The problem when fixing a tilt in an image is determining the amount of rotation to apply. If, when using GIMP, you do a rotation in the "Normal" direction you may have to do a number of attempts until you get the exact amount you want. However, by doing a "Corrective" rotation in the "backwards" direction you can do it in one go. Here's how:
The above shows the test image loaded in GIMP with a grid showing. Look at the horizontal edge of the stage. It's slightly tilted anticlockwise. I pressed Shift-R to bring up the Rotate tool. Here are the Options:
You can see I've selected "Corrective (Backwards)". I've also selected a "Grid" Preview ("Image" did not appear to do anything useful for me - may be a GIMP 2.4.6 bug) and moved the slider to the right to increase the number of grid lines. Then I clicked on the image and pulled the rotation grid around to align it with the stage edge. Here's how the image looked (I'd turned off the red standard grid as the image was too busy with it showing):
Once I was happy, I pressed Rotate. The correction applied was -0.56 degrees:
The result (with grid) was:
The horizontal edge of the stage is aligned correctly, but you can see the vertical edge of the lattice screen is titled. I then used the Perspective Tool (Shift-P), moved the top left box to the right a bit (as described in a previous tutorial) to straighten the right edge of the lattice and cropped the left edge off.
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