OpenCV rgb value for cv::Point in cv::Mat

Question

I've looked at different questions already here on StackOverflow, but none seems to helps. What I want to do is quite simple: I have a cv::Point and I need to get the RGB value for the pixel at that point in a cv::Mat so that I can compare it with a stored RGB value.

Now this should be very easy but I've tried 1001 different ways and it just doesnt work for me.

Someone please help me out of my misery!!

edit: both answers below work! I'm kinda new with C++ and didn't know that outputting a unsigned char through cout gives a questionmark! printf offcourse gives the right value!!

Answer 1

That's really easy. However the documentation of OpenCV is good at hiding the simple answers.

Here is example code:

cv::Mat3b image = imread(filename);
cv::Point point(23, 42);
cv::Vec3b template;
template[0] = 128; template[1] = 12; template[2] = 64;

const cv::Vec3b& bgr = image(point);
if (bgr[0] == template[0] && bgr[1] == template[1] && bgr[2] == template[2])
   std::cout << "Colors match!" << std::endl;

There are probable better ways of dealing with the cv::Vec, but I forgot. See also the OpenCV Cheat Sheet .

Answer 2

It depends on the type of the image/Mat. If the image is just a normal 1 byte for each rgb per pixel, then this is one way to access it. It's pretty efficient, but not the safest way:

// The RGB values are stored in reverse order (i don't know why)
struct RGB { unsigned char b, g, r; };

// Assumes 1 byte for r,g,b
RGB& GetRGB(cv::Mat &mat, cv::Point p)
{
  assert((mat.step/mat.cols) == sizeof(RGB));
  RGB *data = (RGB*)mat.data;
  data += p.y * mat.cols + p.x;
  return *data;
}

If the Mat is of different type, then you will just need to change the structure of RGB to match what you need.

Try changing the type of the image you read in to a normal RGB image with this code:

Mat in, out;
cvtColor(in, out, CV_8UC3);