The GIMP rotation tool is quite good. But I think straightening an image could be done even a little bit easier. I like the way it is done by my former image processing program more than working with the grid of the GIMP rotation tool. So I tried to imitate it by a script.
Usage: place two points on a line or edge of an image element that should be exactly vertical (or horizontal) after rotation. Run the rotation-script. That's all.
The points are easily set by using the path tool: First press "B" to activate the tool, then do single left-clicks (don't hold the mouse key while moving the mouse). After placing the second point a line appears between both points. If you drag the end points (small circles) you can adjust the position of the line.
The script works non-destructively on a copied layer. It uses the sinc-interpolation mode. Sometimes it is recommended to prefer cubic interpolation. I compared both. The results are quite similar. I don't really know, which is better. Have a look at the comparison:
www.psyprasoft.de/heisterbach.xcfOn my GIMP I assigned shortcut „ctrl & R“ to this script, so the rotation can be called even quicklier.
Menu location of the script: <image>Image/Transform/Rotation
Regards,
Bert
GIMP, Image Processing (DIP) and Photography