How to simply get RGB values in OpenCV? - c++

Question

I would like to obtain the RGB values for any pixel I choose or loop. This is currently how I achieve it.
Vec3b color = img.at<Vec3b>(Point(i, j));
and for the loop
for (int i = 0; i < width; i++) {
for (int j = 0; j < height; j++) {
Vec3b color = img.at<Vec3b>(Point(i,j));
img.at<Vec3b>(Point(i, j)) = color;
}
}
But when I apply the Canny/cvtColor function it all messes up. An Unhandled exception at memory location pops up.
I ran more tests, and found that the loop just works fine with the height, but as for the width it works only up (1/3) of the actual width. Most likely to do with Vec3b.
One such solution suggested
unsigned char color = img.at<unsigned char>(Point(i,j));
or
Vec<uchar, 3> color = img2.at <uchar>(Point(i,j));
But in uchar cases, how can i obtain individual RGB from color and how to set color back to pixel?
FULL CODE :
int main() {
VideoCapture cap("Assets/test2.mp4");
if (!cap.isOpened()) {
std::cout << "Problem in reading" << std::endl;
return -1;
}
while (1) {
Mat frame;
cap >> frame;
if (frame.empty()) {
break;
}
frame = Imgfn(frame);
imshow("FRAME", frame);
char c = (char)waitKey(100);
if (c == 27) {
break;
}
}
cap.release();
destroyAllWindows();
return 0;
}
Mat Imgfn(Mat img) {
int width = img.size().width , height = img.size().height;
cvtColor(img, img, COLOR_BGR2GRAY);
GaussianBlur(img, img, Size(3,3),0,0);
Canny(img, img, 50, 150);
for (int i = 0; i < width; i++) {
for (int j = 0; j < height; j++) {
Vec3b color = img.at<Vec3b>(Point(i, j));
color[0] = 0; color[1] = 255; color[2] = 0;
img.at<Vec3b>(Point(i, j)) = color;
// so **Obtain, select, set** color
}
}
return img;
}
[About the mp4][1]
[1]: https://i.stack.imgur.com/9hOBV.png
EDIT - FOUND THE SOLUTION - from one of the answers suggested below. The problem is when cvtColor convert to gray is performed it brings down to just 1 channel from 3. Mat.channels() can be used to check. so a simple uchar would work fine. if 3 channels of RGB are required then one could simply created their own RGB to GRAY function

Answer 1

Currently, how do you choose the type such as { Vec3b, unsigned char, ... } ?
If you access pixels in a known Mat , you know the type at the time and you can use that. Just do it so.
Otherwise, if you are talking about a situation in which you don't know the type of pixels in Mat, you can use Mat::depth(), Mat::channels(), etc to determine the type.