A good way to normalize the lighting of the image:
* Duplicate the layer
* Apply a very heavy gaussian blur. The drawoing should no longer be visible (I use something like 1/5th of the image diagonal)
* Put the top layer in Grain Extract mode.
This is more or less computing a 128-centered difference between the layer and a blurred copy of it. Then you can use Curves with mess impact on the fine detail.
Edit: some success with a different method, use the Wavelet decmpose plugin and decompose to 7 levels, then keep on the Level 4 and the residual. But time to go to work...
* Duplicate the layer
* Apply a very heavy gaussian blur. The drawoing should no longer be visible (I use something like 1/5th of the image diagonal)
* Put the top layer in Grain Extract mode.
This is more or less computing a 128-centered difference between the layer and a blurred copy of it. Then you can use Curves with mess impact on the fine detail.
Edit: some success with a different method, use the Wavelet decmpose plugin and decompose to 7 levels, then keep on the Level 4 and the residual. But time to go to work...