helmut wrote:For straightening an image I'd use custom rotation:
1. Viewer: Image » Rotate » Rotate...
2. Check [x] Show Grid
3. Check [x] Apply to image
4. Change the angle
Though this is a manual process and not an automatic straightening as requested.
The problem with that method at present is that the angle adjustment slider is very sensitive, making it hard to make small angle adjustments without using the digital angle display which is slower. Reducing the sensitivity of the slider, or making the sensitivity adjustable in some way, could make precise angle adjustment much easier.
Out of interest, the program ScanTailor widely used for book scan processing has the following interface that allows really quick and easy fine angle adjustment:
One or other of the blue circles is simply dragged to rotate the image, with a sensitivity that makes it really quick and easy to make accurate adjustments.
But refining the sensitivity of the slider in the existing custom rotation tool would be a good start...
Looking at the XnView MP rotate tool in particular, the response to angle changes also seems very slow which makes adjustments even harder. The ScanTailor tool response is almost instantaneous which makes fine tuning really easy.