gimp seamless sides - start small demo image

How to even out light/dark edges of images with GIMP


I was making a huge seamless texture of a concrete wall, when I found out that the original textures were taken with a light source more powerful at the right side. This gave me images where the left side was darker than the right one. The result of tiling this set of images was a long image with clear edges:
gimp seamless sides - start image