[c++] How to fill Matrix with zeros in OpenCV?

The code below causes an exception. Why?

#include <opencv2/core/core.hpp>
#include <iostream>

using namespace cv;
using namespace std;

void main() {

    try {
        Mat m1 = Mat(1,1, CV_64F, 0);
        m1.at<double>(0,0) = 0;
    }
    catch(cv::Exception &e) {
        cerr << e.what() << endl;
    }

}

Error is follows:

OpenCV Error: Assertion failed (dims <= 2 && data && (unsigned)i0 < (unsigned)size.p[0] && (unsigned)(i1*DataType<_Tp>::channels) < (unsigned)(size.p[1]*channels()) && ((((sizeof(size_t)<<28)|0x8442211) >> ((DataType<_Tp>::depth) & ((1 << 3
) - 1))*4) & 15) == elemSize1()) in unknown function, file %OPENCV_DIR%\build\include\opencv2\core\mat.hpp, line 537

UPDATE

If tracing this code, I see that constructor line calls the constructor

inline Mat::Mat(int _rows, int _cols, int _type, void* _data, size_t _step)

Why? This prototype has 5 parameters, while I am providing 4 arguments.

You can use this to fill zeroes in a Mat object already containing data:

image1 = Scalar::all(0);

For eg, if you use it this way:

Mat image,image1;
image = imread("IMG_20160107_185956.jpg", CV_LOAD_IMAGE_COLOR);   // Read the file

if(! image.data )                              // Check for invalid input
{
    cout <<  "Could not open or find the image" << std::endl ;
    return -1;
}
cvtColor(image,image1,CV_BGR2GRAY);
image1 = Scalar::all(0);

It will work fine. But you cannot use this for uninitialised Mat. For that you can go for other options mentioned in above answers, like

Mat drawing = Mat::zeros( image.size(), CV_8UC3 );

Mat img;

img=Mat::zeros(size of image,CV_8UC3);

if you want it to be of an image img1

img=Mat::zeros(img1.size,CV_8UC3);

If You are more into programming with templates, You may also do it this way...

template<typename _Tp>
... some algo ...
cv::Mat mat = cv::Mat_<_Tp>::zeros(rows, cols);
mat.at<_Tp>(i, j) = val;

How to fill Matrix with zeros in OpenCV?

To fill a pre-existing Mat object with zeros, you can use Mat::zeros()

Mat m1 = ...;
m1 = Mat::zeros(1, 1, CV_64F);

To intialize a Mat so that it contains only zeros, you can pass a scalar with value 0 to the constructor:

Mat m1 = Mat(1,1, CV_64F, 0.0);
//                        ^^^^double literal

The reason your version failed is that passing 0 as fourth argument matches the overload taking a void* better than the one taking a scalar.

use cv::mat::setto

img.setTo(cv::Scalar(redVal,greenVal,blueVal))

You can choose filling zero data or create zero Mat.

1. Filling zero data with setTo():

   img.setTo(Scalar::all(0));
   

2. Create zero data with zeros():

   img = zeros(img.size(), img.type());
   

The img changes address of memory.

I presume you are talking about filling zeros of some existing mat? How about this? :)

mat *= 0;