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

[James Almer <jamrial@gmail.com>]

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> From bfd5500a5448ad468d32994816e8a55c0d4a2428 Mon Sep 17 00:00:00 2001
> From: Jack Lau <jacklau1222gm@gmail.com>
> Date: Tue, 4 Feb 2025 21:39:20 +0800
> Subject: [PATCH] examples/transcoding: Fix time_base handling
> 
> The `dec_ctx->time_base` was incorrectly default set to 0/60, while `enc_ctx->time_base` was derived from `dec_ctx->framerate`. This mismatch could cause incorrect video duration in the output.
> 
> This patch aligns `enc_ctx->time_base` with `dec_ctx->time_base` to prevent rescaling issues and ensure correct video duration.
> ---
>  doc/examples/transcoding.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/doc/examples/transcoding.c b/doc/examples/transcoding.c
> index 013f89fc7d..847bdb7e1a 100644
> --- a/doc/examples/transcoding.c
> +++ b/doc/examples/transcoding.c
> @@ -172,7 +172,7 @@ static int open_output_file(const char *filename)
>                  else
>                      enc_ctx->pix_fmt = dec_ctx->pix_fmt;
>                  /* video time_base can be set to whatever is handy and supported by encoder */
> -                enc_ctx->time_base = av_inv_q(dec_ctx->framerate);
> +                enc_ctx->time_base = dec_ctx->time_base;

AVCodecContext.time_base is not used for decoding.

>              } else {
>                  enc_ctx->sample_rate = dec_ctx->sample_rate;
>                  ret = av_channel_layout_copy(&enc_ctx->ch_layout, &dec_ctx->ch_layout);
> @@ -180,7 +180,7 @@ static int open_output_file(const char *filename)
>                      return ret;
>                  /* take first format from list of supported formats */
>                  enc_ctx->sample_fmt = encoder->sample_fmts[0];
> -                enc_ctx->time_base = (AVRational){1, enc_ctx->sample_rate};
> +                enc_ctx->time_base = dec_ctx->time_base;
>              }
>  
>              if (ofmt_ctx->oformat->flags & AVFMT_GLOBALHEADER)
> -- 
> 2.45.2
> 

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