1. A method of printing comprising: determining a print defect state of a printing system, the determining of the print defect state comprising: printing at least one test patch; sensing gray level variations in the printed test patch; and determining periodic print defects based on the sensed gray level variations; where the determined print defect state includes a periodic print defect, selecting a screen from a plurality of screens, the selected screen having a halftone print parameter which has been determined to mask the determined periodic print defect, wherein the printing system comprises a plurality of marking engines and wherein: determining of the print defect state includes determining the print defect state of each of the plurality of marking engines; and the printing of images according to the selected halftone print parameter includes selecting a marking engine from the plurality of marking engines for printing of the images; and printing images according to the selected screen.