% DERIVATIVE7 - 7-Tap 1st and 2nd discrete derivatives
%
% This function computes 1st and 2nd derivatives of an image using the 7-tap
% coefficients given by Farid and Simoncelli. The results are significantly
% more accurate than MATLAB's GRADIENT function on edges that are at angles
% other than vertical or horizontal. This in turn improves gradient orientation
% estimation enormously.
%
% Usage: [gx, gy, gxx, gyy, gxy] = derivative7(im, derivative specifiers)
%
% Arguments:
% im - Image to compute derivatives from.
% derivative specifiers - A comma separated list of character strings
% that can be any of 'x', 'y', 'xx', 'yy' or 'xy'
% These can be in any order, the order of the
% computed output arguments will match the order
% of the derivative specifier strings.
% Returns:
% Function returns requested derivatives which can be:
% gx, gy - 1st derivative in x and y
% gxx, gyy - 2nd derivative in x and y
% gxy - 1st derivative in y of 1st derivative in x
%
% Examples:
% Just compute 1st derivatives in x and y
% [gx, gy] = derivative7(im, 'x', 'y');
%
% Compute 2nd derivative in x, 1st derivative in y and 2nd derivative in y
% [gxx, gy, gyy] = derivative7(im, 'xx', 'y', 'yy')
%
% See also: DERIVATIVE5
% Reference: Hany Farid and Eero Simoncelli "Differentiation of Discrete
% Multi-Dimensional Signals" IEEE Trans. Image Processing. 13(4): 496-508 (2004)
% Copyright (c) 2010 Peter Kovesi
% www.peterkovesi.com
%
% Permission is hereby granted, free of charge, to any person obtaining a copy
% of this software and associated documentation files (the "Software"), to deal
% in the Software without restriction, subject to the following conditions:
%
% The above copyright notice and this permission notice shall be included in
% all copies or substantial portions of the Software.
%
% The Software is provided "as is", without warranty of any kind.
%
% April 2010
% May 2019 - Correction to calculation of dxy. Thanks to Sebastian Gesemann.
function varargout = derivative7(im, varargin)
varargin = varargin(:);
varargout = cell(size(varargin));
% 7-tap interpolant and 1st and 2nd derivative coefficients
p = [ 0.004711 0.069321 0.245410 0.361117 0.245410 0.069321 0.004711];
d1 = [ 0.018708 0.125376 0.193091 0.000000 -0.193091 -0.125376 -0.018708];
d2 = [ 0.055336 0.137778 -0.056554 -0.273118 -0.056554 0.137778 0.055336];
% Compute derivatives. Note that in the 1st call below MATLAB's conv2
% function performs a 1D convolution down the columns using p then a 1D
% convolution along the rows using d1. etc etc.
for n = 1:length(varargin)
if strcmpi('x', varargin{n})
varargout{n} = conv2(p, d1, im, 'same');
elseif strcmpi('y', varargin{n})
varargout{n} = conv2(d1, p, im, 'same');
elseif strcmpi('xx', varargin{n})
varargout{n} = conv2(p, d2, im, 'same');
elseif strcmpi('yy', varargin{n})
varargout{n} = conv2(d2, p, im, 'same');
elseif strcmpi('xy', varargin{n}) || strcmpi('yx', varargin{n})
varargout{n} = conv2(d1, d1, im, 'same');
else
error('''%s'' is an unrecognized derivative option',varargin{n});
end
end