Open an input image containing a rectangular area with one of the corners missing ( out of image ).
For example, a picture of a framed painting on a wall, which the photographer did not completely capture in the photo.
Use the warp transformation to straighten the image. One of the 4 warp reference points is out of the input image.
Perform coarse alignment of the 4 points using a zoomed-out magnification.
Set zooming of the input image to 100% ( 1:1 ).
Visit each of the 4 reference points for precise manual alignment using the keyboard shortcuts 1-2-3-4.
The endpoint which is out of the image area cannot be displayed using its keyboard shortcut.
Suggestion :
Allow zooming 1:1 on out-of-image endpoints.
There is no easy way to add this feature and if added, it would allow scrolling outside the image which would be confusing and unexpected. On the other hand, you can just add a border to the input image before warping if you want to precisely position control points outside the image.