10-22-2017, 10:18 AM
There is a possibility to order the created gradient, but in any case what you ask seems impossible to me, because an image can have the luminosity L derived from different R,G,B combinations:
L = xR+yG+zB (x, y, z being coefficients of R, G, B values)
L = xR+yG+zB (x, y, z being coefficients of R, G, B values)