The Marr-Hildreth edge detection method operates by convolving the image with the Laplacian of Gaussian. Laplacian of Gaussian (LoG) is a second derivative of a Gaussian filter. After reading the book, the LoG can be broken up into two steps. First, smooth the image with a Gaussian filter, and second, convolve the image with a Laplacian mask. The Laplacian mask I used was taken from the book shown below. I used the 3x3 mask with a value of 8 in the middle surrounded by -1.