how to save current frame and previous frame

edit

 VideoCapture cap(0);
 Mat curr, prev;
 if(!cap)return -1;
 cap>>curr;
 curr.copyTo(prev);
 while(1)
 {
      cap>>curr;
      imwrite("current.png",curr);
      imwrite("previous.png", prev);
      curr.copyTo(prev);
      if(waitKey(5)==27)break; //Esc pressed
 }