[FFmpeg-devel] [PATCH] examples/transcoding: Fix time_base handling

[Jack Lau]

To be clear, i want to give an example, i use a 10s duration, 30fps video.
The ifmt_ctx->streams[stream_index]->time_base is same as
ofmt_ctx->streams[stream_index]->time_base after initializing.
The stream->dec_ctx->time_base is default 0,60 but
stream->enc_ctx->time_base is set to av_inv_q(dec_ctx->framerate) so it's
0,30

174                 /* video time_base can be set to whatever is handy and
supported by encoder */
175                 enc_ctx->time_base = av_inv_q(dec_ctx->framerate);

so the twice rescale is this:
input pkt in_tb: 15360   out_tb: 60
output pkt in_tb: 30     out_tb: 15360

so i get one 20s duration and 15fps video(audio duration is normal because
the input's audio sample ratio is 44100)

So i think the problem is that the enc_ctx->time_base shouldn't set to
av_inv_q(dec_ctx->framerate)

On Wed, Feb 5, 2025 at 10:29 AM Jack Lau <jacklau1222gm at gmail.com> wrote:

>
> AVCodecContext.time_base is not used for decoding.
>
>
>
> Thank you for your reply. I understand that time_base is not used during
> decoding, but the transcoding code calls av_packet_rescale_ts twice, once
> before decoding and once after encoding, as shown below:
>
> 540         if (filter_ctx[stream_index].filter_graph) {
> 541             StreamContext *stream = &stream_ctx[stream_index];
> 542
> 543             av_log(NULL, AV_LOG_DEBUG, "Going to reencode&filter the
> frame\n");
> 544
> 545             av_packet_rescale_ts(packet,
>
>
> 546
>  ifmt_ctx->streams[stream_index]->time_base,
> 547                                  stream->dec_ctx->time_base);
> 548             ret = avcodec_send_packet(stream->dec_ctx, packet);
>
> 448         /* prepare packet for muxing */
>
>
> 449         enc_pkt->stream_index = stream_index;
> 450         av_packet_rescale_ts(enc_pkt,
> 451                              stream->enc_ctx->time_base,
> 452
>  ofmt_ctx->streams[stream_index]->time_base);
> 453
> 454         av_log(NULL, AV_LOG_DEBUG, "Muxing frame\n");
> 455         /* mux encoded frame */
> 456         ret = av_interleaved_write_frame(ofmt_ctx, enc_pkt);
>
> If dec_ctx->time_base and enc_ctx->time_base are not consistent, it can
> lead to incorrect packet duration, which in turn causes issues with the
> output file's duration.
>