| Message ID | MN2PR13MB275255C8B4A6B06598E3158DBAB80@MN2PR13MB2752.namprd13.prod.outlook.com |
|---|---|
| State | Superseded |
| Headers | show |
Am Mi., 4. Sept. 2019 um 22:43 Uhr schrieb Soft Works <softworkz@hotmail.com>: > > > This allows having a video stream as reference stream when using the segment muxer Please add the command line that needs this patch. Carl Eugen
> -----Original Message----- > From: ffmpeg-devel <ffmpeg-devel-bounces@ffmpeg.org> On Behalf Of > Carl Eugen Hoyos > > Am Mi., 4. Sept. 2019 um 22:43 Uhr schrieb Soft Works > <softworkz@hotmail.com>: > > > > > > This allows having a video stream as reference stream when using the > > segment muxer > > Please add the command line that needs this patch. > > Carl Eugen The short version would be this: ------------------ Allows mapping a video stream as reference stream for creating vtt segments: ffmpeg -i INPUT -map 0:3 -c:0 webvtt -map 0:0 -c:v:0 copy -f segment -segment_format webvtt -segment_time 6 -write_empty_segments 1 -y "sub_segment3%d.vtt" ------------------ Is this sufficient? Otherwise, here's the long story: THE REQUIREMENT I need to do HLS segmentation for video/audio while additionally creating one or more subtitle playlists and segments. Very simplified it’s like this: ffmpeg -i INPUT -map 0:0 -map 0:2 -c:v:0 copy -c:a:0 copy -f segment -segment_format mpegts -segment_time 6 -y "segment%d.ts" -map 0:3 -c:0 webvtt -f segment -segment_format webvtt -segment_time 6 -write_empty_segments 1 -y "sub_segment3%d.vtt" THE PROBLEM The mpegts segments are being created in a constant and regular way. But not necessearily in the case of subtitles: When hitting a time range without any subtitle event, the segmenter stops generating subtitle segments, even when write_empty_segments is configured. It’s not that write_empty_segments would not be working, though: As soon as a new subtitle event reaches the muxer, it catches up by creating the missed empty segments all at once. This might be OK for precreating content for HLS/VOD scenarios but it’s a behavioral bug when you rely on having accurate results while the process is running. THE CAUSE That’s rather trivial: When there are no subtitle events, there are no packets arriving at the segment muxer (for vtt) and there’s probably nothiing that the segment muxer could do about it. A REASONABLE APPROACH? After some rather ugly ideas I found that the best way to bring some more action into the vtt segmenter might be to map the video stream in addition to the subtitle stream – not with the purpose of outputting the video – just to ensure regular activity in the segment muxer. Also, the segment muxer has implementation for having a ‚reference_stream‘ and the video stream is supposed to be this anyway. Remains one problem: The WebVTT muxer (libavformat/webvttenc.c) errors when there’s more than a single stream. ("Exactly one WebVTT stream is needed.) And that’s what this commit does: Removed the single-stream limit from the webvtt muxer and simply discard packets from other streams instead of failing. softworkz
diff --git a/libavformat/webvttenc.c b/libavformat/webvttenc.c index 61b7f54622..be85c03130 100644 --- a/libavformat/webvttenc.c +++ b/libavformat/webvttenc.c @@ -49,8 +49,8 @@ static int webvtt_write_header(AVFormatContext *ctx) AVCodecParameters *par = ctx->streams[0]->codecpar; AVIOContext *pb = ctx->pb; - if (ctx->nb_streams != 1 || par->codec_id != AV_CODEC_ID_WEBVTT) { - av_log(ctx, AV_LOG_ERROR, "Exactly one WebVTT stream is needed.\n"); + if (par->codec_id != AV_CODEC_ID_WEBVTT) { + av_log(ctx, AV_LOG_ERROR, "First stream must be WebVTT.\n"); return AVERROR(EINVAL); } @@ -68,6 +68,9 @@ static int webvtt_write_packet(AVFormatContext *ctx, AVPacket *pkt) int id_size, settings_size; uint8_t *id, *settings; + if (pkt->stream_index != 0) + return 0; + avio_printf(pb, "\n"); id = av_packet_get_side_data(pkt, AV_PKT_DATA_WEBVTT_IDENTIFIER,