The disclosure concerns a method of simulating the image projected by a mask during photolithography including determining by a processor (702), taking into account the thickness of a masking layer of a mask, a near-field transmission amplitude curve of light passing through the mask across at least one pattern boundary in the initial mask layout; calculating by the processor, for each of a plurality of zones, average values of the curve; and simulating by a simulator (708) the image projected by the initial mask layout during the photolithography based on the average values.