Contrast and Brightness in C++

Syntax#

void cv::Mat::convertTo(OutputArray m, int rtype,double alpha = 1,double beta = 0)const

Parameters#

Parameter	Details
m	output matrix; if it does not have a proper size or type before the operation, it is reallocated
rtype	desired output matrix type or, rather, the depth since the number of channels are the same as the input has; if rtype is negative, the output matrix will have the same type as the input
alpha	optional scale factor. This changes the contrast of an image. Values below 1 decrease the contrast and above one increases the contrast
beta	optional delta added to the scaled values. Positive values increases the brightness and negative values decreases the brightnes
## Remarks#
Contrast:

Contrast is the difference in luminance or colour that makes an object (or its representation in an image or display) distinguishable. The higher the difference between a pixel and its neighbors the higher the contrast is in that area.

Brightness:

In other words, brightness is the perception elicited by the luminance of a visual target. In terms of pixels, the higher the value of a pixel is the brighter that pixel is.

Contrast and Brightness adjustments:

g(i,j)= α.f(i,j)+β

f(x) as the source image pixels and g(x) as the output image pixels.

i and j indicates that the pixel is located in the i-th row and j-th column.

The parameters α > 0 and β are often called the gain and bias parameters; sometimes these parameters are said to control contrast and brightness respectively.

Opencv has a function called convertTo() which can apply these two operations.

Sources: https://docs.opencv.org/trunk/d3/d63/classcv_1_1Mat.html#adf88c60c5b4980e05bb556080916978b https://opencv-srf.blogspot.ca/2013/07/change-contrast-of-image-or-video.html https://opencv-srf.blogspot.ca/2013/07/change-brightness.html

Adjusting brightness and contrast of an image in c++

// main.cpp : Defines the entry point for the console application.
//
#include "opencv2/highgui/highgui.hpp"
#include <iostream>

using namespace cv;
using namespace std;

int main(int argc, const char** argv)

{

    Mat img = imread("lena30.jpg", CV_LOAD_IMAGE_COLOR); //open and read the image


    if (img.empty())
    {
        cout << "Image cannot be loaded..!!" << endl;
        return -1;
    }

    Mat img_higher_contrast;
    img.convertTo(img_higher_contrast, -1, 2, 0); //increase the contrast (double)

    Mat img_lower_contrast;
    img.convertTo(img_lower_contrast, -1, 0.5, 0); //decrease the contrast (halve)

    Mat img_higher_brightness;
    img.convertTo(img_higher_brightness, -1, 1, 20); //increase the brightness by 20 for each pixel 

    Mat img_lower_brightness;
    img.convertTo(img_lower_brightness, -1, 1, -20); //decrease the brightness by 20 for each pixel 

    //create windows
    namedWindow("Original Image", CV_WINDOW_AUTOSIZE);
    namedWindow("High Contrast", CV_WINDOW_AUTOSIZE);
    namedWindow("Low Contrast", CV_WINDOW_AUTOSIZE);
    namedWindow("High Brightness", CV_WINDOW_AUTOSIZE);
    namedWindow("Low Brightness", CV_WINDOW_AUTOSIZE);
    //show the image
    imshow("Original Image", img);
    imshow("High Contrast", img_higher_contrast);
    imshow("Low Contrast", img_lower_contrast);
    imshow("High Brightness", img_higher_brightness);
    imshow("Low Brightness", img_lower_brightness);

    waitKey(0); //wait for key press
    destroyAllWindows(); //destroy all open windows
    return 0;
}

Output of the program: