opencv - image multiplication

Question

hi, i\'m trying to play a little bit with Mat class. I want to do a product element wise between two images, the c++/opencv port of MATLAB immultiply.

This is my cod

Answer 1

Thanks to Wajih comment i have done some basic test, and some basic debug, and i got i work perfectly. I think this could become a mini tutorial on alpha blending and image multiply, but for now is only a few lines of commented code.

note that the 2 images must be of the same size.. and for sure some error checking should be done for a solid code..

Hope it helps someone! And, of course, if you have some hints to make this code more readable or more compact (one-liner guys are very appreciate!) or efficient.. just comment, thank you a lot!

#include 
#include 
#include "opencv2/imgproc/imgproc.hpp"

#include 

using namespace cv;
using namespace std;

void printMinMax(Mat m, string name) {
    double minVal; 
    double maxVal; 
    Point minLoc; 
    Point maxLoc;

    if(m.channels() >1) {
        cout << "ERROR: matrix "<(3,3) << 0,1,2,3,4,5,6,7,255);
    cout << "test Mat 3x3" << endl << imgA << endl;

    // not that if a value exceed 255 it is truncated at value%256 
    imgA = (Mat_(3,3) << 0,1, 258 ,3,4,5,6,7,255);
    cout << "test Mat 3x3 with last element truncated to 258%256=2" << endl << imgA << endl;

    // create a second matrix
    imgB = Mat(3, 3, CV_8UC1);
    imgB = (Mat_(3,3) << 0,1,2,3,4,5,6,7,8);

    // now the matrix product. we are multiplying a value that can goes from 0-255 with another 0-255 value..
    // the edge cases are "min * min" and "max * max", 
    // that means: our product is a function that return a value in the domain 0*0-255*255 ; 0-65025
    // ah, ah! this number exceed the Mat U8C1 domain!, we need different data types. 
    // we need a bigger one.. let's say 32FC1 

    Mat imgA_32FC1 = imgA.clone();
    imgA_32FC1.convertTo(imgA_32FC1, CV_32FC1);
    Mat imgB_32FC1 = imgB.clone();
    imgB_32FC1.convertTo(imgB_32FC1, CV_32FC1);

    // after conversion.. value are scaled?
    cout << "imgA after conversion:" << endl << imgA_32FC1 << endl;
    cout << "imgB after conversion:" << endl << imgB_32FC1 << endl;

    product = imgA_32FC1.mul( imgB_32FC1 );
    // note: the product values are in the range 0-65025
    cout << "the product:" << endl << product << endl;

    // now, this does not have much sense, because we started from a 0-255 range Mat and now we have a 0-65025 that is nothing..
    // it is not uchar range and it is not float range (that is a lot bigger than that)
    // so, we can normalize back to 0-255
    // what do i mean with 'normalize' now?
    // i mean: scale all values for a constant that maps 0 to 0 and 65025 to 255..
    product.convertTo(product, CV_32FC1, 1.0f/65025.0f * 255);
    // but it is still a 32FC1.. not as the start matix..
    cout << "the product, normalized back to 0-255, still in 32FC1:" << endl << product << endl;
    product.convertTo(product, CV_8UC1);
    cout << "the product, normalized back to 0-255, now int 8UC1:" << endl << product << endl;

    cout << "-----------------------------------------------------------" << endl;

    // real stuffs now.
    imgA = imread("test1.jpg"); 
    cvtColor(imgA, imgA, CV_BGR2GRAY);

    imgB = imread("test2.jpg"); 
    cvtColor(imgB, imgB, CV_BGR2GRAY);

    imgA_32FC1 = imgA.clone();
    imgA_32FC1.convertTo(imgA_32FC1, CV_32FC1);
    imgB_32FC1 = imgB.clone();
    imgB_32FC1.convertTo(imgB_32FC1, CV_32FC1);

    product = imgA_32FC1.mul( imgB_32FC1 );
    printMinMax(product, "product");
    product.convertTo(product, CV_32FC1, 1.0f/65025.0f * 255);
    product.convertTo(product, CV_8UC1);

    // concat two images in one big image
    imgAB = Mat( max(imgA.rows,imgB.rows), imgA.cols+imgB.cols, imgA.type());
    imgA.copyTo(imgAB(Rect(0, 0, imgA.cols, imgA.rows)));
    imgB.copyTo(imgAB(Rect(imgA.cols, 0, imgB.cols, imgB.rows)));

    namedWindow("originals", CV_WINDOW_AUTOSIZE);
    namedWindow("product", CV_WINDOW_AUTOSIZE);

    while( true )
    {
        char c = (char)waitKey(10);

        if( c == 27 )
            { break; }

        imshow( "originals", imgAB );
        imshow( "product", product );
    }

    return 0;
}