Assume we know the width and height of the image as w and h. We want to color the rectangular region near the middle of the picture to be purely black. The width of the black box is a, and the height is b. The black box should be flush on the middle point of the picture and should flush with the top of the image as well. In other words, the leftmost column of the black box overlaps the exact center column of the image. You can assume that that w is always odd such as 101.