01-05 20:14:58 -0600, Marth64 wrote:
> Signed-off-by: Marth64 <marth64@proxyid.net>
> 
> 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 file for later CC processing or conversion. As a result,

file file

> 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.
> 
> A free specification of RCWT can be found here:
> https://github.com/CCExtractor/ccextractor/blob/master/docs/BINARY_FILE_FORMAT.TXT

> This muxer implements the specification as of 01/05/2024, which has

nit: use 2023-01-05 or EU format (2024/01/05) to avoid ambiguity

> 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:
> (1)  This muxer will identify as "FF" as the writing program identifier, so
> as to be honest about the output's origin.
> 
> (2)  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.
> 
> (3)  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).

This is a nice highlight and should be probably partially moved to
muxers.texi to expose this information to users (although many/most
are not documented, we should start to do so).

> ---
>  libavformat/Makefile     |   1 +
>  libavformat/allformats.c |   1 +
>  libavformat/rcwtenc.c    | 203 +++++++++++++++++++++++++++++++++++++++
>  tests/fate/subtitles.mak |   3 +
>  tests/ref/fate/sub-rcwt  |   1 +

missing Changelog entry, and I don't remember if new elements addition
entails a minor library bump (probably it should)

>  5 files changed, 209 insertions(+)
>  create mode 100644 libavformat/rcwtenc.c
>  create mode 100644 tests/ref/fate/sub-rcwt
> 
> diff --git a/libavformat/Makefile b/libavformat/Makefile
> index 45dba53044..03c2c70e67 100644
> --- a/libavformat/Makefile
> +++ b/libavformat/Makefile
> @@ -489,6 +489,7 @@ 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 dc2acf575c..fb14f15739 100644
> --- a/libavformat/allformats.c
> +++ b/libavformat/allformats.c
> @@ -388,6 +388,7 @@ 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..f70a80b175
> --- /dev/null
> +++ b/libavformat/rcwtenc.c
> @@ -0,0 +1,203 @@
> +/*
> + * 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
> + */
> +

> +/*
> + * 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 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.
> + *
> + * A free specification of RCWT can be found here:
> + * https://github.com/CCExtractor/ccextractor/blob/master/docs/BINARY_FILE_FORMAT.TXT
> + * This muxer implements the specification as of 01/05/2024, 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:
> + * (1)  This muxer will identify as "FF" as the writing program identifier, so
> + *      as to be honest about the output's origin.
> + * (2)  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.
> + * (3)  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).
> + */

ditto

> +
> +#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 * sizeof(uint8_t)

or just use 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));

nit: weird indent

> +    }
> +
> +    rcwt_init_cluster(avf);
> +}
> +
> +static int rcwt_write_header(AVFormatContext *avf)
> +{

> +    if (avf->nb_streams != 1
> +            || avf->streams[0]->codecpar->codec_type != AVMEDIA_TYPE_SUBTITLE
> +            || avf->streams[0]->codecpar->codec_id != AV_CODEC_ID_EIA_608) {

nit+: weird indent

> +        av_log(avf, AV_LOG_ERROR,

> +               "RCWT supports only one CC (608/708) stream\n");

this could be more explicit:
"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;

nit: no need to shift

> +        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], sizeof(uint8_t));

indent

> +        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
> +};

No more comments from me, thanks.