#include <iostream>
#include <fstream>
using namespace std;

#pragma pack(2)        //两字节对齐，否则bmp_fileheader会占16Byte
struct bmp_fileheader
{
    unsigned short    bfType;        //若不对齐，这个会占4Byte
    unsigned long    bfSize;
    unsigned short    bfReverved1;
    unsigned short    bfReverved2;
    unsigned long    bfOffBits;
};

struct bmp_infoheader
{
    unsigned long    biSize;
    unsigned long    biWidth;
    unsigned long    biHeight;
    unsigned short    biPlanes;
    unsigned short    biBitCount;
    unsigned long    biCompression;
    unsigned long    biSizeImage;
    unsigned long    biXPelsPerMeter;
    unsigned long    biYpelsPerMeter;
    unsigned long    biClrUsed;
    unsigned long    biClrImportant;
};

fstream input_file;
fstream output_file;

struct bmp_fileheader bfh;
struct bmp_infoheader bih;

unsigned char *src_buff;
unsigned char *dst_buff;

void read_bmp_fileheader()
{

    input_file.read((char*)&bfh,sizeof(struct bmp_fileheader));

}

void read_bmp_infoheader()
{

    input_file.read((char*)&bih,sizeof(struct bmp_infoheader));

}

void read_bmp_data()
{
    src_buff=new unsigned char[bih.biHeight*bih.biWidth*3];

    input_file.read((char*)src_buff,sizeof(unsigned char)*bih.biHeight*bih.biWidth*3);
    
}

void bmp_rgb2gray()
{
    dst_buff=new unsigned char[bih.biHeight*bih.biWidth];
    int tmp;
    unsigned long j=0;
    for (unsigned long i=0;i<bih.biHeight*bih.biWidth*3;i+=3)
    {
        tmp=(src_buff[i]+src_buff[i+1]+src_buff[i+2])/3;
        dst_buff[j++]=(unsigned char)tmp;
    }
}

void write_bmp_fileheader()
{

    bfh.bfSize=bfh.bfOffBits+256*4+bih.biWidth*bih.biHeight;        //256*4是调色板占的字节数
    bfh.bfOffBits=bfh.bfOffBits+256*4;

    output_file.write((char*)&bfh,sizeof(struct bmp_fileheader));

}

void write_bmp_infoheader()
{
    bih.biBitCount=8;            //8位灰度图

    output_file.write((char*)&bih,sizeof(struct bmp_infoheader));
}

void write_bmp_paltte()        //灰度图像需要调色板
{
    unsigned char j=0;
    unsigned char z=0;
    for (int i=0;i<1024;i+=4)
    {
        output_file.write((char*)&j,sizeof(unsigned char));
        output_file.write((char*)&j,sizeof(unsigned char));
        output_file.write((char*)&j,sizeof(unsigned char));

        output_file.write((char*)&z,sizeof(unsigned char));
        j++;
    }
}

void write_bmp_data()
{

    output_file.write((char*)dst_buff,sizeof(unsigned char)*bih.biWidth*bih.biHeight);    

}

int main()
{
    input_file.open("lena.bmp",ios::binary|ios::in);
    output_file.open("lena_gray.bmp",ios::binary|ios::out);

    read_bmp_fileheader();
    read_bmp_infoheader();
    read_bmp_data();

    bmp_rgb2gray();

    write_bmp_fileheader();
    write_bmp_infoheader();
    write_bmp_paltte();
    write_bmp_data();

    input_file.close();
    output_file.close();
    
    cout<<"good job."<<endl;
    cin.get();
    
    delete[] src_buff;
    delete[] dst_buff;
    return 0;
}

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