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 saturate_cast does and why it is useful
- Get some cool info about pixel transformations
Theory
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
Code
- The following code performs the operation
System Message: WARNING/2 (g(i,j) = \alpha \cdot f(i,j) + \beta)
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:
#include <cv.h>
#include <highgui.h>
#include <iostream>
using namespace cv;
double alpha; /**< Simple contrast control */
int beta; /**< Simple brightness control */
int main( int argc, char** argv )
{
/// Read image given by user
Mat image = imread( argv[1] );
Mat new_image = Mat::zeros( image.size(), image.type() );
/// Initialize values
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;
/// Do the operation new_image(i,j) = alpha*image(i,j) + 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 );
}
}
}
/// Create Windows
namedWindow("Original Image", 1);
namedWindow("New Image", 1);
/// Show stuff
imshow("Original Image", image);
imshow("New Image", new_image);
/// Wait until user press some key
waitKey();
return 0;
}
Explanation
We begin by creating parameters to save
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and
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to be entered by the user:
We load an image using imread and save it in a Mat object:
Mat image = imread( argv[1] );
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 Mat::zeros returns a Matlab-style zero initializer based on image.size() and image.type()
Now, to perform the operation
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LaTeX2e <2014/05/01>
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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
System Message: WARNING/2 (\alpha \cdot p(i,j) + \beta)
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This is pdfTeX, Version 3.14159265-2.6-1.40.15 (TeX Live 2014-OpenBSD_Ports) (preloaded format=latex)
restricted \write18 enabled.
entering extended mode
(./math.tex
LaTeX2e <2014/05/01>
Babel <3.9k> and hyphenation patterns for 78 languages loaded.
(/usr/local/share/texmf-dist/tex/latex/base/article.cls
Document Class: article 2007/10/19 v1.4h Standard LaTeX document class
(/usr/local/share/texmf-dist/tex/latex/base/size12.clo))
(/usr/local/share/texmf-dist/tex/latex/base/inputenc.sty
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can give values out of range or not integers (if
System Message: WARNING/2 (\alpha)
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This is pdfTeX, Version 3.14159265-2.6-1.40.15 (TeX Live 2014-OpenBSD_Ports) (preloaded format=latex)
restricted \write18 enabled.
entering extended mode
(./math.tex
LaTeX2e <2014/05/01>
Babel <3.9k> and hyphenation patterns for 78 languages loaded.
(/usr/local/share/texmf-dist/tex/latex/base/article.cls
Document Class: article 2007/10/19 v1.4h Standard LaTeX document class
(/usr/local/share/texmf-dist/tex/latex/base/size12.clo))
(/usr/local/share/texmf-dist/tex/latex/base/inputenc.sty
! LaTeX Error: File `utf8x.def’ not found.
Type X to quit or <RETURN> to proceed,
or enter new name. (Default extension: def)
Enter file name:
! Emergency stop.
<read *>
l.165 \endinput
^^M
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Transcript written on math.log.
is float), we use saturate_cast to make sure the values are valid.
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 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
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