We use an operation called dilation to achieve thickening. This is the operation that makes a shape thicker by adding boundary layers to all the shapes in the image:

Here is the code to do it:

Mat performDilation(Mat inputImage, int dilationElement, int dilationSize)
{
    Mat outputImage;
    int dilationType;

    if(dilationElement == 0)
        dilationType = MORPH_RECT;
    else if(dilationElement == 1)
        dilationType = MORPH_CROSS;
    else if(dilationElement == 2)
        dilationType = MORPH_ELLIPSE;

    // Create the structuring element for dilation
    Mat element = getStructuringElement(dilationType, Size(2*dilationSize + 1, 2*dilationSize + 1), Point(dilationSize, dilationSize));

    // Dilate the image using the structuring element
    dilate(inputImage, outputImage, element);

    // Return the output image
    return outputImage;
}