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Commit7ca9d9c

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Merge pull request#27878 from 0lekW:ffmpeg-negative-dts-seeking-fix
Fix frame seeking with negative DTS values in FFMPEG backend#27878**Merge with extra**:opencv/opencv_extra#1289Fixes#27819Fixes#23472Accompanied by PR onopencv/opencv_extra#1289The FFmpeg backend fails to correctly seek in H.264 videos that contain negative DTS values in their initial frames. This is a valid encoding practice used by modern video encoders (such as DaVinci Resolve's current export) where B-frame reordering causes the first few frames to have negative DTS values.When picture_pts is unavailable (AV_NOPTS_VALUE), the code falls back to using pkt_dts:`picture_pts = packet_raw.pts != AV_NOPTS_VALUE_ ? packet_raw.pts : packet_raw.dts;`If this DTS value is negative (which is legal per H.264 spec), it propagates through the frame number calculation:`frame_number = dts_to_frame_number(picture_pts) - first_frame_number;`This results in negative frame numbers, messing up seeking operations.Solution implemented in this branch is a timestamp normalization similar to FFmpegs -avoid_negative_ts_make_zero flag:- Calculate a global offset once on the first decoded frame by getting the minimum timestamp in either: - Container start_time - Stream start_time - First observed timestamp (PTS, then DTS).- Apply the offset consistently to all timestamps, shifting negative values to begin at 0 while keeping relative timing.- Simplify timestamp converters to remove `start_time` subtractions since timestamps are pre-normalized.This also includes a new test `videoio_ffmpeg.seek_with_negative_dts`This test verifies that seeking behavior performs as expected on a file which has negative DTS values in the first frames. A PR on opencv_extra accompanies this one with that testing file:opencv/opencv_extra#1279```opencv_extra=ffmpeg-videoio-negative-dts-test-data```<cut/>### Pull Request Readiness ChecklistSee details athttps://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request- [x] I agree to contribute to the project under Apache 2 License.- [x] To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV- [x] The PR is proposed to the proper branch- [x] There is a reference to the original bug report and related work- [ ] There is accuracy test, performance test and test data in opencv_extra repository, if applicable Patch to opencv_extra has the same branch name.- [x] The feature is well documented and sample code can be built with the project CMake
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‎modules/videoio/src/cap_ffmpeg_impl.hpp‎

Lines changed: 103 additions & 12 deletions
Original file line numberDiff line numberDiff line change
@@ -524,6 +524,17 @@ inline static std::string _opencv_ffmpeg_get_error_string(int error_code)
524524
returnstd::string("Unknown error");
525525
}
526526

527+
staticinlineint64_tto_avtb(int64_t ts, AVRational tb)
528+
{
529+
returnav_rescale_q(ts, tb, AV_TIME_BASE_Q);
530+
}
531+
532+
staticinlineint64_tfrom_avtb(int64_t ts_avtb, AVRational tb)
533+
{
534+
returnav_rescale_q(ts_avtb, AV_TIME_BASE_Q, tb);
535+
}
536+
537+
527538
structCvCapture_FFMPEG
528539
{
529540
boolopen(constchar* filename,int index,const Ptr<IStreamReader>& stream,const VideoCaptureParameters& params);
@@ -563,6 +574,10 @@ struct CvCapture_FFMPEG
563574
int64_t pts_in_fps_time_base;
564575
int64_t dts_delay_in_fps_time_base;
565576

577+
/// Timestamp offset in AV_TIME_BASE units for normalization
578+
int64_t ts_offset_avtb =0;
579+
bool ts_offset_decided =false;
580+
566581
AVIOContext * avio_context;
567582

568583
AVPacket packet;
@@ -623,6 +638,8 @@ void CvCapture_FFMPEG::init()
623638
picture_pts = AV_NOPTS_VALUE_;
624639
pts_in_fps_time_base =0;
625640
dts_delay_in_fps_time_base =0;
641+
ts_offset_avtb =0;
642+
ts_offset_decided =false;
626643
first_frame_number = -1;
627644
memset( &rgb_picture,0,sizeof(rgb_picture) );
628645
memset( &frame,0,sizeof(frame) );
@@ -1705,15 +1722,74 @@ bool CvCapture_FFMPEG::grabFrame()
17051722
if (picture_pts == AV_NOPTS_VALUE_) {
17061723
int64_t dts =0;
17071724
if (!rawMode) {
1708-
picture_pts = picture->CV_FFMPEG_PTS_FIELD != AV_NOPTS_VALUE_ && picture->CV_FFMPEG_PTS_FIELD !=0 ? picture->CV_FFMPEG_PTS_FIELD : picture->pkt_dts;
1709-
if(frame_number ==0) dts = picture->pkt_dts;
1710-
}
1711-
else {
1712-
const AVPacket& packet_raw = packet.data !=0 ? packet : packet_filtered;
1713-
picture_pts = packet_raw.pts != AV_NOPTS_VALUE_ && packet_raw.pts !=0 ? packet_raw.pts : packet_raw.dts;
1725+
picture_pts = (picture->CV_FFMPEG_PTS_FIELD != AV_NOPTS_VALUE_)
1726+
? picture->CV_FFMPEG_PTS_FIELD
1727+
: picture->pkt_dts;
1728+
if (frame_number ==0) dts = picture->pkt_dts;
1729+
}else {
1730+
const AVPacket& packet_raw = (packet.data !=0) ? packet : packet_filtered;
1731+
picture_pts = (packet_raw.pts != AV_NOPTS_VALUE_)
1732+
? packet_raw.pts
1733+
: packet_raw.dts;
17141734
if (frame_number ==0) dts = packet_raw.dts;
1715-
if (picture_pts <0) picture_pts =0;
17161735
}
1736+
1737+
// Decide timestamp offset once on first frame to normalize all timestamps to start at zero.
1738+
// This handles videos with negative DTS values (e.g., from B-frame reordering) or non-zero
1739+
// start_time. Similar to FFmpeg's -avoid_negative_ts make_zero option.
1740+
if (!ts_offset_decided)
1741+
{
1742+
int64_t min_start_avtb = INT64_MAX;
1743+
1744+
// Check container start_time (already in AV_TIME_BASE units)
1745+
if (ic && ic->start_time != AV_NOPTS_VALUE_)
1746+
{
1747+
min_start_avtb = ic->start_time;
1748+
}
1749+
1750+
// Check stream start_time
1751+
AVStream* st = ic->streams[video_stream];
1752+
if (st->start_time != AV_NOPTS_VALUE_)
1753+
{
1754+
int64_t s =to_avtb(st->start_time, st->time_base);
1755+
if (s < min_start_avtb) min_start_avtb = s;
1756+
}
1757+
1758+
// Check first observed timestamp (PTS preferred, else DTS from frame 0)
1759+
int64_t first_ts_stream = picture_pts;
1760+
if (first_ts_stream == AV_NOPTS_VALUE_ && dts != AV_NOPTS_VALUE_)
1761+
{
1762+
first_ts_stream = dts;
1763+
}
1764+
if (first_ts_stream != AV_NOPTS_VALUE_)
1765+
{
1766+
int64_t t =to_avtb(first_ts_stream, st->time_base);
1767+
if (t < min_start_avtb) min_start_avtb = t;
1768+
}
1769+
1770+
// Compute offset to shift negative timestamps to zero
1771+
ts_offset_avtb = (min_start_avtb != INT64_MAX && min_start_avtb <0) ? -min_start_avtb :0;
1772+
ts_offset_decided =true;
1773+
}
1774+
1775+
// Apply normalization to picture_pts
1776+
if (picture_pts != AV_NOPTS_VALUE_)
1777+
{
1778+
int64_t t =to_avtb(picture_pts, video_st->time_base);
1779+
t += ts_offset_avtb;
1780+
picture_pts =from_avtb(t, video_st->time_base);
1781+
}
1782+
1783+
// Also normalize dts
1784+
if (dts != AV_NOPTS_VALUE_)
1785+
{
1786+
int64_t t =to_avtb(dts, video_st->time_base);
1787+
t += ts_offset_avtb;
1788+
dts =from_avtb(t, video_st->time_base);
1789+
}
1790+
1791+
1792+
17171793
#if LIBAVCODEC_BUILD >= CALC_FFMPEG_VERSION(54, 1, 0) || LIBAVFORMAT_BUILD >= CALC_FFMPEG_VERSION(52, 111, 0)
17181794
AVRational frame_rate = video_st->avg_frame_rate;
17191795
#else
@@ -2120,8 +2196,13 @@ int64_t CvCapture_FFMPEG::dts_to_frame_number(int64_t dts)
21202196

21212197
doubleCvCapture_FFMPEG::dts_to_sec(int64_t dts)const
21222198
{
2123-
return (double)(dts - ic->streams[video_stream]->start_time) *
2124-
r2d(ic->streams[video_stream]->time_base);
2199+
const AVStream* st = ic->streams[video_stream];
2200+
int64_t ts = dts;
2201+
2202+
if (ts_offset_avtb ==0 && st->start_time != AV_NOPTS_VALUE_)
2203+
ts -= st->start_time;
2204+
2205+
return ts *r2d(st->time_base);
21252206
}
21262207

21272208
voidCvCapture_FFMPEG::get_rotation_angle()
@@ -2174,9 +2255,19 @@ void CvCapture_FFMPEG::seek(int64_t _frame_number)
21742255
{
21752256
int64_t _frame_number_temp =std::max(_frame_number-delta, (int64_t)0);
21762257
double sec = (double)_frame_number_temp /get_fps();
2177-
int64_t time_stamp = ic->streams[video_stream]->start_time;
2178-
double time_base =r2d(ic->streams[video_stream]->time_base);
2179-
time_stamp += (int64_t)(sec / time_base +0.5);
2258+
2259+
AVStream* st = ic->streams[video_stream];
2260+
int64_t time_stamp = st->start_time;
2261+
double time_base =r2d(st->time_base);
2262+
int64_t ts_norm = (int64_t)(sec / time_base +0.5);
2263+
2264+
if (ts_offset_avtb !=0) {
2265+
// map normalized target back to original demux timeline
2266+
time_stamp += ts_norm -from_avtb(ts_offset_avtb, st->time_base);
2267+
}else {
2268+
time_stamp += ts_norm;
2269+
}
2270+
21802271
if (get_total_frames() >1)av_seek_frame(ic, video_stream, time_stamp, AVSEEK_FLAG_BACKWARD);
21812272
if(!rawMode)
21822273
avcodec_flush_buffers(context);

‎modules/videoio/test/test_ffmpeg.cpp‎

Lines changed: 57 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -1055,6 +1055,63 @@ TEST(ffmpeg_cap_properties, set_pos_get_msec)
10551055
EXPECT_EQ(cap.get(CAP_PROP_POS_MSEC),0.0);
10561056
}
10571057

1058+
// Test that seeking twice to the same frame in videos with negative DTS
1059+
// does not result in negative position or timestamp values
1060+
// related issue: https://github.com/opencv/opencv/issues/27819
1061+
TEST(videoio_ffmpeg, seek_with_negative_dts)
1062+
{
1063+
if (!videoio_registry::hasBackend(CAP_FFMPEG))
1064+
throwSkipTestException("FFmpeg backend was not found");
1065+
1066+
const std::string filename =findDataFile("video/negdts_h264.mp4");
1067+
VideoCapturecap(filename, CAP_FFMPEG);
1068+
1069+
if (!cap.isOpened())
1070+
throwSkipTestException("Video stream is not supported");
1071+
1072+
// after open, a single grab() should not yield negative POS_MSEC.
1073+
ASSERT_TRUE(cap.grab());
1074+
EXPECT_GE(cap.get(CAP_PROP_POS_MSEC),0.0) <<"Negative ts immediately after open+grab()";
1075+
1076+
ASSERT_TRUE(cap.set(CAP_PROP_POS_FRAMES,0));
1077+
(void)cap.get(CAP_PROP_POS_FRAMES);
1078+
1079+
constint framesToProbe[] = {2,3,4,5};
1080+
1081+
for (int f : framesToProbe)
1082+
{
1083+
// Reset to frame 0
1084+
ASSERT_TRUE(cap.set(CAP_PROP_POS_FRAMES,0));
1085+
cap.get(CAP_PROP_POS_FRAMES);
1086+
1087+
// Seek to target frame
1088+
ASSERT_TRUE(cap.set(CAP_PROP_POS_FRAMES, f));
1089+
constdouble posAfterFirstSeek = cap.get(CAP_PROP_POS_FRAMES);
1090+
1091+
// Seek to the same frame again
1092+
ASSERT_TRUE(cap.set(CAP_PROP_POS_FRAMES, f));
1093+
constdouble posAfterSecondSeek = cap.get(CAP_PROP_POS_FRAMES);
1094+
constdouble tsAfterSecondSeek = cap.get(CAP_PROP_POS_MSEC);
1095+
1096+
EXPECT_GE(posAfterSecondSeek,0)
1097+
<<"Frame index became negative after second seek to frame" << f
1098+
<<" (first seek gave" << posAfterFirstSeek <<")";
1099+
EXPECT_GE(tsAfterSecondSeek,0.0)
1100+
<<"Timestamp became negative after second seek to frame" << f;
1101+
1102+
// Per-iteration decode check: grab() + ts non-negative
1103+
ASSERT_TRUE(cap.grab());
1104+
EXPECT_GE(cap.get(CAP_PROP_POS_MSEC),0.0) <<"Negative timestamp after grab() at frame" << f;
1105+
1106+
// Verify that reading a frame works and position advances
1107+
Mat frame;
1108+
ASSERT_TRUE(cap.read(frame));
1109+
ASSERT_FALSE(frame.empty());
1110+
EXPECT_GE(cap.get(CAP_PROP_POS_FRAMES), f);
1111+
}
1112+
}
1113+
10581114
#endif// WIN32
10591115

1116+
10601117
}}// namespace

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