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!

Shouldn't `24` be `25` instead??? :P – herohuyongtao Mar 19 '14 at 18:13 — herohuyongtao, Mar 19 '14 at 18:13

score 23 · Answer 1 · answered Jul 27 '13 at 04:04

23

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

answered Jul 27 '13 at 04:04

Aurelius

11,111
3
52
69

7

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 imgPlanes(3); split(matImage, imgPlanes); Mat matImageK = imgPlanes[0]; – TimZaman Oct 16 '13 at 10:26
is there no `/` operator? – simplename Jan 06 '22 at 05:24

score 4 · Answer 2 · answered Dec 16 '17 at 21:05

4

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

answered Dec 16 '17 at 21:05

J.E.Tkaczyk

557
1
8
19

score 3 · Answer 3 · answered Sep 24 '18 at 08:31

3

For big Mats you should use forEach.

If C++11 is available:

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

answered Sep 24 '18 at 08:31

Black Arrow

375
3
12

4

why should this be better than using the builtin matrix-scalar multiplication or cv::multiply? – Jack White Feb 14 '20 at 11:06

score 1 · Answer 4 · answered Jul 27 '13 at 08:45

1

something like this.

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

answered Jul 27 '13 at 08:45

jumpdiffusion

704
1
7
22

score 1 · Answer 5 · edited Jun 04 '17 at 01:07

1

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

edited Jun 04 '17 at 01:07

HDJEMAI

9,436
46
67
93

answered Jun 03 '17 at 23:52

Syed Safdar Ali

41
5

Opencv multiply scalar and matrix

5 Answers5

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