Drawing the boundary

We achieve this using a morphological gradient. This is the operation that draws the boundary around a shape by taking the difference between the dilation and erosion of an image:

Let's look at the function to perform morphological gradient:

Mat performMorphologicalGradient(Mat inputImage, int morphologyElement, int morphologySize)
{
    Mat outputImage, tempImage1, tempImage2;
    int morphologyType;

    if(morphologyElement == 0)
        morphologyType = MORPH_RECT;
    else if(morphologyElement == 1)
        morphologyType = MORPH_CROSS;
    else if(morphologyElement == 2)
        morphologyType = MORPH_ELLIPSE;

    // Create the structuring element for erosion
    Mat element = getStructuringElement(morphologyType, Size(2*morphologySize + 1, 2*morphologySize + 1), Point(morphologySize, morphologySize));
    
    // Apply morphological gradient to the image using the structuring element
    dilate(inputImage, tempImage1, element);
    erode(inputImage, tempImage2, element);

    // Return the output image
    return tempImage1 - tempImage2;
}