An image processing method suitable for a printer unit, includes an error diffusion halftoning process arranged for quantizing and diffusing each pixel of an image including a set of subtractive primary colors (C′, M′, Y′), in an image including a quantized printer image including a set of ink drops (DC, DM, DY, Dc, Dm) of respective ink channels to be printed. The method comprises the step of determining, for each pixel, an input variable value (S) representing a measure of solvent quantity that should be ejected by the printer unit for each pixel, the input variable value being computed on the basis of the value of a corresponding pixel of the image including the set of subtractive primary colors (C′, M′, Y′), and of inputting the determined input variable value (S) in the error diffusion halftoning process together with the values of the corresponding pixel. The invention further relates to the apparatus embodying the method.