As CMOS technology continues to shrink, 2D edge placement errors in lithography can't be ignored especially for some key patterns. The pixel patterns of CMOS image sensor require high lithography fidelity since they will influence the optical properties and electric properties of the device significantly. So the corner rounding effect of pixel patterns should be of special concern. Generally, corner rounding effect in lithography can be compensated by adding serifs at the corner. Serifs can be generated by rule based OPC method or model based OPC method. In this article, a novel model based OPC method to add serifs to corners is introduced. This method first retargets the corners by replacing the corner vertex with a small rectangle placed along 45 degree with the x-y axis. After that, model based OPC is run based on the retargeted patterns. Then we simulate the lithography contour of the corners by commercial OPC software. The geometry of lithography contour is quantitatively evaluated according to the value of corner pullback and the deviation of edge rippling. The relation between the geometry of corner image contour and retarget pattern is investigated. This method has potentials in design of experiment of pixel layout patterns of CMOS image sensors.