As subhalf wavelength optical lithography is pursued, the variety of imaging requirements increases. This can result in complex combinations of various resolution enhancement techniques. Optimization based on simple standards or rules is not possible. Problems often arise as analysis is carried out in a spatial domain, where mask and image properties are evaluated using sizing or dimensional evaluations. A more appropriate perspective for image optimization is that of the lens pupil, in a spatial frequency domain. In this paper, we describe the common characteristics of resolution enhancement, beyond the historical comparisons of alternating phase shifting masking (PSM) and strong OAI. Enhancement techniques including assist feature optical proximity correction (OPC), custom illumination, attenuated PSM, and pupil filtering are described from a spatial frequency standpoint where each can be utilized to take advantage of strengths and avoid weaknesses. As a result of this type of analysis, we will also describe an alternative OPC method in which assist features of varying tone, referred to as gray bars, provide significant image improvement.