## Copyright (C) 2010 Andrew Kelly, IPS Radio & Space Services

##

## This program is free software; you can redistribute it and/or modify it under

## the terms of the GNU General Public License as published by the Free Software

## Foundation; either version 3 of the License, or (at your option) any later

## version.

##

## This program is distributed in the hope that it will be useful, but WITHOUT

## ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

## FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more

## details.

##

## You should have received a copy of the GNU General Public License along with

## this program; if not, see <http://www.gnu.org/licenses/>.

## -*- texinfo -*-

## @deftypefn {Function File} {@var{fcc} = } fchcode (@var{bound})

## Determine the Freeman chain code for a boundary.

##

## @code{fchcode} computes the Freeman chain code for the @var{n}-connected

## boundary @var{bound}. @var{n} must be either 8 or 4.

##

## @var{bound} is a K-by-2 matrix containing the row/column coordinates of points

## on the boundary. Optionally, the first point can be repeated as the last point,

## resulting in a (K+1)-by-2 matrix.

##

## @var{fcc} is a structure containing the following elements.

##

## @example

## x0y0 = Row/column coordinates where the code starts (1-by-2)

## fcc = Freeman chain code (1-by-K)

## diff = First difference of fcc (1-by-K)

## @end example

##

## The code uses the following directions.

##

## @example

## 3 2 1

## 4 . 0

## 5 6 7

## @end example

##

## @seealso{bwboundaries}

## @end deftypefn

function fcc = fchcode (bound)

# ensure the boundary start and end points are the same

if (!isempty (bound) && !isequal (bound (1, :), bound (end, :)))

bound = [bound; bound(1, :)];

endif

# number of boundary points

n = max (0, rows (bound)-1);

# structure in which to return results

fcc = struct (...

'x0y0', zeros (1, n), ...

'fcc', zeros (1, n), ...

'diff', zeros (1, n) ...

);

# an empty boundary?

if (isempty (bound))

return;

endif

# direction map

dir = [3, 2, 1; ...

4, NaN, 0; ...

5, 6, 7];

# coordinates

ROW = 1;

COL = 2;

# direction changes as row/column indexes into DIR

ch = 2 + diff (bound, 1, ROW);

# starting point

fcc.x0y0 = bound (1, :);

# chain code

fcc.fcc = dir (sub2ind (size (dir), ch (:, ROW), ch (:, COL)))';

# chain code difference

fcc.diff = mod (diff ([fcc.fcc, fcc.fcc(1)]), 8);

endfunction