1. A non-transitory computer readable storage medium storing a program for causing a computer to generate reticle data for producing a reticle used in an exposure apparatus in which an illumination optical system illuminates a reticle and a projection optical system projects a pattern of the reticle onto a substrate, the program causing the computer to execute processing including the steps of: classifying target patterns to be formed on a substrate into a plurality of direction groups; determining, for each of the plurality of direction groups, a partial light source that is a part of an effective light source distribution formed on a pupil of the projection optical system by the illumination optical system, the partial light source being for resolution of a target pattern belonging to the direction group; executing, for each of a plurality of partial light sources, processing of determining a pattern to be placed on a reticle when each partial light source is used as an illumination condition; and merging patterns determined in the step of executing processing, wherein in the step of classifying target patterns, letting θ(0≦θ<π,θ=2(j−1)Δθ,j =1,. .. , π/(2Δθ)) be an angle between a longitudinal direction of a target pattern and an x-axis in an x-y coordinate system defined on a reticle plane, a target pattern falling within an angle range of θ=2(j−1)Δθ±Δθ is classified into a direction group of a direction j, wherein Δθ is an arbitrary angle.