# Contrast Stretching using C++ and OpenCV: Image Processing

**Theory**

Contrast Stretching is one of the piecewise linear function. Contrast Stretching increases the dynamic range of the grey level in the image being processed.

Points (r1, s1) and (r2, s2) control the shape of the transformation. The selection of control points depends upon the types of image and varies from one image to another image. If r1 = s1 and r2 = s2 then the transformation is linear and this doesn’t affect the image. In other case we can calculate the intensity of output pixel, provided intensity of input pixel is x, as follows

output = s1 / r1 * x

for r1 < x <= r2

output = ((s2 – s1)/(r2 – r1))*(x – r1) + s1

for r2 < x <= L – 1

output = ((L-1 – s2)/(L-1 – r2))*(x – r2) + s2

**Source Code**

#include <iostream> #include <opencv2/core/core.hpp> #include <opencv2/highgui/highgui.hpp> using namespace cv; using namespace std; int computeOutput(int, int, int, int, int); int main() { Mat image = imread("pic.jpg"); Mat new_image = image.clone(); int r1, s1, r2, s2; cout<<"Enter r1: "<<endl; cin>>r1; cout<<"Enter s1: "<<endl; cin>>s1; cout<<"Enter r2: "<<endl; cin>>r2; cout<<"Enter s2: "<<endl; cin>>s2; for(int y = 0; y < image.rows; y++){ for(int x = 0; x < image.cols; x++){ for(int c = 0; c < 3; c++){ int output = computeOutput(image.at<Vec3b>(y,x)[c], r1, s1, r2, s2); new_image.at<Vec3b>(y,x)[c] = saturate_cast<uchar>(output); } } } namedWindow("Original Image", 1); imshow("Original Image", image); namedWindow("New Image", 1); imshow("New Image", new_image); waitKey(); return 0; } int computeOutput(int x, int r1, int s1, int r2, int s2) { float result; if(0 <= x && x <= r1){ result = s1/r1 * x; }else if(r1 < x && x <= r2){ result = ((s2 - s1)/(r2 - r1)) * (x - r1) + s1; }else if(r2 < x && x <= 255){ result = ((255 - s2)/(255 - r2)) * (x - r2) + s2; } return (int)result; }

**Output**

r1 = 70 s1 = 0 r2 = 140 s2 = 255

What are the factors to be considered while choosing r1,s1,r2,s2 values.(Since it varies from image to image,

do we use the histogram plot to choose the r,s values?)

Of course, histogram is one of the best way to choose these parameters.

how to use header of opencv in c++ compiler like codeblocks?

how to take values of r1 s1 r2 s2 from histogram

how to choose the points r1,s1,r2,s2 from histogram plot