Masks from which integrated circuits are manufactured are derived from layers found in integrated circuit designs. The mapping from design layers to mask layers produces masks with various essential properties. The operators that transform the design layers to the mask layers consist of the Boolean operations along with fattening and shrinking. This invention shortens the compute time used in carrying out these operations by keeping all temporary layers in main memory and never on disk, avoiding both disk space and access time as a result. Temporary layers are extra, intermediate layers used in computing the final mask layers. The savings are achieved by slicing the design into slightly overlapping pieces, each of which can fit in main memory, and carrying out the operations upon all layers of each piece before beginning the next piece.