Simen Bjelke wrote:
straight, and that brings me to deskew. What deskew tool should one use, and I hope there are some free alternatives out there.
First let me say that deskewing is not absolutely necessary, even though it makes the result look nicer. In many cases we don't deskew, but publish the slightly skewed image. We also publish the OCR text, and the OCR program (I personally use ABBYY FineReader) internally deskews the image for better recognition, but we don't use its output image, only the output text.
Having said that, we all come from different backgrounds. I'm a C/Unix programmer and so are other members of our core team. If you use Microsoft Windows, I don't know much about which programs are available. One very useful shareware/freeware program is IrfanView that can do most image manipulation operations on single images or entire batches. It can do fine rotation, but it doesn't automatically recognize how much a page is skewed. This means you have to adjust the rotation manually for every image.
Since you ask for a "free" alternative, I suspect you might be interested in programming and algorithms. The problem of deskewing falls into two parts: skew detection and fine rotation. For fine rotation there are many programs, such as pnmrotate or ImageMagick's convert. For skew detection I have found a subroutine which is part of the ClaraOCR free software OCR engine. While ClaraOCR isn't very useful, this subroutine can be isolated and used for skew detection. Tell me if this sounds interesting.