Scanned documents often become skewed (slanted) during scanning because of misfeeds or other alignment errors. Skew is the amount of rotation necessary to return an image to horizontal and vertical alignment. Skew is measured in degrees. Deskewing is a process whereby skew is removed by rotating an image by the same amount as its skew but in the opposite direction. This results in a horizontally and vertically aligned image where the text runs horizontally across the page rather than at an angle.

When an image is not aligned correctly, optical character recognition (OCR) is more difficult and becomes slower and less accurate. Deskewing the documents beforehand can make the OCR process faster and more accurate.

LEADTOOLS offers three command classes that can automatically straighten scanned documents:

• DeskewCheckCommand class
• DeskewCommand class
• DeskewExtendedCommand class

All three command classes have the following features:

• Ability to deskew documents containing pictures
• Option to use high-speed rotation with 1-bit images (faster than normal rotation)
• Option to report the skew angle without performing deskew

Use the DeskewCommand class to automatically straighten scanned documents of any kind. Images can be rotated by as much as 20 degrees in either direction to remove the skew.

Use the DeskewExtendedCommand when more rotation may be necessary (as much as 45 degrees in either direction) or it is desired to specify the maximum angle/resolution of deskew.

Use the DeskewCheckCommand class when performing bank check scanning to obtain more accuracy than provided by the other two classes. Images can be rotated by as much as 20 degrees in either direction. This class provides three different algorithms for performing deskew:

• The same algorithm as the one used in the DeskewCommand
• The bank check algorithm, which considers many features common to standard bank checks in order to determine orientation
• The line detection algorithm, which is an algorithm based on finding the lines in an image

The original color image is shown in the following figure:

The deskewed color image is shown in the following figure:

The original grayscale image is shown in the following figure:

The deskewed grayscale image is shown in the following figure:

The original 1-bit image is shown in the following figure:

The deskewed 1-bit image is shown in the following figure: