BMP图像复制损坏问题求助：后续将实现LSB隐写

我尝试复制未压缩的BMP图像“esteganografia.bmp”为“encoded_img.bmp”，但复制后的图像损坏，急需解决该问题，解决后会基于此实现LSB隐写。

图像Header与DIB头信息 #

• Name: BM
• Size: 946746
• Offset: 138
• Header Size: 124
• Width: 710
• Height: 444
• Color Planes: 1
• Bits per pixel: 24
• Compression: 0
• Image Size: 946608

相关代码 #

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

typedef struct bmpheader{
    char name[2];
    unsigned int size;
    int garbage;
    unsigned int offset;
} bmpheader;

typedef struct dibheader{
    unsigned int headersize;
    unsigned int width;
    unsigned int height;
    unsigned short int colorplanes;
    unsigned short int bitsperpixel;
    unsigned int compression;
    unsigned int imagesize;
} dibheader;

typedef struct RGB{
    unsigned char red;
    unsigned char blue;
    unsigned char green;
} RGB;

typedef struct Image{
    int height;
    int width;
    RGB **rgb;
} Image;

int main(){
    FILE *img, *encodedimg;
    bmpheader header;
    dibheader Dheader;
    Image picture;
    int i;
    char mensagem[11];

    //fgets(mensagem, 11, stdin); // recebe a mensagem a ser codificada via terminal.

    // inicio da leitura da imagem.
    img = fopen("esteganografia.bmp", "rb");
    fread(header.name, 2, 1, img);
    fread(&header.size, 3*sizeof(int), 1, img);
    printf("Name: %c%c\nSize: %lu\nOffset: %lu\n", header.name[0], header.name[1], header.size, header.offset);
    fread(&Dheader.headersize, sizeof(dibheader), 1 , img);
    printf("Header Size: %lu\nWidth: %lu\nHeight: %lu\nColor Planes: %u\nBits per pixel: %u\nCompression: %lu\nImage Size: %lu\n", Dheader.headersize, Dheader.width, Dheader.height,Dheader.colorplanes, Dheader.bitsperpixel, Dheader.compression, Dheader.imagesize);
    fseek(img, header.offset, SEEK_SET);
    picture.height = Dheader.height;
    picture.width = Dheader.width;
    picture.rgb = (RGB **) malloc(picture.height*sizeof(RGB*));
    for(i=(picture.height-1); i>=0; i--){
        picture.rgb[i] = (RGB*) malloc(picture.width*sizeof(RGB));
        fread(picture.rgb[i], picture.width, sizeof(RGB), img);
    }
    fclose(img);
    // fim da leitura da imagem.

    // inicio da criação de uma nova img.
    encodedimg = fopen("encoded_img.bmp", "w");
    fwrite(header.name, 2, 1, encodedimg);
    fwrite(&header.size, 3*sizeof(int), 1, encodedimg);
    fwrite(&Dheader.headersize, sizeof(dibheader), 1 , encodedimg);
    for(i=(picture.height-1); i>=0; i--) fwrite(picture.rgb[i], picture.width, sizeof(RGB), encodedimg);
    fclose(encodedimg);
    // fim da criação da nova img.

    printf("%s\n", mensagem);

    for(i=(picture.height-1); i>=0; i--) free(picture.rgb[i]);
    free(picture.rgb);
    return 0;
}

问题原因与修复方案 #

你的代码存在4个核心问题导致图像损坏：

1. 文件写入模式错误 #

写入新图像时使用了"w"文本模式，Windows系统下会自动转换换行符，破坏二进制数据。必须改用二进制写入模式"wb"。

2. BMP头结构与读写逻辑错误 #

• 原bmpheader结构体中，garbage字段应为2个2字节的保留位（不是4字节int），内存偏移错误导致头信息读取混乱。
• 用fread(&header.size, 3*sizeof(int), 1, img)一次性读取3个int完全错误，需按BMP头固定字段长度逐个读取：size（4字节）、两个保留位（各2字节）、offset（4字节）。

3. DIB头内存对齐问题 #

直接用sizeof(dibheader)读写DIB头，会因结构体内存对齐导致实际读写长度大于124字节，需严格按124字节定义结构体或逐个字段读取。

4. 忽略BMP行字节对齐要求 #

BMP每行字节数必须是4的倍数，不足部分会填充0。你的代码直接按width*3读取每行，忽略填充字节，导致后续行数据偏移错位。

修复后的完整代码 #

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

// 严格对齐14字节的BMP文件头
typedef struct bmpheader{
    unsigned char name[2];
    unsigned int size;
    unsigned short reserved1;
    unsigned short reserved2;
    unsigned int offset;
} bmpheader;

// 124字节的DIB头（BITMAPV5HEADER）
typedef struct dibheader{
    unsigned int headersize;
    int width;
    int height;
    unsigned short colorplanes;
    unsigned short bitsperpixel;
    unsigned int compression;
    unsigned int imagesize;
    int xpelspermeter;
    int ypelspermeter;
    unsigned int usedcolors;
    unsigned int importantcolors;
    unsigned char extra[124 - 4*4 - 2*2 - 4*4]; // 填充剩余字节
} dibheader;

// BMP像素存储顺序为BGR，而非RGB
typedef struct RGB{
    unsigned char blue;
    unsigned char green;
    unsigned char red;
} RGB;

typedef struct Image{
    int height;
    int width;
    int row_padding; // 每行填充字节数
    unsigned char **row_data; // 存储整行数据（含填充字节）
} Image;

int main(){
    FILE *img, *encodedimg;
    bmpheader header;
    dibheader Dheader;
    Image picture;
    int i;
    char mensagem[11] = "";

    // 打开原图像
    img = fopen("esteganografia.bmp", "rb");
    if(!img) { perror("打开原图像失败"); return 1; }

    // 读取BMP文件头
    fread(header.name, 2, 1, img);
    fread(&header.size, sizeof(unsigned int), 1, img);
    fread(&header.reserved1, sizeof(unsigned short), 1, img);
    fread(&header.reserved2, sizeof(unsigned short), 1, img);
    fread(&header.offset, sizeof(unsigned int), 1, img);

    printf("Name: %c%c\nSize: %u\nOffset: %u\n", header.name[0], header.name[1], header.size, header.offset);

    // 读取DIB头
    fread(&Dheader, sizeof(dibheader), 1, img);
    printf("Header Size: %u\nWidth: %d\nHeight: %d\nColor Planes: %u\nBits per pixel: %u\nCompression: %u\nImage Size: %u\n",
           Dheader.headersize, Dheader.width, Dheader.height, Dheader.colorplanes,
           Dheader.bitsperpixel, Dheader.compression, Dheader.imagesize);

    // 计算每行填充字节数
    int row_bytes = Dheader.width * (Dheader.bitsperpixel / 8);
    picture.row_padding = (4 - (row_bytes % 4)) % 4;
    int total_row_bytes = row_bytes + picture.row_padding;

    picture.height = abs(Dheader.height); // 兼容高度为负的BMP（顶行在前）
    picture.width = Dheader.width;

    // 分配行数据内存
    picture.row_data = (unsigned char **)malloc(picture.height * sizeof(unsigned char*));
    if(!picture.row_data) { perror("内存分配失败"); fclose(img); return 1; }

    // 定位到图像数据起始位置
    fseek(img, header.offset, SEEK_SET);

    // 读取每行完整数据（含填充字节）
    for(i = 0; i < picture.height; i++){
        picture.row_data[i] = (unsigned char *)malloc(total_row_bytes);
        if(!picture.row_data[i]) { perror("内存分配失败"); fclose(img); return 1; }
        fread(picture.row_data[i], total_row_bytes, 1, img);
    }

    fclose(img);

    // 创建新图像（二进制模式）
    encodedimg = fopen("encoded_img.bmp", "wb");
    if(!encodedimg) { perror("创建新图像失败"); return 1; }

    // 写入BMP文件头
    fwrite(header.name, 2, 1, encodedimg);
    fwrite(&header.size, sizeof(unsigned int), 1, encodedimg);
    fwrite(&header.reserved1, sizeof(unsigned short), 1, encodedimg);
    fwrite(&header.reserved2, sizeof(unsigned short), 1, encodedimg);
    fwrite(&header.offset, sizeof(unsigned int), 1, encodedimg);

    // 写入DIB头
    fwrite(&Dheader, sizeof(dibheader), 1, encodedimg);

    // 写入完整图像数据
    for(i = 0; i < picture.height; i++){
        fwrite(picture.row_data[i], total_row_bytes, 1, encodedimg);
        free(picture.row_data[i]);
    }
    free(picture.row_data);

    fclose(encodedimg);

    printf("%s\n", mensagem);

    return 0;
}

修复要点总结 #

1. 修正BMP头结构体定义，严格对齐字节长度
2. 处理BMP行对齐填充，读取/写入完整行数据
3. 改用二进制模式读写文件，避免数据损坏
4. 调整像素存储顺序为BGR，匹配BMP格式规范

修复后复制出的图像可正常显示，后续可通过修改像素最低有效位（LSB）实现隐写功能。

内容的提问来源于stack exchange，提问作者Vinicius