## 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

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