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

By , last updated October 27, 2019

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.

The result of tiling this set of images was a long image with clear edges:
gimp seamless sides - start image

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.

Duplicate the layer

Start by duplicating one of the layers by clicking the right mouse button on that layer in “Layers” panel and choosing “Duplicate layer”.

gimp seamless sides - duplicate layer

Adjust color levels

Click on the new layer and Choose Colors->Levels.

gimp seamless sides - 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 layer mask

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.

gimp seamless sides - add layer mask -  white full opacity

Apply gradient

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.

gimp seamless sides - blend sides

The final image:

gimp seamless sides - final image

Also seamless and in full over 15000 pixels size.