Add a useful package (from Source forge) for octave

[CreaPhase.git] / octave_packages / image-1.0.15 / imshear.m

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+## Copyright (C) 2002 Jeff Orchard
+##
+## 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 2
+## 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} {} imshear (@var{M}, @var{axis}, @var{alpha}, @var{bbox})
+## Applies a shear to the image @var{M}.
+##
+## The argument @var{M} is either a matrix or an RGB image.
+##
+## @var{axis} is the axis along which the shear is to be applied, and can
+## be either 'x' or 'y'.
+## For example, to shear sideways is to shear along the 'x' axis. Choosing
+## 'y' causes an up/down shearing.
+##
+## @var{alpha} is the slope of the shear. For an 'x' shear, it is the 
+## horizontal shift (in pixels) applied to the pixel above the 
+## center. For a 'y' shear, it is the vertical shift (in pixels)
+## applied to the pixel just to the right of the center pixel.
+##
+## NOTE: @var{alpha} does NOT need to be an integer.
+##
+## @var{bbox} can be one of 'loose', 'crop' or 'wrap'.
+## 'loose' allows the image to grow to accomodate the new transformed image.
+## 'crop' keeps the same size as the original, clipping any part of the image
+## that is moved outside the bounding box.
+## 'wrap' keeps the same size as the original, but does not clip the part 
+## of the image that is outside the bounding box. Instead, it wraps it back 
+## into the image.
+##
+## If called with only 3 arguments, @var{bbox} is set to 'loose' by default.
+## @end deftypefn
+
+## Author: Jeff Orchard <jjo@sfu.ca>
+## Created: June 2002
+
+function g = imshear(m, axis, alpha, bbox, noshift)
+
+       # The code below only does y-shearing. This is because of
+       # the implementation of fft (operates on columns, but not rows).
+       # So, transpose first for x-shearing.
+       if ( strcmp(axis, "x")==1 )
+               m = m';
+       endif
+
+       if ( nargin < 4 )
+               bbox = "loose";
+               noshift = 0;
+       elseif ( nargin < 5 )
+               noshift = 0;
+       endif
+
+       [ydim_orig xdim_orig] = size(m);
+       if ( strcmp(bbox, "wrap") == 0 )
+               ypad = ceil( (xdim_orig+1)/2 * abs(alpha) );
+               m = impad(m, [0,0], [ypad,ypad]);
+       endif
+
+       [ydim_new xdim_new] = size(m);
+       xcentre = ( xdim_new + 1 ) / 2;
+       ycentre = ( ydim_new + 1 ) / 2;
+
+       # This applies FFT to columns of m (x-axis remains a spatial axis).
+       # Because the way that fft and fftshift are implemented, the origin
+       # will move by 1/2 pixel, depending on the polarity of the image
+       # dimensions.
+       #
+       # If dim is even (=2n), then the origin of the fft below is located
+       # at the centre of pixel (n+1). ie. if dim=16, then centre is at 9.
+       #
+       # If dim is odd (=2n+1), then the origin of the fft below is located
+       # at the centre of pixel (n). ie. if dim=15, then centre is at 8.
+       if ( noshift==1 )
+               M = fft(m);
+       else
+               #M = imtranslate(fft(imtranslate(m, -xcentre, ycentre, "wrap")), xcentre, -ycentre, "wrap");
+               M = fftshift(fft(fftshift(m)));
+       endif
+
+       [ydim xdim] = size(m);
+       x = zeros(ydim, xdim);
+
+       # Find coords of the origin of the image.
+       if ( noshift==1 )
+               xc_coord = 1;
+               yc_coord = 1;
+               l = (1:ydim)' - yc_coord;
+               r = (1:xdim) - xc_coord;
+               if ( strcmp(bbox, "wrap")==1 )
+                       l((ydim/2):ydim) = l((ydim/2):ydim) - ydim;
+                       r((xdim/2):xdim) = r((xdim/2):xdim) - xdim;
+               endif
+       else
+               xc_coord = (xdim+1)/2;
+               yc_coord = (ydim+1)/2;
+               l = (1:ydim)' - yc_coord;
+               r = (1:xdim) - xc_coord;
+       endif
+       x = l * r;
+
+       Ms = M.* exp(2*pi*I*alpha/ydim * x);
+
+       if ( noshift==1 )
+               g = abs(ifft(Ms));
+       else
+               #g = abs(imtranslate( ifft( imtranslate(Ms, -xcentre, ycentre, "wrap") ), xcentre, -ycentre, "wrap"));
+               g = abs( fftshift(ifft(ifftshift(Ms))) );
+       endif
+
+       if ( strcmp(bbox, "crop")==1 )
+               g = g(ypad+1:ydim_orig+ypad, :);
+       endif
+
+       # Un-transpose if x-shearing was wanted
+       if ( strcmp(axis, "x")==1 )
+               g = g';
+       endif
+endfunction