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Is it possible to find IDR frame (CMSampleBuffer) in AVAsset h264 video file?

I'm doing random access sampling from AVAsset of local h264 video file let track = asset.tracks(withMediaType: .video)[0] let assetReader = try! AVAssetReader(asset: asset) let trackOutput = AVAssetReaderTrackOutput(track: track, outputSettings: nil) trackOutput.supportsRandomAccess = true assetReader.add(trackOutput) assetReader.startReading() ... let targetFrameDTS = CMTime(value: 56, timescale: 30) let timeRange = CMTimeRange( start: CMTimeAdd(time, CMTime(value: -1, timescale: 30)), duration: CMTime(value: 2, timescale: 30) ) // reset output to be near target frame decoding time trackOutput.reset(forReadingTimeRanges: [NSValue(timeRange: timeRange)]) while assetReader.status == .reading { guard let sample = trackOutput.copyNextSampleBuffer() else { break } let dts = CMSampleBufferGetDecodeTimeStamp(sample) print("\(dts.value)/\(dts.timescale)") } for some reason with some targetFrameDTS assetReader copyNextSampleBuffer will skip samples. in my particular case the output is ... 47/30 48/30 50/30 51/30 54/30 55/30 57/30 why is it so?