If your image has alpha, then using G’MIC, you could try:
Testing/Reptorian/Grouped Pixel Axis-Based Shift.
@Reptorian, thank you. I installed the GMIC plugin in GIMP & all that, But when I use “Grouped Pixel Axis-Based Shift”, it doesnt do anything to my selected image. And i checked the channel dialog & it says my photo has Alpha. I also found a solution, which is to use Cage Transformation, i’d never used it before. It does the job well.
A copy of the image can be made, and marked-up with a wavy line that we want to be straight and horizontal. A script can then be run that will automatically distort the image to make the line straight and horizontal. The distortion can be applied to the marked-up copy or the original. Other scripts work with two wavy lines, which both become straight and horizontal. Or four wavy lines that become rectangles.
@snibgo, Thanks! Or one could just use cage transformation?
Gimp’s Cage Transformation requires the user to select points, and manually move those points until they seem to be in a straight line.
My scripts automatically transform the image so the points in a wobbly line become exactly straight.
Which tool works best for you will depend on factors not mentioned in your OP. For example:
You might not care if the line becomes exactly straight.
You might have one image to process, or a thousand.
You might be able to automatically find the line that needs to be straightened.
Thank you , very helpful & interesting