This function applies the Laplacian, Sobel, or Robert Cross filter to
the input image img. The filter is applied to each color channel separately. ltype determines
the different matrices for Laplacian filter used in the
literature. ltype == 1 will use:
conv <- matrix(c(-1,-1,-1,-1,-1,
-1,-1,-1,-1,-1,
-1,-1,24,-1,-1,
-1,-1,-1,-1,-1,
-1,-1,-1,-1,-1),5,5)
ltype == 2 will use:
conv <- matrix(c(0,-1,0,-1,4,-1,0,-1,0), 3, 3)
ltype == 3 will use:
conv <- matrix(c(-1,-1,-1,-1,8,-1,-1,-1,-1), 3, 3)
ltype == 4 (default) will use:
conv <- matrix(c(1,-2,1,-2,4,-2,1,-2,1), 3, 3)