Simply hold the camera directly overhead while looking through the viewfinder. Frame the page as much as possible while keeping the horizontal and vertical edges as straight as possible.
一边观察取景器，一边直接将相机举过头顶，尽量在将横边和竖边保持垂直情况下选定图像。
2
Above, the camera is placed perpendicular to the train pavilion bridge. This creates a lot of horizontal and vertical lines in the image as opposed to diagonal lines dominating the composition.
In this instance, the location of camera in the world coordinates is unnecessary to calibrated, and the rotary matrix and horizontal movable matrix is needless too.