.BMP or .DIB (device-independent bitmap) is a bitmapped graphics format used internally by the Microsoft Windows graphics subsystem (GDI), and used commonly as a simple graphics file format on that platform.

Images are generally stored with a color depth of 2 (1-bit), 16 (4-bit), 256 (8-bit), 65,536 (16-bit), or 16.7 million (24-bit). An alpha channel (for transparency) may be stored in a separate file, where it is similar to a greyscale image. 8-bit images can also be greyscale instead of color.

BMP files are usually not compressed, so they are typically much larger than compressed image file formats for the same image. The typical true-color bitmap size in bytes can be calculated as: (width in pixels)×(height in pixels)×3. So an 800×600 image will occupy almost 1.5 megabytes. As such they are generally unsuitable for transferring images on the Internet or other slow or capacity-limited media. Compressed imaged employ RLE algorithm.

Nevertheless, the simplicity of BMP and its widespread familiarity in MS Windows and elsewhere, as well as the fact that this format is well-documented and free of patents, makes it a very common format that image processing programs from many operating systems can read and write.

The X Window System uses a similar .XBM format for true single-bit black and white images, and .XPM (pixelmap) for color images. There is also a .RAW format, which saves raw data with no other information. The Portable Pixmap file format (.PPM) and Truevision TGA (.TGA) formats also exist, but are rarely used. Yet other formats store as "bitmaps" (as opposed to vector graphics), but use compression or color indexes, and thus are not strictly considered true bitmaps.

External links

• Microsoft's MSDN page on Bitmaps
• Wotsit.org's entry on the BMP format
• An introduction to DIBs (Device Independent Bitmaps)