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Fix frame seeking with negative DTS values in FFMPEG backend #27878 **Merge with extra**: https://github.com/opencv/opencv_extra/pull/1289 Fixes https://github.com/opencv/opencv/issues/27819 Fixes https://github.com/opencv/opencv/issues/23472 Accompanied by PR on https://github.com/opencv/opencv_extra/pull/1289 The 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: https://github.com/opencv/opencv_extra/pull/1279 ``` opencv_extra=ffmpeg-videoio-negative-dts-test-data ``` <cut/> ### Pull Request Readiness Checklist See details at https://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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