Multi-primary conversion

A method converts an input image signal (IS) into a drive signal (DS) for driving sub-pixels (SP) of a display device (DD) comprising display pixels (DPI) having at least two sub-pixel groups (SG, SG) being able to contribute to luminance information displayed. The conversion comprises a multi-primary conversion (MPC) which receives the input image signal (IS) and which is performed under a constraint (CO). The constraint (CO) is determined (CD) by substantially matching local display luminances (DL, DL; DLD) associated with the at least two sub-pixel groups (SG, SG) with corresponding local input luminances (L, L; LD) of input pixels (IP) of the input image signal (IS), thereby obtaining a display luminance pattern defined by the display pixels (DPI) corresponding to an input luminance pattern defined by the input pixels (IP) associated with the display pixels (DPI).