Adding (blending) two images using OpenCV¶

Goal¶

In this tutorial you will learn:

what is linear blending and why it is useful;
how to add two images using addWeighted

Theory¶

Note

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

From our previous tutorial, we know already a bit of Pixel operators. An interesting dyadic (two-input) operator is the linear blend operator:

$g(x) = (1 - \alpha)f_{0}(x) + \alpha f_{1}(x)$

By varying $\alpha$ from $0 \rightarrow 1$ this operator can be used to perform a temporal cross-disolve between two images or videos, as seen in slide shows and film productions (cool, eh?)

Code¶

As usual, after the not-so-lengthy explanation, let’s go to the code:

#include <opencv2/opencv.hpp>
#include <iostream>

using namespace cv;

int main( int argc, char** argv )
{
 double alpha = 0.5; double beta; double input;

 Mat src1, src2, dst;

 /// Ask the user enter alpha
 std::cout<<" Simple Linear Blender "<<std::endl;
 std::cout<<"-----------------------"<<std::endl;
 std::cout<<"* Enter alpha [0-1]: ";
 std::cin>>input;

 /// We use the alpha provided by the user if it is between 0 and 1
 if( input >= 0.0 && input <= 1.0 )
   { alpha = input; }

 /// Read image ( same size, same type )
 src1 = imread("../../images/LinuxLogo.jpg");
 src2 = imread("../../images/WindowsLogo.jpg");

 if( !src1.data ) { printf("Error loading src1 \n"); return -1; }
 if( !src2.data ) { printf("Error loading src2 \n"); return -1; }

 /// Create Windows
 namedWindow("Linear Blend", 1);

 beta = ( 1.0 - alpha );
 addWeighted( src1, alpha, src2, beta, 0.0, dst);

 imshow( "Linear Blend", dst );

 waitKey(0);
 return 0;
}

Explanation¶

Since we are going to perform:

$g(x) = (1 - \alpha)f_{0}(x) + \alpha f_{1}(x)$

We need two source images ( $f_{0}(x)$ and $f_{1}(x)$ ). So, we load them in the usual way:
```
src1 = imread("../../images/LinuxLogo.jpg");
src2 = imread("../../images/WindowsLogo.jpg");
```
Warning

Since we are adding src1 and src2, they both have to be of the same size (width and height) and type.
Now we need to generate the $g(x)$ image. For this, the function addWeighted comes quite handy:
```
beta = ( 1.0 - alpha );
addWeighted( src1, alpha, src2, beta, 0.0, dst);
```
since addWeighted produces:

$dst = \alpha \cdot src1 + \beta \cdot src2 + \gamma$

In this case, $\gamma$ is the argument $0.0$ in the code above.
Create windows, show the images and wait for the user to end the program.