Windows BMP Graphics Files

This describes the bitmap graphics-file formats used by the Microsoft Windows operating system V3.x, Window 95 and OS/2.

Bitmap-File Formats

Windows bitmap files are stored in a device-independent bitmap (DIB) format that allows 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. The default filename extension of a Windows DIB file is .BMP.

OS/2 BMP files have a different header and color table encoding. Mark Fleming's PICT / BMP application, and XTND BMP translator will read OS/2 format of BMP file but will not write them.

Images of the World - Vol#1 CD-ROM includes one fully licenced copy of the PICT / BMP application and XTND translators.

BMP formats:

• B&W Bitmap is monochrome and the color table contains two entries. Each bit in the bitmap array represents a pixel. If the bit is clear, the pixel is displayed with the color of the first entry in the color table. If the bit is set, the pixel has the color of the second entry in the table.
• 4bit Bitmap has a maximum of 16 colors. Each pixel in the bitmap is represented by a 4-bit index into the color table array. The first pixel contains the color in the second table entry, and the second pixel contains the color in the sixteenth table entry.
• 8bit Bitmap has a maximum of 256 colors. Each pixel in the bitmap is represented by a 1-byte index into the color table.
• 24bit Bitmap has a maximum of 2^24 colors. Each pixcel is a 3-byte sequence in the bitmap array represents the relative intensities of red, green, and blue.

Bitmap Compression

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.

Compression of 8-Bits-per-Pixel Bitmaps the DIB is compressed using a run-length encoded format for a 256-color bitmap. This format uses two modes: encoded mode and absolute mode. Both modes can occur anywhere throughout a single bitmap.

Compression of 4-Bits-per-Pixel Bitmaps the DIB is compressed using a run-length encoded format for a 16-color bitmap. This format uses two modes: encoded mode and absolute mode. These are different than the 8bit methods.

Macintosh File Type of: BMPp or BINA

MS-DOS extension: .BMP