Methods and apparatus, including computer programs, for processing transparency attributes of digital graphics objects. The techniques result in the color compositing of an arbitrary structure of artwork elements defining the appearance of a page to produce a result color at any position on the page...http://www.google.de/patents/US7505046?utm_source=gb-gplus-sharePatent US7505046 - Preserving opaque-like rendering in transparent 2D graphics using knockout groups
