Simple way to doubleSize or shrink an Image by half

Excerpt from Andrea Vedaldi's code:

function J = doubleSize(I)
[M,N]=size(I) ;
J = zeros(2*M,2*N) ;
J(1:2:end,1:2:end) = I ;
J(2:2:end-1,2:2:end-1) = ...
0.25*I(1:end-1,1:end-1) + ...
0.25*I(2:end,1:end-1) + ...
0.25*I(1:end-1,2:end) + ...
0.25*I(2:end,2:end) ;
J(2:2:end-1,1:2:end) = ...
0.5*I(1:end-1,:) + ...
0.5*I(2:end,:) ;
J(1:2:end,2:2:end-1) = ...
0.5*I(:,1:end-1) + ...
0.5*I(:,2:end) ;

function J = halveSize(I)
J=I(1:2:end,1:2:end) ;