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 the 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, wherein each average value is an amplitude A determined by the following equation: A = ∫ - ∞ ∞ ⁢ g ⁡ ( x ) ⁢ ⅆ x ⅆ where d is the width of the corresponding zone, and g(x) is a function equal to zero outside the corresponding zone and equal to said curve within the corresponding zone; and simulating by a simulator the image projected by said initial mask layout during said photolithography based on the average values.