A Shapes structure is a collection of shapes extracted from an image.
ncol are the dimensions of the image.
interpolation is the convention used to extract level
lines. Currently, the valid values are 0 (module
flst) and 1 (module
The elements are stored consecutively in the array
the_shapes of size
nb_shapes. By convention, the shape at index 0 is the root of the
smallest_shape is an array of size
ncol giving for each pixel the smallest shape in
the tree that contains it. By going upward in the tree, it is possible to know
all the shapes containing a given pixel.