Traditional OPC is essentially an iterated feedback process, in which the position of each target edge is corrected by adjusting a controlling mask edge. However, true optimization adjusts the mask variables collectively, and in so-called SMO approaches (for Source Mask Optimization) the source variables are adjusted as well. Optimized masks often have high edge density if synthesis methods are used in an effort to obtain a more global solution, and the correspondence between individual mask edges and printed target edges becomes less clearcut than in traditionally OPCd masks. Restrictions on phase shift and MEEF tend to reduce this departure from traditional solutions, but they trade off the theoretical performance advantage in dose and focus latitude that phase shift provides for a reduced sensitivity to thick mask topography and to manufacturing error. Mask variables couple across long distances only in the indirect sense of stitched connection across chains of neighbor-to-neighbor interactions, but source variables interact directly across entire masks. Source+mask optimization of large areas therefore involves long-range communication across the parts of the calculation, though the number of source variables involved is small. Tradeoffs between source structure and pattern diversity are illustrated, taking into account the limited (but unknown) number of binding features in a large layout. SMO's exploitation of complex source designs is shown to provide superior solutions to those obtained by mask optimization alone. Moreover, m development work the ability to adjust the source opens up new options in process engineering, and these will become particularly valuable when future exposure tools provide greater flexibility in programmable source control. Such capabilities can be explored in a preliminary way by using programmed multi-scans to compose optimized compound sources with e.g. multiple poles or annular elements.