我用C语言读取一个图片，不处理显示正常。按书上说的提取轮廓，怎么显示不了，请高手帮忙
#include<stdio.h>
#include<math.h>
#include<stdlib.h>
int main()
{
    FILE *fp,*fp1,*fp2;
static     char cx[9]={0 ,0, 0  //算子系数
                      , 0 ,1 ,0,
                        0,0,-1};
static    char cy[9]={0 ,0, 0  //算子系数
                      ,0 ,0 ,1,
                        0,-1,0};
    char d[9];
    int i,j,dat;
    float xx,yy,zz;
    unsigned char image_in[256][256]={0};
    unsigned char image_out[256][256]={0};
    if((fp=fopen("2.jpg","rb"))==NULL)
    {
        printf("open error");
        exit(1);
    }

  fread(image_in,256,256,fp);//读取
for(i=1;i<255;i++)//提取轮廓
   for(j=1;j<255;j++)
   {
       d[0]=image_in[i-1][j-1];
       d[1]=image_in[i-1][j];
       d[2]=image_in[i-1][j+1];
       d[3]=image_in[i][j-1];
       d[4]=image_in[i][j];
       d[5]=image_in[i][j+1];
       d[6]=image_in[i+1][j-1];
       d[7]=image_in[i+1][j];
       d[8]=image_in[i+1][j+1];
       xx=(float)(d[0]*cx[0]+d[1]*cx[1]+d[2]*cx[2]+d[3]*cx[3]+
           d[4]*cx[4]+d[5]*cx[5]+d[6]*cx[6]+d[7]*cx[7]+d[8]*cx[8]);
       yy=(float)(d[0]*cy[0]+d[1]*cy[1]+d[2]*cy[2]+d[3]*cy[3]+
           d[4]*cy[4]+d[5]*cy[5]+d[6]*cy[6]+d[7]*cy[7]+d[8]*cy[8]);
        zz=(float)(5*sqrt(xx*xx+yy*yy));
    dat=(int)zz;
    if(dat>255) dat=255;
    image_out[i][j]=(char)dat;
   }
   fp1=fopen("new.jpg","wb");
   fp2=fopen("old.jpg","wb");
   fwrite(image_out,256,256,fp1);//显示轮廓
   fwrite(image_in,256,256,fp2);//显示原来的图片
   fclose(fp);
   fclose(fp1);
   fclose(fp2);
    return 0;
}

带专业背景的，最好介绍一下