A bitmap index is a special kind of database index that uses bitmaps. Using bitmaps in modeling polygon shapes has many drawbacks. Unlike previous fonts designed for CJK environments, Meiryo contains no embedded bitmaps. Early algorithms for Boolean operations on polygons were based on the use of bitmaps. Run length encoders are often used for crudely compressing bitmaps. Older character sets were stored as bitmaps. ClearType does not alter other graphic display elements (including text already in bitmaps). Computer algorithms to perform color quantization on bitmaps have been studied since the 1970s. This approach reduces the number of bitmaps used regardless of encoding method. Pixelation is a problem unique to bitmaps. A counter-measure was devised using spectral bitmaps from the stored speech of verified users. Infantry units have new commands available and explosions are now bitmaps. Microsoft adopted a different approach than color bitmaps. Block allocation is tracked by free space bitmaps in fixed locations. Indices are usually implemented via B+ trees, R-trees, and bitmaps.