Modify image RGB values in equations

[PDB Jones]

I would ,like to be able to modify image RBG values based on combination of: a) neighbouring RBG values; b) user parameter inputs. I light to experiment with various colour abstractions of my images.

I have not found any expressions in help material, or corresponding forum answers. 

 

Is this something already available? If so, where? Otherwsie could this capability be added to the equations/expressions capability of Affinity Photo.