Main Menu: Transform
> Adjust Perspective
Perspective distortion is a warping or transformation of an object and its surrounding area that differs significantly from what the object would look like with a normal focal length, due to the relative scale of nearby and distant features.
The Adjust Perspective window can be used to change easily the perspective of an image. It shows a reduced view of the active image with eight handlers on the image corners. By dragging those handlers the image perspective can be changed.
The lock/unlock buttons placed between handlers, lock/unlock handlers to change the perspective in symmetrical or assymetrical fashions; either vertically (left or right sides) or horizontally (top or bottom sides).
The Canvas Size button can add some empty space around the image to avoid losing data when part of the image slips outside the original image boundaries.
The Show Grid checkbox turns on or off the grid, which helps to align horizontal or vertical features in an image.
Apply closes the Perspective window and applies its settings to the active image.
When reopening the Perspective window, it will remember its previous perspective settings. In that way it is possible to correct the perspective of a batch of images with similar perspective in an easy way.