BMP is a standard image file format which was historically used by computers running the Windows operating system. The format is more commonly know as Bitmap files. The term bitmap was originally coined in computer programming to refer to a map of bits. Historically when you saved an image on your home PC you might have saved it as a .bhp file, this is becoming less common with .jpeg the preferred image file format with the advent of digital cameras.
Bitmap files are housed in a device-independent bitmap (DIB) format. The term device independent means that the bitmap specifies pixel color in a form independent of the method used by a display to represent color. This enables Windows to show the bitmap on any type of display device - such as your monitor.
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Development of the TGA file format began in the mid eighties. A TGA file is most commonly associated with a TARGA image file (Truevision Advanced Raster Graphics Adapter). TGA files are bitmapped images, they were originally developed for use with Truevision's (now Pinnacle Systems) high-end graphics cards which were designed for video editing. TGA files are often seen in the video and animation industry, as well as by computer game manufacturers who use the TGA format to store texture files.
TGA files can store images with a range of bits, these include 8-bit, 16-bit, 24-bit or 32-bit which is 24-bit colour with transparency. They use a palette to define all available colours in an image. TGA images have typically large file sizes as every pixel is represented by a certain number of bits (dependant on the colour depth). However, they can be compressed using simple lossless run-length encoding, which can result in greatly reduced file sizes.