Bitmap (.BMP) File Format

The Bitmap or .bmp image file format is the Microsoft Windows image file format. Bitmap files are sometimes also saved as .dib files, or 'Device Independent Bitmap' files. The 'Device Independent' part of the name doesn't refer to alternative platforms, but rather the screen display itself - the same colours should be rendered regardless of the technical method of screen rendering employed by the display.

Bitmaps are structured in four parts - the bitmap file header (the type and size of the bitmap file), the bitmap information header (the colour format, compression if used and the file dimensions), the bitmap colour table (which contains details of all the colours present in the bitmap), and the bitmap document array, which contains the actual bits of the bitmap image in a consecutive line-by-line pixel array. This line-by-line image structure is similar to the way a scanner builds an image, and is called 'scan lines' of image data.

Bitmaps can be saved in four different bit depths: 1-bit (monochrome), 4-bit (16 colours), 8-bit (256 colours) and 24-bit (16 million colours). Even in 24-bit colour, the colours are restricted to RGB combinations (similar to a television screen), which makes printing from bitmaps not appropriate as prepress requires CMYK colour separations.

ViewONE and ViewONE Pro are both capable of viewing bitmap .BMP files as standard.