Method and apparatus for determining mask layouts for a spacer-is-dielectric self-aligned double-patterning process

Methods and apparatuses are described for determining mask layouts for printing a design intent on a wafer using a spacer-is-dielectric self-aligned double-patterning process. A system can determine whether a graph corresponding to a design intent is two-colorable. If the graph is not two-colorable, the system can merge one or more pairs of shapes in the design intent to obtain a modified design intent, so that a modified graph corresponding to the modified design intent is two-colorable. The system can then determine a two-coloring for the modified graph. Next, the system can place one or more core shapes in a mandrel mask layout which correspond to vertices in the modified graph that are associated with a selected color in the two-coloring. The system can then place one or more shapes in a trim mask layout for separating the shapes in the design intent that were merged.