Color Palette Considerations for Digital Map Displays

摘要

The Map Data Formatting Facility is creating a library of Compressed AeronauticalChart (CAC) data on a compact disk (CD) media in support of mission planning systems and airborne digital moving-map systems for military aircraft. The CAC library is generated by transforming the Defense Mapping Agency's Equal Arc-Second Raster Chart (ARC) Digitized Raster Graphics data from the ARC projection system into the Navy-specified tessellated spheroid projection system. The data are then color and spatially compressed using a technique known as vector quantization. This report discusses the method by which a standard set of thirty 8-bit color palettes, which are used for the color-compression portion of CAC generation, are built. Briefly, color compression is achieved by reducing the amount of color information from 24 bits per pixel to 8 bits per pixel via a color look-up table. A CAC color palette, which contains 256 entries (including 16 that are reserved for vector graphic overlays), is used to define the color look-up table. In addition, the color palette is written onto the final CAC CD-ROM for decompression operations. Since a single color palette will be used to color compress ARC digitized raster graphics data for one-fifth of the world at a given scale, prudent color selection is vital. Digital maps, Optical storage, Databases, Data compression.