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C语言图像对比度扩展 C语言实现bmp图像对比度扩展

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假设有一幅图,由于成象时光照不足,使得整幅图偏暗(例如,灰度范围从0到63);或者成象时光照过强,使得整幅图偏亮(例如,灰度范围从200到255),我们称这些情况为低对比度,即灰度都挤在一起,没有拉开。灰度扩展的意思就是把你所感性趣的灰度范围拉开,使得该范围内的象素,亮的越亮,暗的越暗,从而达到了增强对比度的目的。

我们可以用下图来说明对比度扩展(contrast stretching)的原理:

图中的横坐标gold表示原图的灰度值,纵坐标gnew表示gold经过对比度扩展后得到了新的灰度值。a,b,c为三段直线的斜率,因为是对比度扩展,所以斜率b>1。g1old和g2old表示原图中要进行对比度扩展的范围,g1new和g2new表示对应的新值。用公式表示为:

实现代码:

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#pragma pack(1)   //全紧凑模式

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;
 }
 //读入彩色bmp图像文件头,信息头和图像数据
 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在这里对应斜率b,因为“b”这个符号被bmp图像结构名占了
 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;
}

代码效果:

原图像:

灰度(100-200)扩展图像:

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