1. A method of simulating the image projected by a mask during photolithography comprising: determining by a processor, 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 said processor, for each of a plurality of zones, average values of said curve; and simulating by a simulator the image projected by said initial mask layout during said photolithography based on the average values.