The internal structure of a region, other than the storage length and its bounding rectangle, is opaque - there are no Apple-published documents available, though the mechanism is outlined in the patent. Regions are implemented using both vertical and horizontal compression. A region is stored as a series of horizontal scan lines ("rasters"), each of which contains a vertical coordinate followed by a list of horizontal inversion coordinates. Each inversion point can be thought of as toggling inclusion in the region for all the points after it: the first point turns the region on, the second turns it off, and so on. Further compression is achieved by storing each line differentially: each line contains only the differences from the previous line rather than a full set of inversion points. Finally, identical adjacent scan lines are efficiently encoded by simply skipping them. In this way, a commonly used region, the rounded corner rectangle, is efficiently encoded, and complex operations such as region composition and image clipping may be done without requiring either extensive processor cycles or large amounts of memory. (The original systems executing QuickDraw code used processors operating at 8 megahertz clock rates and systems had but 128 kilobytes of writable memory.)