An interface controller, situated between a graphics controller and a memory array in a color video display system operable in a read-modify-write mode, configured to detect a select transparency color in whole or in part and to respond by selectively changing the color binary data for the corresponding...http://www.google.com/patents/US4893116?utm_source=gb-gplus-sharePatent US4893116 - Logical drawing and transparency circuits for bit mapped video display controllers
