A novel Sub-Resolution Assist Feature (SRAF) insertion approach is discussed. The method of linearly added SRAFs (laSRAF) is based on the superposition, linear addition in the simplest case, of pre-calculated SRAF usefulness templates. The usefulness templates describe net effect of an elementary SRAF insertion in the proximity of an isolated arbitrary target shape. The method includes two major steps: (i) SRAF usefulness map generation for arbitrary layout comprised of OPC target shapes and (ii) derivation of Mask Rule Check (MRC) compliant SRAF shapes from those usefulness maps. The performance of the laSRAF method measured by the run-time and process window enhancement effect is compared to production quality model-based SRAF insertion implementations and one rule-based realization. The comparison reveals up to 8 times faster SRAF insertion at the same or better SRAF quality with the laSRAF method compared to model-based solutions. LaSRAF has been found to be significantly superior in terms of SRAF quality at the penalty of 2-4x longer run-time compared to rule-based SRAF.