C/C++二进制文件编写

Question

我的任务是将bmp文件放大

k

次。

如果我先写

BITMAPHEADER

然后写像素（

24

位）就可以了。

但反过来不行。

写完我使用的所有像素后：

out.seekp(0, ios_base::beg); 

BITMAPHEADER

写得正确，但我看到添加了额外的像素。

为什么会这样？

此代码有效：

//this works correctly
#include <iostream>
#include <fstream>
#include <vector> 
using namespace std;

#pragma pack(push, 1)
struct BMP {
    int8_t id[2];              
    int32_t filesize;        
    int32_t reserved;        
    int32_t headersize;      
    int32_t infoSize;        
    int32_t width;          
    int32_t height;          
    int16_t biPlanes;        
    int16_t bits;            
    int32_t biCompression;   
    int32_t biSizeImage;     
    int32_t biXPelsPerMeter; 
    int32_t biYPelsPerMeter; 
    int32_t biClrUsed;       
    int32_t biClrImportant; 
};
#pragma pack(pop)

#pragma pack(push, 1)
struct PIXEL {
    char B;
    char G;
    char R;
};
#pragma pack(pop)



int main(){
    int k = 100;
    BMP bitMap;
    BMP cpy;
    PIXEL px;

    ifstream in; in.open("bmp.bmp", ios_base::binary);
    ofstream out; out.open("output.bmp", ios_base::binary );

    in.read((char*)&bitMap, sizeof(BMP));
    in.seekg( bitMap.headersize, ios_base::beg );
    int paddingRead = (4 - bitMap.width * sizeof(PIXEL) % 4) % 4;
    int paddingWrite = (4 - bitMap.width * sizeof(PIXEL) * k % 4) % 4;

    cpy = bitMap;
    cpy.width *= k;
    cpy.height *= k;
    cpy.filesize = cpy.headersize + (cpy.height * cpy.width * sizeof(PIXEL)) + (cpy.height * paddingWrite);

    out.write( (char*)&cpy, sizeof(BMP) );

    for (int row = 1; row <= bitMap.height; row++) {
        vector<PIXEL> Line;
        char str[3] = {};
        for (int j = 0; j < bitMap.width; j++) {
            in.read( (char*)&px, sizeof(PIXEL) );
            Line.push_back(px);
        }
        in.read( str, paddingRead );

        for (int cnt = 1; cnt <= k; cnt++) {//k lines add in this way
            for (PIXEL pixel : Line) {
                for (int x = 1; x <= k; x++)
                    out.write((char*)&pixel, sizeof(PIXEL));
            }
            out.write(str, paddingWrite);
        }
    }


    in.close();
    out.close();
    return 0;
}

但是这个没有：

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

#pragma pack(push, 1)
struct BMP {
    int8_t id[2];              
    int32_t filesize;        
    int32_t reserved;        
    int32_t headersize;      
    int32_t infoSize;        
    int32_t width;          
    int32_t height;          
    int16_t biPlanes;        
    int16_t bits;            
    int32_t biCompression;   
    int32_t biSizeImage;     
    int32_t biXPelsPerMeter; 
    int32_t biYPelsPerMeter; 
    int32_t biClrUsed;       
    int32_t biClrImportant; 
};
#pragma pack(pop)

#pragma pack(push, 1)
struct PIXEL {
    char B;
    char G;
    char R;
};
#pragma pack(pop)



int main(){
    int k = 100;
    BMP bitMap;
    BMP cpy;
    PIXEL px;

    ifstream in; in.open("bmp.bmp", ios_base::binary);
    ofstream out; out.open("output.bmp", ios_base::binary );

    in.read((char*)&bitMap, sizeof(BMP));
    in.seekg( bitMap.headersize, ios_base::beg );
    int paddingRead = (4 - bitMap.width * sizeof(PIXEL) % 4) % 4;
    int paddingWrite = (4 - bitMap.width * sizeof(PIXEL) * k % 4) % 4;

    cpy = bitMap;
    cpy.width *= k;
    cpy.height *= k;
    cpy.filesize = cpy.headersize + (cpy.height * cpy.width * sizeof(PIXEL)) + (cpy.height * paddingWrite);

    //out.write( (char*)&cpy, sizeof(BMP) );//comment this line

    for (int row = 1; row <= bitMap.height; row++) {
        vector<PIXEL> Line;
        char str[3] = {};
        for (int j = 0; j < bitMap.width; j++) {
            in.read( (char*)&px, sizeof(PIXEL) );
            Line.push_back(px);
        }
        in.read( str, paddingRead );

        for (int cnt = 1; cnt <= k; cnt++) {//k lines add in this way
            for (PIXEL pixel : Line) {
                for (int x = 1; x <= k; x++)
                    out.write((char*)&pixel, sizeof(PIXEL));
            }
            out.write(str, paddingWrite);
        }
    }
    out.seekp(0, ios::beg);//added
    out.write( (char*)&cpy, sizeof(BMP) );//added
    in.close();
    out.close();
    return 0;
}