OpenCV: calcOpticalFlowPyrLK nextPts with points, that have negative (with minus) coordinates

OpenCV: calcOpticalFlowPyrLK nextPts with points, that have negative (with
minus) coordinates

It's my peace of source code:
...
cv::VideoCapture cap(fileName);
cap.read(frame);
image_next = frame;
features_next = findShiCorners(image_next);
while (true)
{
image_prev = image_next.clone();
features_prev = features_next;
cap.read(image_next);
if(image_next.empty())
break;
cv::calcOpticalFlowPyrLK(image_prev, image_next, features_prev,
features_next, featuresFound, err);
cv::imshow("Video", image_next);
cv::waitKey(fps);
}
When I see on results point in features_next I find dots with negative
values. How it can be?