Adding (blending) two images using OpenCV

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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-dissolve between two images or videos, as seen in slide shows and film productions (cool, eh?)

Source Code

C++

Download the source code from here.

#include "opencv2/imgcodecs.hpp"
#include "opencv2/highgui.hpp"
#include <iostream>

using namespace cv;

// we're NOT "using namespace std;" here, to avoid collisions between the beta variable and std::beta in c++17
using std::cin;
using std::cout;
using std::endl;

int main( void )
{
   double alpha = 0.5; double beta; double input;

   Mat src1, src2, dst;

   cout << " Simple Linear Blender " << endl;
   cout << "-----------------------" << endl;
   cout << "* Enter alpha [0.0-1.0]: ";
   cin >> input;

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

   src1 = imread( "../data/LinuxLogo.jpg" );
   src2 = imread( "../data/WindowsLogo.jpg" );

   if( src1.empty() ) { cout << "Error loading src1" << endl; return -1; }
   if( src2.empty() ) { cout << "Error loading src2" << endl; return -1; }

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

   imshow( "Linear Blend", dst );
   waitKey(0);

   return 0;
}

Java

Download the source code from here.

import org.opencv.core.*;
import org.opencv.highgui.HighGui;
import org.opencv.imgcodecs.Imgcodecs;

import java.util.Locale;
import java.util.Scanner;

class AddingImagesRun{
    public void run() {
        double alpha = 0.5; double beta; double input;

        Mat src1, src2, dst = new Mat();

        System.out.println(" Simple Linear Blender ");
        System.out.println("-----------------------");
        System.out.println("* Enter alpha [0.0-1.0]: ");
        Scanner scan = new Scanner( System.in ).useLocale(Locale.US);
        input = scan.nextDouble();

        if( input >= 0.0 && input <= 1.0 )
            alpha = input;

        src1 = Imgcodecs.imread("../../images/LinuxLogo.jpg");
        src2 = Imgcodecs.imread("../../images/WindowsLogo.jpg");

        if( src1.empty() == true ){ System.out.println("Error loading src1"); return;}
        if( src2.empty() == true ){ System.out.println("Error loading src2"); return;}

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

        HighGui.imshow("Linear Blend", dst);
        HighGui.waitKey(0);

        System.exit(0);
    }
}

public class AddingImages {
    public static void main(String[] args) {
        // Load the native library.
        System.loadLibrary(Core.NATIVE_LIBRARY_NAME);
        new AddingImagesRun().run();
    }
}

Python

Download the source code from here.

from __future__ import print_function

import cv2 as cv

alpha = 0.5

try:
    raw_input          # Python 2
except NameError:
    raw_input = input  # Python 3

print(''' Simple Linear Blender
-----------------------
* Enter alpha [0.0-1.0]: ''')
input_alpha = float(raw_input().strip())
if 0 <= alpha <= 1:
    alpha = input_alpha
# [load]
src1 = cv.imread(cv.samples.findFile('LinuxLogo.jpg'))
src2 = cv.imread(cv.samples.findFile('WindowsLogo.jpg'))
# [load]
if src1 is None:
    print("Error loading src1")
    exit(-1)
elif src2 is None:
    print("Error loading src2")
    exit(-1)
# [blend_images]
beta = (1.0 - alpha)
dst = cv.addWeighted(src1, alpha, src2, beta, 0.0)
# [blend_images]
# [display]
cv.imshow('dst', dst)
cv.waitKey(0)
# [display]
cv.destroyAllWindows()

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:

C++

   src1 = imread( "../data/LinuxLogo.jpg" );
   src2 = imread( "../data/WindowsLogo.jpg" );

Java

        src1 = Imgcodecs.imread("../../images/LinuxLogo.jpg");
        src2 = Imgcodecs.imread("../../images/WindowsLogo.jpg");

Python

src1 = cv.imread(cv.samples.findFile('LinuxLogo.jpg'))
src2 = cv.imread(cv.samples.findFile('WindowsLogo.jpg'))

We used the following images: LinuxLogo.jpg and 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:

C++

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

Java

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

Python

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

Numpy version of above line (but cv function is around 2x faster):

dst = np.uint8(alpha*(img1)+beta*(img2))

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.

C++

   imshow( "Linear Blend", dst );
   waitKey(0);

Java

        HighGui.imshow("Linear Blend", dst);
        HighGui.waitKey(0);

Python

cv.imshow('dst', dst)
cv.waitKey(0)

Result