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 resulted in images where the left side was darker than the right one.
In this article I will explain how you can fix this.
I start with to layers of images one upon another. They fit each other and the only thing that needs to be fixed is blending of the edges.
Start by duplicating one of the layers by clicking the right mouse button on that layer in “Layers” panel and choosing “Duplicate layer”.
Click on the new layer and Choose Colors->Levels.
Adjust the levels until one of the sides (in my case the lighter one) is totally blended with the neighbor side from another layer.
Click OK when you are satisfied.
Add a transparent layer mask to the duplicated layer. We need this in order to be able to blend the layer with the underlying one.
At this point we have one of the sides (the lighter one) blended OK with the neighbor. Now we need to adjust the other side. To do this click on the duplicated layer and choose a gradient tool. Blend the other side (the darker one) until satisfied.
The final image: