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

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

This question is related to c++ opencv image-processing pixel mat

The answer is


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
    }
}

I would not use .at for performance reasons.

Define a struct:

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

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

RGB& rgb = image.ptr<RGB>(y)[x];

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


just use a reference:

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

Examples related to c++

Method Call Chaining; returning a pointer vs a reference? How can I tell if an algorithm is efficient? Difference between opening a file in binary vs text How can compare-and-swap be used for a wait-free mutual exclusion for any shared data structure? Install Qt on Ubuntu #include errors detected in vscode Cannot open include file: 'stdio.h' - Visual Studio Community 2017 - C++ Error How to fix the error "Windows SDK version 8.1" was not found? Visual Studio 2017 errors on standard headers How do I check if a Key is pressed on C++

Examples related to opencv

Real time face detection OpenCV, Python OpenCV TypeError: Expected cv::UMat for argument 'src' - What is this? OpenCV !_src.empty() in function 'cvtColor' error ConvergenceWarning: Liblinear failed to converge, increase the number of iterations How do I install opencv using pip? Access IP Camera in Python OpenCV ImportError: libSM.so.6: cannot open shared object file: No such file or directory Convert np.array of type float64 to type uint8 scaling values How to import cv2 in python3? cmake error 'the source does not appear to contain CMakeLists.txt'

Examples related to image-processing

Convert np.array of type float64 to type uint8 scaling values dlib installation on Windows 10 OpenCV - Saving images to a particular folder of choice How do I increase the contrast of an image in Python OpenCV OpenCV & Python - Image too big to display TypeError: Image data can not convert to float Extracting text OpenCV c++ and opencv get and set pixel color to Mat cv2.imshow command doesn't work properly in opencv-python How does one convert a grayscale image to RGB in OpenCV (Python)?

Examples related to pixel

c++ and opencv get and set pixel color to Mat Calculating width from percent to pixel then minus by pixel in LESS CSS Count all values in a matrix greater than a value Get pixel's RGB using PIL What's the best way to set a single pixel in an HTML5 canvas? Converting pixels to dp Setting width/height as percentage minus pixels How to get screen dimensions as pixels in Android Convert Pixels to Points How to read the RGB value of a given pixel in Python?

Examples related to mat

Pandas: Creating DataFrame from Series c++ and opencv get and set pixel color to Mat