Re: [FFmpeg-devel] [PATCH v3 4/4] bsf: Add new bitstream filter to set SCTE-35 pts_adjustment when reclocking

[James Almer <jamrial@gmail.com>]

On 7/21/2023 5:37 PM, Devin Heitmueller wrote:
> Because SCTE-35 messages are represented in TS streams as sections
> rather than PES packets, we cannot rely on ffmpeg's standard
> mechanisms to adjust PTS values if reclocking the stream.
> This filter will leverage the SCTE-35 pts_adjust field to
> compensate for any change in the PTS values in the stream.
> 
> See SCTE-35 2019 Sec 9.6 for information about the use of
> the pts_adjust field.
> 
> This filter also tweaks the mpegtsenc mux to automatically
> add it so the user doesn't have to include it manually.
> 
> Thanks to Andreas Rheinhardt for providing feedback/suggestions
> on improving the patch.
> 
> Signed-off-by: Devin Heitmueller <dheitmueller@ltnglobal.com>
> ---
>   doc/bitstream_filters.texi       |   9 ++++
>   libavcodec/Makefile              |   1 +
>   libavcodec/bitstream_filters.c   |   1 +
>   libavcodec/scte35ptsadjust_bsf.c | 103 +++++++++++++++++++++++++++++++++++++++
>   libavformat/mpegtsenc.c          |   2 +
>   5 files changed, 116 insertions(+)
>   create mode 100644 libavcodec/scte35ptsadjust_bsf.c
> 
> diff --git a/doc/bitstream_filters.texi b/doc/bitstream_filters.texi
> index c63c203..068b0c9 100644
> --- a/doc/bitstream_filters.texi
> +++ b/doc/bitstream_filters.texi
> @@ -797,6 +797,15 @@ Remove extradata from all frames.
>   @end table
>   @end table
>   
> +@section scte35ptsadjust
> +This bitstream filter makes use of side data injected by the MPEG-TS demux
> +in order to rewrite the PTS adjust field within SCTE-35 packets.  This
> +ensures the pts_adjust field contains a valid value if the caller changes
> +the timebase of the stream.
> +
> +The bitstream filter is added automatically by the mpegtsenc mux, and no
> +action is required on the part of the user.
> +
>   @section setts
>   Set PTS and DTS in packets.
>   
> diff --git a/libavcodec/Makefile b/libavcodec/Makefile
> index 1b0226c..4c1b312 100644
> --- a/libavcodec/Makefile
> +++ b/libavcodec/Makefile
> @@ -1253,6 +1253,7 @@ OBJS-$(CONFIG_PCM_RECHUNK_BSF)            += pcm_rechunk_bsf.o
>   OBJS-$(CONFIG_PGS_FRAME_MERGE_BSF)        += pgs_frame_merge_bsf.o
>   OBJS-$(CONFIG_PRORES_METADATA_BSF)        += prores_metadata_bsf.o
>   OBJS-$(CONFIG_REMOVE_EXTRADATA_BSF)       += remove_extradata_bsf.o av1_parse.o
> +OBJS-$(CONFIG_SCTE35PTSADJUST_BSF)        += scte35ptsadjust_bsf.o
>   OBJS-$(CONFIG_SETTS_BSF)                  += setts_bsf.o
>   OBJS-$(CONFIG_TEXT2MOVSUB_BSF)            += movsub_bsf.o
>   OBJS-$(CONFIG_TRACE_HEADERS_BSF)          += trace_headers_bsf.o
> diff --git a/libavcodec/bitstream_filters.c b/libavcodec/bitstream_filters.c
> index 1e9a676..60ed164 100644
> --- a/libavcodec/bitstream_filters.c
> +++ b/libavcodec/bitstream_filters.c
> @@ -57,6 +57,7 @@ extern const FFBitStreamFilter ff_pcm_rechunk_bsf;
>   extern const FFBitStreamFilter ff_pgs_frame_merge_bsf;
>   extern const FFBitStreamFilter ff_prores_metadata_bsf;
>   extern const FFBitStreamFilter ff_remove_extradata_bsf;
> +extern const FFBitStreamFilter ff_scte35ptsadjust_bsf;
>   extern const FFBitStreamFilter ff_setts_bsf;
>   extern const FFBitStreamFilter ff_text2movsub_bsf;
>   extern const FFBitStreamFilter ff_trace_headers_bsf;
> diff --git a/libavcodec/scte35ptsadjust_bsf.c b/libavcodec/scte35ptsadjust_bsf.c
> new file mode 100644
> index 0000000..9870737
> --- /dev/null
> +++ b/libavcodec/scte35ptsadjust_bsf.c
> @@ -0,0 +1,103 @@
> +/*
> + * SCTE-35 PTS fixup bitstream filter
> + * Copyright (c) 2023 LTN Global Communications, Inc.
> + *
> + * Author: Devin Heitmueller <dheitmueller@ltnglobal.com>
> + *
> + * Because SCTE-35 messages are represented in TS streams as sections
> + * rather than PES packets, we cannot rely on ffmpeg's standard
> + * mechanisms to adjust PTS values if reclocking the stream.
> + * This filter will leverage the SCTE-35 pts_adjust field to
> + * compensate for any change in the PTS values in the stream.
> + *
> + * See SCTE-35 2019 Sec 9.6 for information about the use of
> + * the pts_adjust field.
> + *
> + * 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 "bsf.h"
> +#include "bsf_internal.h"
> +#include "defs.h"
> +#include "libavutil/intreadwrite.h"
> +
> +static int scte35ptsadjust_filter(AVBSFContext *ctx, AVPacket *pkt)
> +{
> +    const AVTransportTimestamp *transport_ts;
> +    int64_t cur_pts_adjust;
> +    int ret = 0;
> +
> +    ret = ff_bsf_get_packet_ref(ctx, pkt);
> +    if (ret < 0)
> +        return ret;
> +
> +    /* Retrieve the original PTS, which will be used to calculate the pts_adjust */
> +    transport_ts = (AVTransportTimestamp *) av_packet_get_side_data(pkt, AV_PKT_DATA_TRANSPORT_TIMESTAMP, NULL);
> +    if (transport_ts == NULL) {
> +        /* No original PTS specified, so just pass the packet through */
> +        return 0;
> +    }
> +
> +    /* The pts_adjust field is logically buf[4]-buf[8] of the payload */
> +    if (pkt->size < 9) {
> +        ret = AVERROR_INVALIDDATA;
> +        goto fail;
> +    }
> +
> +    /* Extract the current pts_adjust value from the packet */
> +    cur_pts_adjust = ((int64_t) pkt->data[4] & (int64_t) 0x01 << 32) |
> +                     ((int64_t) pkt->data[5] << 24) |
> +                     ((int64_t) pkt->data[6] << 16) |
> +                     ((int64_t) pkt->data[7] << 8) |
> +                     ((int64_t) pkt->data[8] );

I think this is cleaner:

cur_pts_adjust = ((int64_t)(pkt->data[4] & 1) << 32 ) | 
AV_RB32(pkt->data + 5);

> +
> +    av_log(ctx, AV_LOG_DEBUG, "pts=%" PRId64 "(%d/%d) orig_pts=%" PRId64 "(%d/%d) pts_adjust=%" PRId64 "\n",
> +           pkt->pts, pkt->time_base.num, pkt->time_base.den,
> +           transport_ts->pts, transport_ts->time_base.num, transport_ts->time_base.den, cur_pts_adjust);
> +
> +    /* Compute the new PTS adjust value */
> +    cur_pts_adjust -= av_rescale_q(transport_ts->pts, transport_ts->time_base, (AVRational){1, 90000});
> +    cur_pts_adjust += av_rescale_q(pkt->pts, pkt->time_base, (AVRational){1, 90000});
> +    cur_pts_adjust &= (int64_t) 0x1ffffffff;

cur_pts_adjust &= 0x1FFFFFFFFLL;

> +
> +    av_log(ctx, AV_LOG_DEBUG, "new pts_adjust=%" PRId64 "\n", cur_pts_adjust);
> +
> +    ret = av_packet_make_writable(pkt);
> +    if (ret < 0)
> +        goto fail;
> +
> +    /* Insert the updated pts_adjust value */
> +    pkt->data[4] &= 0xfe; /* Preserve top 7 unrelated bits */
> +    pkt->data[4] |= cur_pts_adjust >> 32;
> +    AV_WB32(pkt->data + 5, cur_pts_adjust);
> +
> +fail:
> +    if (ret < 0)
> +        av_packet_unref(pkt);
> +
> +    return ret;
> +}
> +
> +static const enum AVCodecID codec_ids[] = {
> +    AV_CODEC_ID_SCTE_35, AV_CODEC_ID_NONE,
> +};
> +
> +const FFBitStreamFilter ff_scte35ptsadjust_bsf = {
> +    .p.name         = "scte35ptsadjust",
> +    .p.codec_ids    = codec_ids,
> +    .filter         = scte35ptsadjust_filter,
> +};
> diff --git a/libavformat/mpegtsenc.c b/libavformat/mpegtsenc.c
> index c6cd1fd..48d7833 100644
> --- a/libavformat/mpegtsenc.c
> +++ b/libavformat/mpegtsenc.c
> @@ -2337,6 +2337,8 @@ static int mpegts_check_bitstream(AVFormatContext *s, AVStream *st,
>                                 (st->codecpar->extradata_size > 0 &&
>                                  st->codecpar->extradata[0] == 1)))
>               ret = ff_stream_add_bitstream_filter(st, "hevc_mp4toannexb", NULL);
> +    } else if (st->codecpar->codec_id == AV_CODEC_ID_SCTE_35) {
> +        ret = ff_stream_add_bitstream_filter(st, "scte35ptsadjust", NULL);
>       }
>   
>       return ret;
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