Storing images having semi-transparent pixels via alpha regions

An image storage and processing method constructs, clips, fills, and combines arbitrary 2-dimensional shapes in an advanced graphics system. The method supports processing of anti-aliased images by compressing the image into a novel AlphaRegion data structure. AlphaRegion encodes into the data structure both the partially transparent as well as the fully opaque alpha values of an image according to rectangular bands. A band is a contiguous series of scan lines that have the same pattern of alpha values in a single direction. AlphaRegion encoding results in a more compact representation than possible in a conventional bitmap, but without losing the anti-aliasing features. AlphaRegion can be constructed either by supersampling sub-scan lines of an image, from the scan lines of a gray-scale image, from an existing conventional 1-bit Region or alpha mask buffer, or directly from a scan-line conversion of a geometric shape. AlphaRegion stores image data into three arrays, yInfo, xInfo and alphaData. Methods provided with AlphaRegion include rendering methods for filling or clipping the area on the drawing surface and methods for combining two AlphaRegions using boolean combination operators OR, AND, XOR, SUBTRACT, or SUBTRACT_FROM.