One embodiment of the present invention provides a system that computes translational differentials between a database-image and a scanned-image of a photomask. During operation, the system receives a noise-free database-image and a scanned-image that is generated by an imaging process. Next, the system determines whether the database-image has complex geometry and whether the database-image is a line-and-space pattern. If the database-image is a line-and-space pattern, the system first converts the scanned-image into a polarity-enhanced-image. The system then computes translational differentials by performing a correlation using the database-image and the polarity-enhanced-image. On the other hand, if the database-image is not a line-and-space pattern, the system computes translational differentials by performing a correlation using the database-image and the scanned-image. Note that the accuracy of the computed translational differentials is significantly increased because the system chooses a technique for computing the translational differentials which is suitable for the type of pattern present in the database-image.