今天突然遇到了這麼個奇怪的問題，之前一直沒有注意到。
imshow()之後要加waitKey()才能正確顯示圖像。
官方說明如下：
A common mistake for opencv newcomers is to call cv::imshow() in a loop through video frames, without following up each draw with cv::waitKey(30). In this case, nothing appears on screen, because highgui is never given time to process the draw requests from
cv::imshow().
參考