I have a photo of a pencil drawing made on paper, taken with a hotshoe flash on the DSLR and two polarising filters (cross polariser technique for canvas photography).
The problem now is that when getting closer to the drawing, lighting becomes uneven since the lens casts a shadow on part of the image. This creates a gradient over the image that is approximately, but not quite, linear.
Below is an example image; the original is around 3 cm wide.
How can this be edited such that the background is evenly white?
Scanned results below, uncorrected and corrected. While this looks quite promising at first, it unfortunately is full resolution what you see here (600 px wide); the scanner can do 300 dpi, and the drawing is roughly one inch wide. This is a bit less compared to the 5000 px from camera.
Also, the uncorrected scan hints that graphite indeed reflects a good part of the light, as opposed to the cross-polariser technique.