BMP file Format

BMP file Format

6.1.1 Introduction

The bitmap file (Bitmap-file,bmp) format is the image file storage format used by Windows, which is supported by all image processing software running in the Windows environment. The previous BMP bitmap file format for Windows 3.0 is related to the display device, so it is referred to as a device-dependent bitmap (device-Dependent bitmap,ddb) file format. The BMP bitmap file format after Windows 3.0 is independent of the display device, so this BMP bitmap file format is called a device-independent bitmap (device-independent bitmap,dib) Format to allow Windows to display BMP bitmap files on any type of display device. BMP bitmap File The default file name extension is BMP or BMP.

6.1.2 File Structure

A bitmap file can be seen as consisting of 4 parts: a bitmap file header (Bitmap-file header), a bitmap information header (bitmap-information header), a color table, and a byte array that defines a bitmap. Their names and symbols are shown in table 6-01.

Table 6-01 BMP image File component names and symbols

The composition of the bitmap file	Structure name	Symbol
Bitmap file Header (Bitmap-file header)	Bitmapfileheader	Bmfh
Bitmap Information Header (bitmap-information header)	Bitmapinfoheader	Bmih
Colored tables (color table)	Rgbquad	Acolors[]
Image data Array byte	BYTE	Abitmapbits[]

The bitmap file structure can be synthesized in table 6-02.

Table 6-02 Summary of bitmap file structure contents

	Offset amount	Name of the domain	Size	Content
Image file Head	0000h	Identifier (Identifier)	2 bytes	The two-byte content is used to identify the type of bitmap: ' BM ': Windows 3.1x, up, NT, ... ' BA ': OS/2 Bitmap Array ' CI ': OS/2 Color Icon ' CP ': OS/2 Color Pointer ' IC ': OS/2 Icon ' PT ': OS/2 Pointer
	0002h	File Size	1 DWORD	The size of the entire file represented in bytes
	0006h	Reserved	1 DWORD	Reserved, set to 0
	000Ah	Bitmap Data Offset	1 DWORD	The offset between the data starting from the beginning of the file in the bitmap (bitmap data)
	000Eh	Bitmap Header Size	1 DWORD	The length of the bitmap information header (Bitmap info header), used to describe the color of the bitmap, the compression method, and so on. The following lengths indicate: 28h-windows 3.1x, N/A, NT, ... 0CH-OS/2 1.x F0H-OS/2 2.x
	0012h	Width	1 DWORD	The width of the bitmap, in pixels
	0016h	Height	1 DWORD	The height of the bitmap, in pixels
	001Ah	Planes	1 word	Number of bits of bitmap
Image Information Head	001Ch	Bits Per Pixel	1 word	The number of bits per pixel 1-monochrome bitmap 4-16 Color Bitmap 8-256 Color Bitmap 16-16bit (high color) bitmap 24-24bit (True Color) bitmap 32-32bit (True Color) bitmap
	001Eh	Compression	1 DWORD	Compression Description: 0-none (also using BI_RGB) 1-rle 8-bit/pixel (also using bi_rle4) 2-rle 4-bit/pixel (also using Bi_rle8) 3-bitfields (also using bi_bitfields)
	0022h	Bitmap Data Size	1 DWORD	The size of the bitmap data represented by the number of bytes. The number must be a multiple of 4
	0026h	Hresolution	1 DWORD	Horizontal resolution expressed in pixels/meters
	002Ah	Vresolution	1 DWORD	Vertical resolution expressed in pixels/meters
	002Eh	Colors	1 DWORD	The number of colors used by the bitmap. If 8-bit/pixel is represented as 100h or 256.
	0032h	Important Colors	1 DWORD	Specifies the number of important colors. When the value of the field equals the number of colors, it means that all colors are equally important
Palette data	0036h