Slimming the shapes

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

Let's take a 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;
}

You can check out the complete code in the .cpp files to understand...