How to whiten photographed drawing paper in darktable without losing color?

[Donatzsky (Nis Donatzsky)]

Here’s how to implement the technique @snibgo mentions in darktable:

How to balance exposure with external raster mask?

A great thank you Todd, actually that works using divide, I misunderstood at first! And I am surprised how well that works, even though I have not been using a tripod, so the background does not align perfectly, the overall light distribution is much better. What I did different is that I had just used the tone curve and pulled up the left hand side and pulled down the right side to invert the background image, as I was not sure about the exact parameters using negadoctor. For example, this is …