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

[Jack Lau via ffmpeg-devel <ffmpeg-devel@ffmpeg.org>]

From: Jack Lau <jacklau1222gm@gmail.com>

The `dec_ctx->time_base` was incorrectly default set by avcodec_open2(), while `enc_ctx->time_base` was derived from `dec_ctx->framerate`. This mismatch could cause incorrect video duration in the output.

This patch corrects the issue by adjusting the `enc_ctx->time_base` calculation to account for `ticks_per_frame`, ensuring that the time base is consistent between the decoder and encoder contexts.
---
 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..09c2ad4b62 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 = av_inv_q(av_mul_q(dec_ctx->framerate, (AVRational){dec_ctx->ticks_per_frame, 1}));
             } 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 = (AVRational){1, dec_ctx->sample_rate};
             }
 
             if (ofmt_ctx->oformat->flags & AVFMT_GLOBALHEADER)
-- 
2.48.1

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