BMP (Windows Bitmap) Image Format

Format name: Microsoft Windows Bitmap
File extension: .bmp, .dib, .rle, .rl8, .vga

BMP (an abbreviation derived from "BitMaP") is a native bitmap format in Windows because it most closely corresponds to the internal format used by Windows for storing its bitmap arrays. BMP files most often have the .bmp extension, though sometimes the .rle extension is used. The .rle extension usually indicates that the bitmap information was compressed using one of the two RLE (run-length encoding) methods available for BMP files.

In a BMP file, information about each pixel's color is encoded in 1, 4, 8, 16, or 24 bits (bits per pixel). The number of bits per pixel, also called “color depth,” determines the maximum number of colors in a picture. A color depth of 1 bit per pixel means that only 2 colors can be used in the picture. A color depth of 24 bits per pixel limits the maximum number of colors by approx. 16.7 million.

Take a look at the picture, which shows the structure of a typical BMP format file that contains a 256-color (8 bits per pixel) image. The file consists of four basic sections: a bitmap file header, a bitmap information header, a color table (palette), and pixel array data. The bitmap file header contains information about the file, including the starting address of the pixel array data. The bitmap information header contains information about the image stored in the file, such as image height and width (in pixels). The color table stores RGB (red, green, and blue) values used for creating specific colors. If a graphics card doesn't support more than 256 colors, the software that reads and displays BMP files can programmatically set such RGB values in the card's palette for precise color rendering.

The format of bitmap array data in a BMP file depends on the number of bits used for encoding data on each pixel's color. If the number of colors in an image is limited by 256, each pixel in the bitmap array is described by one byte (8 bits). Such a pixel description doesn't represent any RGB value but indicates the entry point in the BMP file's color table. For example, if the first RGB value in the color table is R/G/B=255/0/0, then pixel 0 in the bitmap array will represent the pure red color. Pixel values are stored from left to right, usually starting from the bottom row of the image. In a 256-color BMP file, the first byte of the bitmap array is an index for the color of the leftmost pixel in the bottom row; the second byte, for the color of the next pixel; and so on. If the number of bytes in each row is odd, then an additional byte is added for each row to align the bitmap array data by 16-bit boundaries.

Not all BMP files have a structure like that shown in the picture. For example, BMP files with a color depth of 16 or 24 bits per pixel do not use color tables because pixel values in their bitmap arrays directly represent RGB values. Internal formats of file sections may differ too. For instance, bitmap array data in some 16- or 256-color BMP files may be compressed using the RLE algorithm, with replaces each sequence of repeated pixels by the number of pixels and their color value. Windows also supports BMP files inherited from OS/2, which use formats of the bitmap information header and palette different from those in Windows.