一個BMP文件可以包含4部分
1，文件頭是14字節長的數據結構，定義如下：
typedef struct _BFHEADER
{
unsigned short magic;
unsigned int size;
unsigned int reserved;
unsigned int bitOffset;
}bfHeader;
2.bmp 信息結構，結構定義如下：
typedef struct _bmpInfoHead
{
unsigned int headSize;
unsigned int width;
unsigned int height;
unsigned short planes;
unsigned short bpp;
unsigned int compress;
unsigned int imageSize;
unsigned int PelsPerMeterX;
unsigned int PelsPerMeterY;
unsigned int ClrUsed;
unsigned int ClrImportant;
unsigned int RedMask;
unsigned int GreenMask;
unsigned int BlueMask;
unsigned int AlphaMask;
unsigned int CsType;
unsigned int Endpoints[9]; // see http://msdn2.microsoft.com/en-us/library/ms536569.aspx
unsigned int GammaRed;
unsigned int GammaGreen;
unsigned int GammaBlue;
}bmpInfoHead;
3.調色板palette
typedef struct _BGRA {
unsigned char Blue;
unsigned char Green;
unsigned char Red;
unsigned char Alpha;
} BGRA;
4.pixel Data
一個完整的簡單BMP reader程序如下：
#include<stdio.h>
#include<stdlib.h>
/*
Pixel format
The 1-bit per pixel (1bpp) format supports 2 distinct colors, (for example: black and white). The pixel values are stored in each bit, with the first (left-most) pixel in the most-significant bit of the first byte.[4] Each bit is an index into a table of 2 colors. An unset bit will refer to the first color table entry, and a set bit will refer to the last (second) color table entry.
The 2-bit per pixel (2bpp) format supports 4 distinct colors and stores 4 pixels per 1 byte, the left-most pixel being in the two most significant bits (Windows CE only:[19]). Each pixel value is a 2-bit index into a table of up to 4 colors.
The 4-bit per pixel (4bpp) format supports 16 distinct colors and stores 2 pixels per 1 byte, the left-most pixel being in the more significant nibble.[4] Each pixel value is a 4-bit index into a table of up to 16 colors.
The 8-bit per pixel (8bpp) format supports 256 distinct colors and stores 1 pixel per 1 byte. Each byte is an index into a table of up to 256 colors.
The 16-bit per pixel (16bpp) format supports 65536 distinct colors and stores 1 pixel per 2-byte WORD. Each WORD can define the alpha, red, green and blue samples of the pixel.
The 24-bit pixel (24bpp) format supports 16,777,216 distinct colors and stores 1 pixel value per 3 bytes. Each pixel value defines the red, green and blue samples of the pixel (8.8.8.0.0 in RGBAX notation). Specifically, in the order: blue, green and red (8 bits per each sample).[4]
The 32-bit per pixel (32bpp) format supports 4,294,967,296 distinct colors and stores 1 pixel per 4-byte DWORD. Each DWORD can define the alpha, red, green and blue samples of the pixel.
*/#pragma pack(push, 1)
typedef struct _BFHEADER
{
unsigned short magic;
unsigned int size;
unsigned int reserved;
unsigned int bitOffset;
}bfHeader;typedef struct _bmpInfoHead
{
unsigned int headSize;
unsigned int width;
unsigned int height;
unsigned short planes;
unsigned short bpp;
unsigned int compress;
unsigned int imageSize;
unsigned int PelsPerMeterX;
unsigned int PelsPerMeterY;
unsigned int ClrUsed;
unsigned int ClrImportant;
unsigned int RedMask;
unsigned int GreenMask;
unsigned int BlueMask;
unsigned int AlphaMask;
unsigned int CsType;
unsigned int Endpoints[9]; // see http://msdn2.microsoft.com/en-us/library/ms536569.aspx
unsigned int GammaRed;
unsigned int GammaGreen;
unsigned int GammaBlue;
}bmpInfoHead;typedef struct _RGBA {
unsigned char Red;
unsigned char Green;
unsigned char Blue;
unsigned char Alpha;
} RGBA;typedef struct _BGRA {
unsigned char Blue;
unsigned char Green;
unsigned char Red;
unsigned char Alpha;
} BGRA;
FILE *inFd;
bfHeader fHead;
bmpInfoHead bmpInfo;BGRA *paletteTable;
BGRA *pixelData;
unsigned char *pixelLine;int main(void)
{
unsigned int ColorTableSize = 0;
unsigned int linePixelLength;
int count;
int rdLen;
//fgetc();
inFd = fopen("testimg.bmp", "rb");
rdLen = fread(&fHead, sizeof(fHead), 1,inFd);
rdLen = fread(&bmpInfo, sizeof(bmpInfo), 1,inFd);
switch (bmpInfo.bpp)
{
case 2:
break;
case 4:
ColorTableSize = 16;
break;
case 8:
ColorTableSize = 256;
case 16:
break;
case 32:
//true color each bit save RGB, do not need palette
break;
default:
break;
}paletteTable = (BGRA *) malloc(ColorTableSize * sizeof(BGRA));
pixelData = (BGRA *)malloc(sizeof(BGRA)*bmpInfo.height*bmpInfo.width);
count=ftell(inFd);
count = sizeof(fHead) + sizeof(bmpInfo);
fseek(inFd, count, SEEK_SET);//from begin;
//Read color map
count= bmpInfo.ClrUsed * sizeof(BGRA);
rdLen = fread(paletteTable, count, 1, inFd);linePixelLength = (bmpInfo.width*bmpInfo.bpp / 8);
pixelLine = (unsigned char *)malloc(linePixelLength);
fseek(inFd, fHead.bitOffset, SEEK_SET);//from begin;
BGRA *p = pixelData;
unsigned char color;
for (unsigned int h = 0; h < bmpInfo.height; h++)
{
rdLen = fread(pixelLine, linePixelLength, 1, inFd);
for (unsigned int w = 0; w < bmpInfo.width; w++)
{
color = pixelLine[w];
p->Blue = paletteTable[color].Blue;
p->Green = paletteTable[color].Green;
p->Red = paletteTable[color].Red;
p++;
}
}count++;
}