1. A computer implemented method for lithography simulation comprising: specifying a position for measuring a difference between a lithography image and a target pattern; generating one or more critical dimension (CD) marker candidates for the target pattern by using a template having one or more CD markers; and selecting, by using a processor, a CD marker candidate from the one or more CD marker candidates as a CD marker for the target pattern, in which selecting the CD marker candidate comprises combining a spatial region specification, a decision using a number, information of a circuit that is realized by a layout pattern of the lithography image, CD measurement results, and a gradient of pattern density.