Sustainability of Digital Formats: Planning for Library of Congress Collections

BMP is a simple raster graphics image file format designed to store bitmap digital images independently of a display device, originally and primarily on Microsoft Windows and OS/2 operating systems. The format is sometimes known as Device-Independent Bitmap (DIB), since, when loaded into memory using Windows software, the image is held as a DIB structure. Several backwards-compatible versions have been introduced with new versions of Windows. As of late 2011, the current version of BMP is version 5, introduced with Windows 98 in 1998. This description focuses on version 5, but is relevant for earlier versions, since the structure has remained similar for backwards compatibility. The Wikipedia article on BMP has a table describing the differences between versions.

To quote from part 1 of a 1995 article by David Charlap in Dr. Dobbs's Journal, "Unlike other image-file formats like GIF (CompuServe's Graphic Interchange File format) and JPEG (Joint Photographic Experts Group), the BMP file format was not designed to be portable. Instead, it was designed to easily work with the Windows API using the same structures that Windows applications use to manipulate in-memory bitmaps. As the API changed, so did the BMP file format." BMP is used primarily for icons, screen grabs, and other purposes within Windows.

The BMP File Format version 5 is capable of storing 2D digital images of arbitrary width, height, and resolution, both monochrome and color, in various color depths (1, 4, 8, 16, 24, 32, or 64 bits per pixel). Colors may be indexed. Optional features include compression (specific lossless compression algorithms that are fast to decompress), alpha channels, and ICC color profiles. In practice most BMP images are uncompressed, and 16 bpp and 32bpp images are always uncompressed.

The structure of a BMP file incorporates two headers. The first is a 14-byte BITMAPFILEHEADER. This specifies the type of bitmap file, the size of the file, and the position (offset) in the file where the pixel data begins. This header simply permits a check for the file type and for damage. The second header, known in general as a DIB header, supplies technical information needed to render the image, such as bits-per-pixel, image height and width in pixels, compression method. The second header will have a different name, size and different set of elements recorded depending on the version of the specification. These possible header names include BITMAPCOREHEADER, BITMAPINFOHEADER, BITMAPV4HEADER, and BITMAPV5HEADER. The BITMAPINFOHEADER and BITMAPV5HEADER have superseded the BITMAPCOREHEADER and BITMAPV4HEADER. The Later versions of the DIB header added new elements at the end of the header. If an ICC profile is specified, the DIB header provides a pointer to the the beginning of the ICC information. If a color table is used it is at the end of the DIB header. Following the color table, which is mandatory for color depths of 8 or less, comes the image data. Finally, ICC Profile information, either embedded or as a reference by path to another file.

Microsoft has never published a specification document for BMP. Information about structure and data encoding methods has been included in a number of programmer's references, manuals, online help facilities, and files associated with the Microsoft Windows Software Development Kits (SDKs). However, the format is well understood and has been described in detail at various times in ways that many consider more helpful. Many of these descriptions do not cover the latest version. As of June 2021, Microsoft makes information available through Microsoft Docs. A selected list of documentation sources follows:

• Bitmap Storage from the MSDN Library [https://docs.microsoft.com/en-us/windows/win32/gdi/bitmap-storage]
• Wikipedia page for BMP [https://en.wikipedia.org/wiki/BMP_file_format]
• Description from 1994 in Encyclopedia of Graphic File Formats [https://www.fileformat.info/format/bmp/egff.htm]. Covers version 1-4.
• Microsoft description for version 3 [https://www.fileformat.info/format/bmp/spec/e27073c25463436f8a64fa789c886d9c/view.htm]