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/*
* 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 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:
* (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).
*
* 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 "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_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_pos = 0;
rcwt->cluster_pts = AV_NOPTS_VALUE;
}
static void rcwt_flush_cluster(AVFormatContext *avf)
{
RCWTContext *rcwt = avf->priv_data;
if (rcwt->cluster_pos > 0) {
avio_wl64(avf->pb, rcwt->cluster_pts);
avio_wl16(avf->pb, rcwt->cluster_pos / RCWT_BLOCK_SIZE);
avio_write(avf->pb, rcwt->cluster_buf, rcwt->cluster_pos);
}
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;
if (pkt->size < RCWT_BLOCK_SIZE)
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 - RCWT_BLOCK_SIZE;) {
uint8_t cc_valid;
uint8_t cc_type;
if (rcwt->cluster_pos == RCWT_CLUSTER_MAX_BLOCKS * RCWT_BLOCK_SIZE) {
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 (!(cc_valid || cc_type == 3)) {
i++;
continue;
}
memcpy(&rcwt->cluster_buf[rcwt->cluster_pos], &pkt->data[i], 3);
rcwt->cluster_pos += 3;
i += 3;
}
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
};
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