假設(shè)有一幅圖，由于成象時光照不足，使得整幅圖偏暗(例如，灰度范圍從0到63)；或者成象時光照過強，使得整幅圖偏亮(例如，灰度范圍從200到255)，我們稱這些情況為低對比度，即灰度都擠在一起，沒有拉開?；叶葦U(kuò)展的意思就是把你所感性趣的灰度范圍拉開，使得該范圍內(nèi)的象素，亮的越亮，暗的越暗，從而達(dá)到了增強對比度的目的。

我們可以用下圖來說明對比度擴(kuò)展(contrast stretching)的原理：

圖中的橫坐標(biāo)gold表示原圖的灰度值，縱坐標(biāo)gnew表示gold經(jīng)過對比度擴(kuò)展后得到了新的灰度值。a,b,c為三段直線的斜率，因為是對比度擴(kuò)展，所以斜率b>1。g1old和g2old表示原圖中要進(jìn)行對比度擴(kuò)展的范圍，g1new和g2new表示對應(yīng)的新值。用公式表示為：

實現(xiàn)代碼：

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#pragma pack(1)   //全緊湊模式

typedef struct {
 unsigned char bfType[2];
 unsigned int bfSize;
 unsigned short bfReserved1;
 unsigned short bfReserved2;
 unsigned int bfOffBits;
}bitmapFileHeader;

typedef struct {
 unsigned int biSize;
 unsigned int biWidth;
 unsigned int biHeight;
 unsigned short biPlanes;
 unsigned short biBitCount;
 unsigned int biCompression;
 unsigned int biSizeImage;
 unsigned int biXPixPerMeter;
 unsigned int biYPixPerMeter;
 unsigned int biClrUsed;
 unsigned int biClrImportant;
}bitmapInfoHeader;


typedef struct{
 unsigned char rgbBlue;
 unsigned char rgbGreen;
 unsigned char rgbRed;
 unsigned char rgbReserved;

}rgbQUAD;

typedef struct{
 bitmapFileHeader bfHeader;
 bitmapInfoHeader biHeader;
 rgbQUAD palette[256];
 unsigned char *imgData;
}bmp;

int main(){
 FILE *fp;
 if((fp=fopen("d:\Temp\\test_gray.bmp","rb"))==NULL){
  perror("can not open file!");
  return -1;
 }
 //讀入彩色bmp圖像文件頭，信息頭和圖像數(shù)據(jù)
 bitmapFileHeader bfHeader;
 fread(&bfHeader,14,1,fp);
 bitmapInfoHeader biHeader;
 fread(&biHeader,40,1,fp);
 int imSize=biHeader.biSizeImage;
 int width=biHeader.biWidth;
 int height=biHeader.biHeight;
 int bitCount=biHeader.biBitCount;
 int lineBytes=(width*bitCount+31)/32*4;
 
 fseek(fp,bfHeader.bfOffBits,SEEK_SET);
 unsigned char*imageData=(unsigned char*)malloc(imSize*sizeof(unsigned char));
 fread(imageData,imSize*sizeof(unsigned char),1,fp); 
 fclose(fp);

 bmp b;
 memcpy(&(b.bfHeader),&bfHeader,sizeof(bfHeader));
 memcpy(&(b.biHeader),&biHeader,sizeof(biHeader));
 b.imgData=(unsigned char*)malloc(sizeof(unsigned char)*imSize);
 memset(b.imgData,0,sizeof(unsigned char)*imSize);
 for(int i=0;i<256;i++){
  b.palette[i].rgbBlue=i;
  b.palette[i].rgbGreen=i;
  b.palette[i].rgbRed=i;
 }
 
 int i,j,temp;
 double m=1.5,g1=100.0,g2=200.0; //m在這里對應(yīng)斜率b，因為“b”這個符號被bmp圖像結(jié)構(gòu)名占了
 double a=(255.0-m*(g2-g1))/(255.0-(g2-g1));
 for(i=0;i<height;i++){
  for(j=0;j<width;j++){
   temp=imageData[lineBytes*i+j];
   if(temp<g1)
    b.imgData[lineBytes*i+j]=(unsigned char)(a*temp);
   else if(temp<g2)
    b.imgData[lineBytes*i+j]=(unsigned char)(g1+m*(temp-g1));
   else 
    b.imgData[lineBytes*i+j]=(unsigned char)(g2+a*(temp-g2));

  }
 }
 
 char savePath[]="D:\Temp\\save_test.bmp";
 FILE *f_save=fopen(savePath,"wb");
 if(f_save==NULL){
  perror("can not open file!");
  return -2;
 }

 fwrite(&b.bfHeader,sizeof(bitmapFileHeader),1,f_save);
 fwrite(&b.biHeader,sizeof(bitmapInfoHeader),1,f_save);
 fwrite(&b.palette,1024,1,f_save);
 fwrite(b.imgData,sizeof(unsigned char)*b.biHeader.biSizeImage,1,f_save);
 fclose(f_save);
 
 free(imageData);
 free(b.imgData);
 getchar();
 return 0;
}

代碼效果：

原圖像：