OpenCV Access pixel value

Accessing pixel intensity values

In order to get pixel intensity value, you have to know the type of an image and the number of channels. Here is an example for a single channel grey scale image (type 8UC1) and pixel coordinates x and y:

Scalar intensity = img.at<uchar>(y, x);

intensity.val[0] contains a value from 0 to 255. Note the ordering of x and y. Since in OpenCV images are represented by the same structure as matrices, we use the same convention for both cases - the 0-based row index (or y-coordinate) goes first and the 0-based column index (or x-coordinate) follows it. Alternatively, you can use the following notation:

Scalar intensity = img.at<uchar>(Point(x, y));

Now let us consider a 3 channel image with BGR color ordering (the default format returned by imread):

Vec3b intensity = img.at<Vec3b>(y, x);uchar blue = intensity.val[0];uchar green = intensity.val[1];uchar red = intensity.val[2];

You can use the same method for floating-point images (for example, you can get such an image by running Sobel on a 3 channel image):

Vec3f intensity = img.at<Vec3f>(y, x);float blue = intensity.val[0];float green = intensity.val[1];float red = intensity.val[2];

The same method can be used to change pixel intensities:

img.at<uchar>(y, x) = 128;

There are functions in OpenCV, especially from calib3d module, such as projectPoints, that take an array of 2D or 3D points in the form of Mat. Matrix should contain exactly one column, each row corresponds to a point, matrix type should be 32FC2 or 32FC3 correspondingly. Such a matrix can be easily constructed from std::vector:

vector<Point2f> points;//... fill the arrayMat pointsMat = Mat(points);

One can access a point in this matrix using the same method Mat::at :

Point2f point = pointsMat.at<Point2f>(i, 0);