c++ and opencv get and set pixel color to Mat

Question

I'm trying to set a new color value to some pixel into a cv::Mat image my code is below:

    Mat image = img;
    for(int y=0;y<img.rows;y++)
    {
        for(int x=0;x<img.cols;x++)
        {
        Vec3b color = image.at<Vec3b>(Point(x,y));
        if(color[0] > 150 && color[1] > 150 && color[2] > 150)
        {
            color[0] = 0;
            color[1] = 0;
            color[2] = 0;
            cout << "Pixel >200 :" << x << "," << y << endl;
        }
        else
        {
            color.val[0] = 255;
            color.val[1] = 255;
            color.val[2] = 255;
        }
    }
    imwrite("../images/imgopti"+to_string(i)+".tiff",image);

It seems to get the good pixel in output (with cout) however in the output image (with imwrite) the pixel concerned aren't modified. I have already tried using color.val[0].. I still can't figure out why the pixel colors in the output image dont change. thanks

score 89 · Accepted Answer · edited Jan 13 '20 at 10:47

You did everything except copying the new pixel value back to the image.

This line takes a copy of the pixel into a local variable:

Vec3b color = image.at<Vec3b>(Point(x,y));

So, after changing color as you require, just set it back like this:

image.at<Vec3b>(Point(x,y)) = color;

So, in full, something like this:

Mat image = img;
for(int y=0;y<img.rows;y++)
{
    for(int x=0;x<img.cols;x++)
    {
        // get pixel
        Vec3b & color = image.at<Vec3b>(y,x);

        // ... do something to the color ....
        color[0] = 13;
        color[1] = 13;
        color[2] = 13;

        // set pixel
        //image.at<Vec3b>(Point(x,y)) = color;
        //if you copy value
    }
}

score 21 · Answer 2 · answered Apr 11 '14 at 07:13

21

just use a reference:

Vec3b & color = image.at<Vec3b>(y,x);
color[2] = 13;

answered Apr 11 '14 at 07:13

berak

39,159
9
91
89

2

because the problem was about a color Mat. in the uchar case, ofc. use `uchar& c = image.at(y,x)` – berak Aug 02 '14 at 14:54

score 17 · Answer 3 · edited Aug 17 '21 at 19:11

17

I would not use .at for performance reasons.

Define a struct:

//#pragma pack(push, 2) //not useful (see comments below)
struct BGR {
    uchar blue;
    uchar green;
    uchar red;  };

And then use it like this on your cv::Mat image:

BGR& bgr = image.ptr<BGR>(y)[x];

image.ptr(y) gives you a pointer to the scanline y. And iterate through the pixels with loops of x and y

edited Aug 17 '21 at 19:11

seht97

67
6

answered Apr 11 '14 at 13:59

Flocke

1,004
17
23

What is the sense of setting `pack` value to 2 if `sizeof(uchar)` is 1? – Antony Hatchkins Aug 02 '14 at 16:01
1

The compiler will add padding Bytes to a struct. The setting of pack changes those paddings so that the data is aligned in another way.Edit: See this post http://stackoverflow.com/questions/3318410/pragma-pack-effect – Flocke Aug 03 '14 at 07:34
3

`pragma` has no effect on a struct made solely from `uchar`s. Read a nice explanation [here](http://publib.boulder.ibm.com/infocenter/compbgpl/v9v111/index.jsp?topic=/com.ibm.xlcpp9.bg.doc/compiler_ref/pragma_pack.htm) for details. – Antony Hatchkins Aug 03 '14 at 15:16
Thx good link. Seem the pragma really is useless in this case :) thx! – Flocke Aug 03 '14 at 16:57
You're welcome :) And nice answer: less magic, more efficient. – Antony Hatchkins Aug 03 '14 at 17:50
You actually want to call this struct BGR, or similiar, since by default OpenCV stores the bands in BGR format. – trieck Sep 13 '20 at 21:49
Yes, the struct-name is in the wrong order. No biggy in my opinion though... but yes, you are correct :) – Flocke Sep 14 '20 at 09:46

c++ and opencv get and set pixel color to Mat

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