BMP format Analysis

BMP file format Analysis

BMP (Bitmap-file) is a Windows Image File Format. All image processing software running in Windows supports the BMP image file format. In Windows, Image Rendering is based on BMP. The BMP image file format before Windows 3.0 is related to the display device. Therefore, the BMP image file format is called the device-Dependent Bitmap File Format. BMP image files after Windows 3.0 have nothing to do with the display device. Therefore, this BMP image file format is called device-independent Bitmap (DIB) format (Note: After Windows 3.0, the DDB bitmap still exists in the system. functions such as bitblt () are based on the DDB bitmap, but if you want to save the image to a disk file in BMP format, microsoft strongly recommends that you store images in Dib format) to enable Windows to display stored images on any type of display device. The default file extension of a BMP bitmap file is BMP or BMP (sometimes it uses. DiB or. RLE as the extension ).

6.1.2 file structure

A bitmap file is composed of four parts: Bitmap-file header, bitmap-Information header, and color table) and the byte array that defines the bitmap. It has the following format.

Composition of Bitmap files	Structure name	Symbol
Bitmap-File Header)	Bitmapfileheader	Bmfh
Bitmap-Information Header)	Bitmapinfoheader	Bmih
Color Table)	Rgbquad	Acolors []
Image Data array bytes	Byte	Abitmapbits []

The structure of the bitmap file can be integrated in Table 6-01.

Table 01 structure Summary of Bitmap files

	Offset	Domain Name	Size	Content
Image files Header	0000 h	File ID	2 bytes	The two-byte content is used to identify the bitmap type: 'Bm ': Windows 3.1x, 95, NT ,... 'Ba': OS/2 bitmap Array 'Cies': OS/2 color icon 'Cp': OS/2 color pointer 'Ic ': OS/2 icon 'Pt': OS/2 pointer Note: Because OS/2 systems are not widely used, you only need to determine the first "BM" in programming.
	0002 H	File Size	1 DWORD	Size of the entire file in bytes
	0006 H	Reserved	1 DWORD	Reserved, must be set to 0
	000ah	Bitmap data offset	1 DWORD	Offset between the data (bitmap data) starting from the file to the graph data
	000eh	Bitmap header size	1 DWORD	The length of the bitmap info header, used to describe the color and compression method of the bitmap. The following length: 28 h-Windows 3.1x, 95, NT ,... 0ch-OS/2 1.x F0h-OS/2 2.x Note: in Windows 95, 98, 2000, and other operating systems, the length of the Bitmap header is not necessarily 28 h, because Microsoft has developed a new BMP file format, the structure of the information header changes greatly and the length is extended. Therefore, it is best not to directly use the constant 28 h, but to read the value from a specific file. This ensures program compatibility.
	0012 H	Width	1 DWORD	The width of the bitmap, in pixels.
	0016 H	Height	1 DWORD	The height of the bitmap, in pixels.
	001ah	Planes	1 word	Bitmap bit number (Note: This value will always be 1)
Image Information Header	001ch	Bits per pixel	1 word	Number of digits in each pixel 1-monochrome Bitmap (in fact, there are two colors, black and white by default. You can define these two colors by yourself) 4-16 color bitmap 8-256-color bitmap 16-16 bit high-color bitmap 24-24bit true color bitmap 32-32bit enhanced true color bitmap
	001eh	Compression	1 DWORD	Compression description: 0-no compression (in bi_rgb format) 1-RLE 8-use the 8-bit RLE compression method (represented by bi_rle8) 2-RLE 4-use the 4-bit RLE compression method (represented by bi_rle4) 3-bitfields-bit domain storage method (represented by bi_bitfields)
	0022 H	Bitmap data size	1 DWORD	The size of the bitmap data expressed by the number of cells. The value must be a multiple of 4.
	0026 H	Hresolution	1 DWORD	Horizontal resolution in pixels/meters
	002ah	Vresolution	1 DWORD	Vertical Resolution in pixels/meters
	002eh	Colors	1 DWORD	The number of colors used by the bitmap. For example, 8-bit/pixel represents 256 h or.
	0032 H	Important colors	1 DWORD	Specify the number of important colors. When the value of this field is equal to the number of colors (or equal to 0), it indicates that all colors are equally important.
Color Palette data	Varies with the BMP version.	Palette	N * 4 byte	Color Palette specification. For each table item in the color palette, use the following methods to describe the values of RGB: 1 byte for the blue component 1 byte for green component 1 byte for red weight 1 byte for padding (set to 0)
Image Data	The size varies depending on the BMP version and palette size.	Bitmap data	Xxx bytes	The size of this field depends on the compression method, image size, and image bit depth. It contains all bitmap data bytes, which may be the index number of the color palette, it may also be the actual RGB value, which is determined based on the bit depth value in the Image Information header.