I have to compare aerial photographies with geomagnetic data. The photos are georeferenced scanned slides showing structures on the surface. The magnetic data have a certain unit, but the photos do not. For a comparision I need a unit for the structures in the photos (like Delta E colour difference). I vectorized the structures of interest. Now I would like to use some kind of function to calculate the colour difference between a structure within the polygon and the surrounding soil.
Does anyone know such a function in QGIS?
I thought about converting the photos into greyscale images first, to compare the greyscale cell-values. But it would be much better to use the color images.