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Setting Deskew

Setting Deskew
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Sometimes your images are not straight but rotated by a certain degree that you do not know. In other words, those images have a skew. The degree of the rotation might vary from image to image. PPP can straighten out these images for you if you wish so. This straightening operation is called deskew. The deskew can be turned on and off by checking or unchecking the Deskew check box (Figure 6-24) on the right side of the Cleanup page.


Figure 6-24. Deskew Option

If deskew is turned on, PPP will automatically deskew all of your images. Unfortunately, this operation takes a lot of time. If your images are straight, uncheck this option. You only need to turn on the deskew if your images have a skew.


The deskew is always a two-phase operation. PPP first detects the skew, then it rotates the image. The skew detection can be based on the horizontal lines, on the letters, or on the combination of the two. You have to choose from one of the following skew detection options:


  • Line detection only: PPP will search for long horizontal lines and tries to measure the skew from the longest straight line.
  • Text detection only: PPP will search for a paragraph formed by long rows of letters. The longest row will be used for skew detection.
  • First line then text detection: PPP will search for long horizontal lines first. If there are no lines in the image, it will search for text.
  • First text then line detection: PPP will search for text in the image. If there is no text detectable, it will search for long horizontal lines.


It is not recommended to straighten out a very small skew, because the rotation takes a lot of time, it might shift your image, and you are usually unable to see the difference between a perfectly straight image and an image that is very close to straight. Therefore you can set up a Minimum skew. If the detected skew is less than the minimal skew, PPP will not rotate the image at all. Exception will not be generated in this case. If you want to do the rotation even if the skew is very small, enter 0 here. This means PPP will always deskew the image, except when the skew is exactly 0.0 degree.


If the skew is too big, it usually means that something is wrong with the image. When you are scanning a large number of images, it is possible that some of them are not perfectly straight. If, however, the skew is suspiciously big, you can be very positive that something is wrong with the skew detection. Your image might be totally bad, or PPP might have detected a wrong skew. For example, a long line that has an angle can easily fake the line detection of PPP, thus producing extremely bad output. Therefore we introduced an option called Maximum skew. If the skew is bigger than this value, PPP will perform the deskew, but it will send the image to the Exception List. The quality control operator will then decide if the output image is acceptable or bad. Bad images can be corrected manually with the built-in editor, or they can be re-processed with the deskew turned off.


It is possible that PPP can not detect the skew at all. For example, if the image does not contain long horizontal lines and does not even contain any text, then no skew will be detected. You can decide what to do in this case. There is a check box titled Unable to detect skew in the Report in Exception List if... group box. If this option is checked, PPP will send an image to the Exception List whenever the skew can not be detected. If this option is turned off, no exception will be generated. If PPP can not detect the skew, it will simply skip the image and will not perform a deskew.



See also:


Setting Report Options


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