When documents scanning and the acquisition plan is not parallel to the plane on which the document, the image obtained suffers of keystone effect, or a distortion due to the effect of perspective. In essence, the lines on the document are parallel, in the scanned image are oblique and convergent.

This effect can be, for example, if you try to photograph a paper document with the camera of your phone, with a webcam or a digital camera.

To solve this optical aberration can use the keystone correction, sometimes also called keystone correction effect or perspective.

To perform keystone correction, the system identifies the trapeze implies that the paper acquired by searching for edges and automatically calculates a transformation matrix to convert it into a rectangle.

Once the affine transformation was performed and was made a crop automatically, that is an automatic cropping of the image, to exclude the area outside the rectangle so reconstructed, the effect is no longer visible and the document has a very more like a scanned document with a normal document scanner.