尝试对 PPM 图像进行基本的隐写术。
我已经完成了基本算法。读入文件，检查头部以P6开头，得到图像宽高，像素数据。
我一共需要四个方法：ReadPPM、WritePPM、WriteMsg 和 ReadMsg。
我有 ReadImg 和 WriteImg 方法，但我卡住的地方是我的 WriteMsg 方法。这是基本的隐写术，只是将字符串的每一位写入每个字节的最后一位。假设前 8 个字节包含被隐藏字符串的大小，然后每个字节开始隐藏消息。
我的想法是创建一个庞大的数组，其中包含字符串大小的二进制代码，然后是字符串本身的二进制代码。我只是想弄清楚如何获取该数组并将其添加到图像中的每个字节。
任何帮助深表感谢。这是我当前的代码：
#include<stdio.h>
#include<stdlib.h>
typedef struct {
unsigned char red,green,blue;
} PPMPixel;
typedef struct {
int x, y;
PPMPixel *data;
} PPMImage;
#define CREATOR "RPFELGUEIRAS"
#define RGB_COMPONENT_COLOR 255
static PPMImage *readPPM(const char *filename)
{
char buff[16];
PPMImage *img;
FILE *fp;
int c, rgb_comp_color;
//open PPM file for reading
fp = fopen(filename, "rb");
if (!fp) {
fprintf(stderr, "Unable to open file '%s'\n", filename);
exit(1);
}
//read image format
if (!fgets(buff, sizeof(buff), fp)) {
perror(filename);
exit(1);
}
//check the image format
if (buff[0] != 'P' || buff[1] != '6') {
fprintf(stderr, "Invalid image format (must be 'P6')\n");
exit(1);
}
//alloc memory form image
img = (PPMImage *)malloc(sizeof(PPMImage));
if (!img) {
fprintf(stderr, "Unable to allocate memory\n");
exit(1);
}
//check for comments
c = getc(fp);
while (c == '#') {
while (getc(fp) != '\n') ;
c = getc(fp);
}
ungetc(c, fp);
//read image size information
if (fscanf(fp, "%d %d", &img->x, &img->y) != 2) {
fprintf(stderr, "Invalid image size (error loading '%s')\n", filename);
exit(1);
}
//read rgb component
if (fscanf(fp, "%d", &rgb_comp_color) != 1) {
fprintf(stderr, "Invalid rgb component (error loading '%s')\n", filename);
exit(1);
}
//check rgb component depth
if (rgb_comp_color!= RGB_COMPONENT_COLOR) {
fprintf(stderr, "'%s' does not have 8-bits components\n", filename);
exit(1);
}
while (fgetc(fp) != '\n') ;
//memory allocation for pixel data
img->data = (PPMPixel*)malloc(img->x * img->y * sizeof(PPMPixel));
if (!img) {
fprintf(stderr, "Unable to allocate memory\n");
exit(1);
}
//read pixel data from file
if (fread(img->data, 3 * img->x, img->y, fp) != img->y) {
fprintf(stderr, "Error loading image '%s'\n", filename);
exit(1);
}
fclose(fp);
return img;
}
void writePPM(const char *filename, PPMImage *img)
{
FILE *fp;
//open file for output
fp = fopen(filename, "wb");
if (!fp) {
fprintf(stderr, "Unable to open file '%s'\n", filename);
exit(1);
}
//write the header file
//image format
fprintf(fp, "P6\n");
//comments
fprintf(fp, "# Created by %s\n",CREATOR);
//image size
fprintf(fp, "%d %d\n",img->x,img->y);
// rgb component depth
fprintf(fp, "%d\n",RGB_COMPONENT_COLOR);
// pixel data
fwrite(img->data, 3 * img->x, img->y, fp);
fclose(fp);
}
void writeMsg(PPMImage *img, char *s)
{
int i;
int len;
len = sizeof(s);
if (img)
{
j = 0;
for (i=0; i < img->x * img->y; i++)
{
while(j < 8)
{
if(len & 0x80)
{
img->data[i].red= img->data[i].red | 0x01;
}
else
{
img->data[i].red= img->data[i].red & 0xFE;
}
len=len << 1;
j++;
if (len & 0x80)
{
img->data[i].green= img->data[i].green | 0x01;
}
else
{
img->data[i].green= img->data[i].green & 0xFE;
}
len = len << 1;
j++;
if (len & 0x80)
{
img->data[i].blue= img->data[i].blue | 0x01;
}
else
{
img->data[i].blue= img->data[i].blue & 0xFE;
}
j++;
}
}
}
}