Opencv multiply scalar and matrix

Question

I have been trying to achieve something which should pretty trivial and is trivial in Matlab.

Using methods of OpenCV, I want to simply achieve something such as:

cv::Mat sample = [4 5 6; 4 2 5; 1 4 2];      
sample = 5*sample;

After which sample should just be:

[20 25 30; 20 10 25; 5 20 10]

I have tried scaleAdd, Mul, Multiply and neither allow a scalar multiplier and require a matrix of the same "size and type". In this scenario I could create a Matrix of Ones and then use the scale parameter but that seems so very extraneous

Any direct simple method would be great!

Congreve

Shouldn't 24 be 25 instead??? :P – Equalizer

What language is the code in? – Epicritic

Answer 1

OpenCV does in fact support multiplication by a scalar value with overloaded operator*. You might need to initialize the matrix correctly, though.

float data[] = {1 ,2, 3,
                4, 5, 6,
                7, 8, 9};
cv::Mat m(3, 3, CV_32FC1, data);
m = 3*m;  // This works just fine

If you are mainly interested in mathematical operations, cv::Matx is a little easier to work with:

cv::Matx33f mx(1,2,3,
               4,5,6,
               7,8,9);
mx *= 4;  // This works too

Claman

This function only works for single channeled images. This therefore does not work generally. You would have to split all the channels first and then recombine then, eg //(3) Split the RGB image vector<Mat> imgPlanes(3); split(matImage, imgPlanes); Mat matImageK = imgPlanes[0]; – Sheronsherourd

is there no / operator? – Annam

Answer 2

For java there is no operator overloading, but the Mat object provides the functionality with a convertTo method.

Mat dst= new Mat(src.rows(),src.cols(),src.type());
src.convertTo(dst,-1,scale,offset);

Doc on this method is here

Passant

Answer 3

For big Mats you should use forEach.

If C++11 is available:

m.forEach<double>([&](double& element, const int position[]) -> void
{
element *= 5;
}
);

Shelbashelbi

why should this be better than using the builtin matrix-scalar multiplication or cv::multiply? – Anesthesiologist

Answer 4

something like this.

Mat m = (Mat_<float>(3, 3)<<
                    1, 2, 3,
                    4, 5, 6,
                    7, 8, 9)*5;

Kingcup

Answer 5

Mat A = //data;//source Matrix
Mat B;//destination Matrix
Scalar alpha = new Scalar(5)//factor
Core.multiply(A,alpha,b);

Mayfield