The BMP file format
BMP is a meanwhile outdated image file format for computers running the Windows operating system. The format was developed by Microsoft for storing bitmap files in a device-independent bitmap (DIB) format that would allow Windows to display the bitmap on any type of display device. 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.
Since BMP is a fairly simple file format, its structure is pretty straightforward. Each bitmap file contains:
- a bitmap-file header: this contains information about the type, size, and layout of a device-independent bitmap file.
- a bitmap-information header which specifies the dimensions, compression type, and color format for the bitmap.
- a color table, defined as an array of RGBQUAD structures, contains as many elements as there are colors in the bitmap. The color table is not present for bitmaps with 24 color bits because each pixel is represented by 24-bit red-green-blue (RGB) values in the actual bitmap data area.
- an array of bytes that defines the bitmap bits. These are the actual image data, represented by consecutive rows or ‘scan lines’ of the bitmap. Each scan line consists of consecutive bytes representing the pixels in the scan line, in left-to-right order.
BMP files always contain RGB data. The file can be:
- 1-bit: 2 colors (monochrome)
- 4-bit: 16 colors
- 8-bit: 256 colors.
- 24-bit: 16777216 colors, mixes 256 tints of Red with 256 tints of Green and Blue
Windows versions 3.0 and later support run-length encoded (RLE) formats for compressing bitmaps that use 4 bits per pixel and 8 bits per pixel.
The default filename extension of a Windows DIB file is .BMP.
Use in a prepress environment
Everything that can be done with BMP, can also be done with TIFF files (or EPS or PNG). Because TIFF is an established and more versatile file format for prepress applications, it is better to avoid BMP-files for prepress production. BMP is also limited to RGB-images only while CMYK data are often prefered in prepress.
What is a BMP file?
Files having extension .BMP represent Bitmap Image files that are used to store bitmap digital images. These images are independent of graphics adapter and are also called device independent bitmap (DIB) file format. This independency serves the purpose of opening the file on multiple platforms such as Microsoft Windows and Mac. The BMP file format can store data as two-dimensional digital images in both monochrome as well as color format with various colour depths.
BMP File Format Specifications
Device Independent Bitmaps act as an aid to exchanging bitmaps between devices and applications. Due to the continuous evolution of this file format, the information contained in the headers can be different as per the version of Bitmap. A single bitmap file consists of fixed as well as variable-size structures in a specific sequence.