Immersion technology has successfully extends the application of ArF lithography in the semiconductor. However, as we further push the k1 factor below 0.3, the patterning fidelities degrade significantly. In this paper, a novel method to quantify the mask fidelity of complex 2D patterns is proposed. With this method, the critical dimension (CD) error of both edge placement error (EPE) and corner rounding can be well described by using 2 indices "bias" and "blur" respectively. The "bias" is defined as the CD offset between the mask and the targets, and the "blur" is a derived term that can well represent the mask rounding. These 2 indices are not only able to describe the mask quality but also able to link with model parameters that are used in optical proximity correction (OPC) and some other applications. In this paper, we demonstrate the methodology and quantify the actual mask quality on the complex and critical 2D patterning in the advanced nodes.