quadrant.m

function outp = quadrant(inp, whichQuadrant)
% extract one quadrant of a square matrix

% % test
% qs = {'top', 'bottom', 'left', 'right'};
% for q = 1:length(qs),
%     subplot(2,2,q); surf(quadrant(randn(100, 100), qs{q})); axis square;
%     title(qs{q});
% end

if ~exist('inp', 'var'); inp = randn(100, 100); end
if ~exist('whichQuadrant', 'var'); whichQuadrant = 'north'; end

% make axes
x = linspace(-1, 1, size(inp, 1));
y = linspace(-1, 1, size(inp, 2));

% make a grid
[xgr, ygr]  = meshgrid(x,y);
[th, ~]     = cart2pol(xgr, ygr);
th          = wrapTo360(rad2deg(th));

% select one quadrant
switch whichQuadrant
    case 'north'
        idx = (th > 225 & th <= 315);
    case 'south'
        idx = (th > 45 & th <= 135);
    case 'west'
        idx = (th > 135 & th <= 225);
    case 'east'
        idx = (th > 315 | th <= 45);
end

% now output the original matrix with only this quadrant *not* set to zero
outp                = inp;
outp((idx == 0))    = 0; % use idx as a mask
assert(all(size(outp) == size(inp)), 'something went horribly wrong');

end