BMP

Revision as of 01:25, 9 September 2013

BMP is a family of raster image file formats primarily used 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.

Though seemingly a simple format, it is complicated by its many different versions, lack of an official specification, lack of any version control process, and ambiguities and contradictions in the documentation.

Images are usually uncompressed, but RLE compression can be used under some conditions. JPEG, PNG, and Huffman 1D compression are also theoretically possible, but rarely supported.

Identifiers

No MIME type has been officially registered. Strings found in practice are: image/bmp; image/x-bmp; image/x-ms-bmp

The usual filename extension is .bmp. Extensions .rle (for RLE-compressed images) and .dib are also sometimes seen. Reportedly, .rl4 and .rl8 have also been used. The .dib extension sometimes means that the file lacks a file header. The .ddb extension is used only for v1 files.

Identification

All common types of BMP files start with bytes 'B' 'M' ?? ?? 00 00 00 00.

Well-known versions

BMP files (except for v1) use a common 14-byte file header, named BITMAPFILEHEADER. Following that is the "Info header", which begins with a 4-byte integer indicating its size. The Info header size (mostly) reveals the version of BMP that the file uses.

Windows BMP v2

• Info header size: 12
• Info header name: BITMAPCOREHEADER or OS21XBITMAPHEADER
• PRONOM: fmt/115

Also known as OS/2 BMP 1.0.

Some sources say that the height and width fields in Windows BMP v2 are signed integers (which would make the Windows BMP v2 and OS/2 BMP 1.0 formats slightly different), but modern documentation says they are unsigned integers.

Windows BMP v3

• Info header size: 40
• Info header name: BITMAPINFOHEADER
• PRONOM: fmt/116, fmt/117

This is by far the most widely used version of BMP. It was introduced with Windows 3.x. Windows NT extended it to support 16 and 32 bits/pixel.

Windows CE also extended it, for example to allow 2 bits/pixel, but its extensions were not migrated to the BMP v4 and v5 formats.

It is apparently possible for OS/2 BMP 2.0 format to masquerade as Windows BMP v3. The upshot is that if the compression type is 3 and the bit depth is 1, or the compression type is 4 and the bit depth is 24, then the file should be treated as OS/2 BMP 2.0.

Windows BMP v4

• Info header size: 108
• Info header name: BITMAPV4HEADER
• PRONOM: fmt/118

Introduced with Windows 95. Adds support for transparency and colorimetry.

Windows BMP v5

• Info header size: 124
• Info header name: BITMAPV5HEADER
• PRONOM: fmt/119

Introduced with Windows 98. Adds support for ICC profiles.

Other versions

Windows BMP v1

• PRONOM: fmt/114

This is the bitmap file format used by Windows 1.0. It's not really a BMP format. It has a 10-byte file header, and is also called "DDB" (Device-Dependent Bitmap).

OS/2 BMP 2.0

(see also OS/2 BMP)

• Info header size: 16–64 (16, 40, 48, and 64 may be most common)
• Info header name: BITMAPCOREHEADER2 or OS22XBITMAPHEADER
• PRONOM: x-fmt/270

OS/2 BMP 2.0 defines several file subtypes; here we are describing only the "Bitmap" subtype (files with a signature of "BM"). For other subtypes, see OS/2 BMP.

The header size can be reduced from its full size of 64 bytes. Omitted fields are assumed to have a value of zero.

The fields in the first 40 bytes of the header are (nearly) identical to those in Windows BMP v3, v4, and v5. The remaining fields are different.

OS/2 BMP 2.0 supports compression types "Huffman 1D" and "RLE24", unlike any other version of BMP.

BITMAPV2INFOHEADER

• Info header size: 52
• Info header name: BITMAPV2INFOHEADER

No details known.

BITMAPV3INFOHEADER

• Info header size: 56
• Info header name: BITMAPV3INFOHEADER

No details known.

Symbol definitions

Here are the definitions, from the Windows SDKs, of some of the symbols used in the BMP documentation.

All integers use little-endian byte order.

Specifications

• BITMAPFILEHEADER
• BITMAPCOREHEADER
• BITMAPINFOHEADER
• BITMAPINFOHEADER (Windows CE 5.0)
• BITMAPV4HEADER
• BITMAPV5HEADER

Sample images

• Example BMP images (all Windows v3 except as indicated)
• BMP Suite Image List

Resources

• BMP (file format): Wikipedia
• Bitmap Image File (BMP), Version 5, from Library of Congress resource on Sustainability of Digital Formats
• Microsoft Windows Bitmap File Format Summary From Encyclopedia of Graphics File Formats at FileFormat.Info (1996)
• OS/2 Bitmap File Format Summary From Encyclopedia of Graphics File Formats at FileFormat.Info (1996)