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I'm creating an mp4 file at 60 FPS from a measurement which gives me frames at a variable FPS. I've done this by adding the same frame until the timestamp it give's me a new frame. Even the frame count * FPS should give me a specific duration time the final file has a smaller duration time. For example from a duration time of 8 minutes the final file has only 5 minutes. I use opencv 3.4.3 with H264 codec. Why is this happens?

This is the function:

 void ExporterChannel::ProcessMFCImage(Timestamp _timestamp, const std::string& _filename, const std::string& _data)

 static bool first = true;
 static long long accumulated_time_error = 0;
 static long long first_timestamp;

 static structs::SensorNearData::MFC::CameraImage mfc_image_struct;
 static pb::SensorNearData::MFC::CameraImage mfc_image_pb;

 if (first)
 
 first = false;
 mfc_image_pb.ParseFromString(_data);
 pb2structs::convert(mfc_image_pb, mfc_image_struct);
 first_timestamp = _timestamp;
 Size frameSize(mfc_image_struct.image_width, mfc_image_struct.image_height);
 bool color_mode_rgb = (config_.color == "true");
 if (color_mode_rgb == true)
 
 video_file.open(_filename, CV_FOURCC('H', '2', '6', '4'), FPS, frameSize, true);
 
 else
 
 video_file.open(_filename, CV_FOURCC('H', '2', '6', '4'), FPS, frameSize, false);
 
 // check if we succeeded
 if (!video_file.isOpened())
 
 ExporterException("Could not open the output video file for writen");
 return;
 
 HandleMFCImage(mfc_image_struct);
 
 else
 
 if ((long long)_timestamp < (first_timestamp + STEP))
 
 accumulated_time_error += first_timestamp + STEP - (long long)_timestamp;
 
 else
 
 while ((long long)_timestamp >= (first_timestamp + 2 * STEP))
 
 HandleMFCImage(mfc_image_struct);
 first_timestamp += STEP;
 
 accumulated_time_error += (long long)_timestamp - (first_timestamp + STEP);
 if (accumulated_time_error >= STEP)
 
 HandleMFCImage(mfc_image_struct);
 accumulated_time_error -= STEP;
 
 
 mfc_image_pb.ParseFromString(_data);
 pb2structs::convert(mfc_image_pb, mfc_image_struct);
 first_timestamp = (long long)_timestamp;
 HandleMFCImage(mfc_image_struct);