Slimming the shapes

We achieve this effect using an operation called erosion. This is the operation that makes a shape thinner by peeling the boundary layers of all the shapes in the image:

Let's look at the function that performs morphological erosion:

Mat performErosion(Mat inputImage, int erosionElement, int erosionSize)
{

    Mat outputImage;
    int erosionType;

    if(erosionElement == 0)
        erosionType = MORPH_RECT;
    else if(erosionElement == 1)
        erosionType = MORPH_CROSS;
    else if(erosionElement == 2)
        erosionType = MORPH_ELLIPSE;

    // Create the structuring element for erosion
    Mat element = getStructuringElement(erosionType, Size(2*erosionSize + 1, 2*erosionSize + 1), Point(erosionSize, erosionSize));

    // Erode the image using the structuring element
    erode(inputImage, outputImage, element);

    // Return the output image
    return outputImage...