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Stop fabricating a 30 fps fallback in seek() #62

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Merged
kmatzen merged 1 commit into from
Jul 18, 2026
Merged

Stop fabricating a 30 fps fallback in seek() #62

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36 changes: 20 additions & 16 deletions src/VideoReader.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -338,19 +338,28 @@ bool VideoReader::seek(int64_t frame_number) {
(frame_number < current_frame_) || (frame_number - current_frame_ > 50);

if (need_keyframe_seek) {
int64_t target_ts;
if (stream->avg_frame_rate.num != 0 && stream->avg_frame_rate.den != 0) {
target_ts = av_rescale_q(frame_number,
av_inv_q(stream->avg_frame_rate),
stream->time_base);
} else {
AVRational fps = stream->r_frame_rate;
if (fps.num == 0 || fps.den == 0) {
fps = {30, 1};
}
target_ts = av_rescale_q(frame_number, av_inv_q(fps), stream->time_base);
// Keyframe seeking needs a frame rate twice: to turn the requested
// frame index into a target timestamp, and to turn the landing
// timestamp back into an index. If the container declares none there
// is no honest way to do either, so fail here rather than proceed on a
// fabricated rate -- getFPS() reports 0.0 in exactly this case, and the
// two must not disagree.
AVRational seek_fps =
(stream->avg_frame_rate.num != 0 && stream->avg_frame_rate.den != 0)
? stream->avg_frame_rate
: stream->r_frame_rate;

if (seek_fps.num == 0 || seek_fps.den == 0) {
detail::log(LogLevel::Error)
<< "framewright::VideoReader: Cannot seek by frame number without a frame rate"
<< std::endl;
return false;
}

const double frame_duration = av_q2d(av_inv_q(seek_fps));
const int64_t target_ts =
av_rescale_q(frame_number, av_inv_q(seek_fps), stream->time_base);

auto seek_to_keyframe = [&]() -> bool {
int ret =
av_seek_frame(formatCtx_, videoStreamIndex_, target_ts, AVSEEK_FLAG_BACKWARD);
Expand All @@ -376,11 +385,6 @@ bool VideoReader::seek(int64_t frame_number) {
return false;
}

AVRational seek_fps =
stream->avg_frame_rate.num != 0 ? stream->avg_frame_rate : stream->r_frame_rate;
double frame_duration =
(seek_fps.num != 0 && seek_fps.den != 0) ? av_q2d(av_inv_q(seek_fps)) : 0.0;

// Without a usable timestamp there is no way to know where we landed.
// Rewind to the start so the reader is left in a position it can
// honestly report, then fail.
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