## -*- texinfo -*-
## @deftypefn {Function File} {} imshear (@var{M}, @var{axis}, @var{alpha}, @var{bbox})
## Applies a shear to @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
## 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