Example of keystoning:

Cause of keystone effect:

The keystone effect is an optical distortion that is created when the camera is not at a 90º angle to the surface of the page. It results in one side of the image being stretched and the other side being compressed.

Preventing Keystoning

Keystoning can be prevented with camera mounts that have been accurately adjusted. Tripods will need to be adjusted each time they are setup, whereas permanent mounts fixed to the book scanner only need to be correctly installed the first time.

A simple way to check angles is with a carpenter's level that has 45º indicators. Make sure that the base of the book scanner is level, that the cradle opens to a 45º angle, and that the camera points down at a 45º angle.

Fixing Keystoning

An image that has a keystoning effect can be corrected with software. While preventing keystoning at a hardware level is generally preferred, sometimes it may be impossible or too time consuming to recapture a series of images, or one may deliberately allow keystoning http://www.diybookscanner.org/forum/viewtopic.php?f=1&t=669&hilit=keystone .

At the moment, most software such as Gimp, Lightroom3, or Photoshop require human intervention to manually correct keystoning. In theory, though, a series of images from the same hardware setup could be corrected in a batch process once the necessary values are found.

Currently ScanTailor is developing an automated dewarping feature that should also correct keystoning without manual intervention.