vectorCross3d.m

function c = vectorCross3d(a,b)
%VECTORCROSS3D Vector cross product faster than inbuilt MATLAB cross.
%
%   C = vectorCross3d(A, B) 
%   returns the cross product of the 3D vectors A and B, that is: 
%       C = A x B
%   A and B must be N-by-3 element vectors. If either A or B is a 1-by-3
%   row vector, the result C will have the size of the other input and will
%   be the  concatenation of each row's cross product. 
%
%   Example
%     v1 = [2 0 0];
%     v2 = [0 3 0];
%     vectorCross3d(v1, v2)
%     ans =
%         0   0   6
%
%
%   Class support for inputs A,B:
%      float: double, single
%
%   See also DOT.

%   Sven Holcombe

% needed_colons = max([3, length(size(a)), length(size(b))]) - 3;
% tmp_colon = {':'};
% clnSet = tmp_colon(ones(1, needed_colons));
% 
% c = bsxfun(@times, a(:,[2 3 1],clnSet{:}), b(:,[3 1 2],clnSet{:})) - ...
%     bsxfun(@times, b(:,[2 3 1],clnSet{:}), a(:,[3 1 2],clnSet{:}));

sza = size(a);
szb = size(b);

% Initialise c to the size of a or b, whichever has more dimensions. If
% they have the same dimensions, initialise to the larger of the two
switch sign(numel(sza) - numel(szb))
    case 1
        c = zeros(sza);
    case -1
        c = zeros(szb);
    otherwise
        c = zeros(max(sza, szb));
end

c(:) =  bsxfun(@times, a(:,[2 3 1],:), b(:,[3 1 2],:)) - ...
        bsxfun(@times, b(:,[2 3 1],:), a(:,[3 1 2],:));