Changing the contrast and brightness of an image!

Goal

In this tutorial you will learn how to:

• Access pixel values
• Initialize a matrix with zeros
• Learn what cv::saturate_cast does and why it is useful
• Get some cool info about pixel transformations

Theory

Note

The explanation below belongs to the book Computer Vision: Algorithms and Applications by Richard Szeliski

Image Processing

• A general image processing operator is a function that takes one or more input images and produces an output image.
• Image transforms can be seen as:
  • Point operators (pixel transforms)
  • Neighborhood (area-based) operators

Pixel Transforms

• In this kind of image processing transform, each output pixel's value depends on only the corresponding input pixel value (plus, potentially, some globally collected information or parameters).
• Examples of such operators include brightness and contrast adjustments as well as color correction and transformations.

Brightness and contrast adjustments

• Two commonly used point processes are multiplication and addition with a constant:

  \[g(x) = \alpha f(x) + \beta\]

• The parameters \(\alpha > 0\) and \(\beta\) are often called the gain and bias parameters; sometimes these parameters are said to control contrast and brightness respectively.

• You can think of \(f(x)\) as the source image pixels and \(g(x)\) as the output image pixels. Then, more conveniently we can write the expression as:

  \[g(i,j) = \alpha \cdot f(i,j) + \beta\]

  where \(i\) and \(j\) indicates that the pixel is located in the i-th row and j-th column.

Code

• The following code performs the operation \(g(i,j) = \alpha \cdot f(i,j) + \beta\) :

  #include <opencv2/opencv.hpp>
  #include <iostream>
  
  using namespace cv;
  
  double alpha; /*< Simple contrast control */
  int beta;  /*< Simple brightness control */
  
  int main( int argc, char** argv )
  {
      Mat image = imread( argv[1] );
      Mat new_image = Mat::zeros( image.size(), image.type() );
  
      std::cout<<" Basic Linear Transforms "<<std::endl;
      std::cout<<"-------------------------"<<std::endl;
      std::cout<<"* Enter the alpha value [1.0-3.0]: ";std::cin>>alpha;
      std::cout<<"* Enter the beta value [0-100]: "; std::cin>>beta;
  
      for( int y = 0; y < image.rows; y++ ) {
          for( int x = 0; x < image.cols; x++ ) {
              for( int c = 0; c < 3; c++ ) {
                  new_image.at<Vec3b>(y,x)[c] =
                  saturate_cast<uchar>( alpha*( image.at<Vec3b>(y,x)[c] ) + beta );
              }
          }
      }
  
      namedWindow("Original Image", 1);
      namedWindow("New Image", 1);
  
      imshow("Original Image", image);
      imshow("New Image", new_image);
  
      waitKey();
      return 0;
  }

Explanation

1. We begin by creating parameters to save \(\alpha\) and \(\beta\) to be entered by the user:

   double alpha;
   int beta;

2. We load an image using cv::imread and save it in a Mat object:

   Mat image = imread( argv[1] );

3. Now, since we will make some transformations to this image, we need a new Mat object to store it. Also, we want this to have the following features:
   • Initial pixel values equal to zero
   • Same size and type as the original image

     Mat new_image = Mat::zeros( image.size(), image.type() );

     We observe that cv::Mat::zeros returns a Matlab-style zero initializer based on image.size() and image.type()
4. Now, to perform the operation \(g(i,j) = \alpha \cdot f(i,j) + \beta\) we will access to each pixel in image. Since we are operating with RGB images, we will have three values per pixel (R, G and B), so we will also access them separately. Here is the piece of code:

   for( int y = 0; y < image.rows; y++ ) {
       for( int x = 0; x < image.cols; x++ ) {
           for( int c = 0; c < 3; c++ ) {
               new_image.at<Vec3b>(y,x)[c] =
                 saturate_cast<uchar>( alpha*( image.at<Vec3b>(y,x)[c] ) + beta );
           }
       }
   }

   Notice the following:
   • To access each pixel in the images we are using this syntax: image.at<Vec3b>(y,x)[c] where y is the row, x is the column and c is R, G or B (0, 1 or 2).
   • Since the operation \(\alpha \cdot p(i,j) + \beta\) can give values out of range or not integers (if \(\alpha\) is float), we use cv::saturate_cast to make sure the values are valid.
5. Finally, we create windows and show the images, the usual way.

   namedWindow("Original Image", 1);
   namedWindow("New Image", 1);
   
   imshow("Original Image", image);
   imshow("New Image", new_image);
   
   waitKey(0);

Note

Instead of using the for loops to access each pixel, we could have simply used this command:

image.convertTo(new_image, -1, alpha, beta);

where cv::Mat::convertTo would effectively perform *new_image = a*image + beta*. However, we wanted to show you how to access each pixel. In any case, both methods give the same result but convertTo is more optimized and works a lot faster.

Result

• Running our code and using \(\alpha = 2.2\) and \(\beta = 50\)

  $ ./BasicLinearTransforms lena.jpg
  Basic Linear Transforms
  -------------------------
  * Enter the alpha value [1.0-3.0]: 2.2
  * Enter the beta value [0-100]: 50

• We get this: