Size of Matrix OpenCV

Question

I know this might be very rudimentary, but I am new to OpenCV. Could you please tell me how to obtain the size of a matrix in OpenCV?. I googled and I am still searching, but if any of you know the answer, please help me.

Size as in number of rows and columns.

And is there a way to directly obtain the maximum value of a 2D matrix?

Michael O · Accepted Answer · 2021-04-20T00:43:06.773

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cv:Mat mat;
int rows = mat.rows;
int cols = mat.cols;

cv::Size s = mat.size();
rows = s.height;
cols = s.width;

edited Apr 20 '21 at 00:43

answered Dec 25 '12 at 06:29

Michael O

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Link to documentation: http://docs.opencv.org/modules/core/doc/basic_structures.html#mat-size – Rasim Dec 25 '12 at 06:50
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Are those two exact equivalents? – alx - recommends codidact Mar 09 '20 at 19:40
What's the difference between those? The note seems confusing, mentioning a stride for some reason. – hans Apr 13 '21 at 13:39
1

Yeah, someone edited the answer to add the weirdness about stride. I've rolled that back. – Michael O Apr 20 '21 at 00:43
Why does the documentation list `size` as a public attribute and not as a function like you have it? https://docs.opencv.org/4.x/d3/d63/classcv_1_1Mat.html#a146f8e8dda07d1365a575ab83d9828d1 – simplename Jan 07 '22 at 21:10

score 21 · Answer 2 · answered Feb 23 '14 at 07:19

21

Note that apart from rows and columns there is a number of channels and type. When it is clear what type is, the channels can act as an extra dimension as in CV_8UC3 so you would address a matrix as

uchar a = M.at<Vec3b>(y, x)[i];

So the size in terms of elements of elementary type is M.rows * M.cols * M.cn

To find the max element one can use

Mat src;
double minVal, maxVal;
minMaxLoc(src, &minVal, &maxVal);

answered Feb 23 '14 at 07:19

Vlad

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This is the only answer that also addresses finding the largest element in an OpenCV Mat. – rayryeng Nov 15 '17 at 19:04

score 15 · Answer 3 · answered Jun 16 '16 at 14:43

15

For 2D matrix:

mat.rows – Number of rows in a 2D array.

mat.cols – Number of columns in a 2D array.

Or: C++: Size Mat::size() const

The method returns a matrix size: Size(cols, rows) . When the matrix is more than 2-dimensional, the returned size is (-1, -1).

For multidimensional matrix, you need to use

int thisSizes[3] = {2, 3, 4};
cv::Mat mat3D(3, thisSizes, CV_32FC1);
// mat3D.size tells the size of the matrix 
// mat3D.size[0] = 2;
// mat3D.size[1] = 3;
// mat3D.size[2] = 4;

Note, here 2 for z axis, 3 for y axis, 4 for x axis. By x, y, z, it means the order of the dimensions. x index changes the fastest.

answered Jun 16 '16 at 14:43

user1914692

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just to be clear, there is no `Mat::size()` member method, rather a [`Mat::size`](https://docs.opencv.org/3.4/d3/d63/classcv_1_1Mat.html#a146f8e8dda07d1365a575ab83d9828d1) member variable of type [`MatSize`](https://docs.opencv.org/3.4/df/d63/structcv_1_1MatSize.html). The latter overloads the parentheses operator [`MatSize::operator()`](https://docs.opencv.org/3.4/df/d63/structcv_1_1MatSize.html#abdc35c3d1d636243a9fa4b7e3a4810fe) to return a [`Size`](https://docs.opencv.org/3.4/d6/d50/classcv_1_1Size__.html) object – Amro Feb 02 '19 at 20:59
As of today, I do use the `Mat::size()` function of opencv 4.7. So I believe this function added later after year 2019. – ollydbg23 Mar 21 '23 at 01:57

score 5 · Answer 4 · edited Nov 06 '19 at 20:13

5

If you are using the Python wrappers, then (assuming your matrix name is mat):

mat.shape gives you an array of the type- [height, width, channels]
mat.size gives you the size of the array

Sample Code:

import cv2
mat = cv2.imread('sample.png')
height, width, channel = mat.shape[:3]
size = mat.size

edited Nov 06 '19 at 20:13

Cody Gray - on strike

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answered Dec 15 '17 at 21:23

nimesh00

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score 4 · Answer 5 · edited Dec 17 '16 at 18:55

4

A complete C++ code example, may be helpful for the beginners

#include <iostream>
#include <string>
#include "opencv/highgui.h"

using namespace std;
using namespace cv;

int main()
{
    cv:Mat M(102,201,CV_8UC1);
    int rows = M.rows;
    int cols = M.cols;

    cout<<rows<<" "<<cols<<endl;

    cv::Size sz = M.size();
    rows = sz.height;
    cols = sz.width;

    cout<<rows<<" "<<cols<<endl;
    cout<<sz<<endl;
    return 0;
}

edited Dec 17 '16 at 18:55

Eldelshell

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answered Oct 20 '16 at 06:06

MD. Nazmul Kibria

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how do we get the depth of the matrix in cpp? – sai Jul 02 '20 at 12:59
.channels() command will give you depth – Saurav Solanki Sep 23 '21 at 11:25

Size of Matrix OpenCV

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