function  y = idct(c)
% Inverse DCT.   Columnwise if c is a matrix

% Version 11.12.2003.  INCBOX

[m n] = size(c);  alpha = sqrt(2/m);
x = ([1:m]' - 0.5)*(pi/m);       % grid as column vector
y = repmat(c(1,:)/sqrt(m),m,1);  % initialize with constant term
for  k = 2 : m
  if  any(c(k,:) ~= 0)
    y = y + cos((k-1)*x) * (alpha*c(k,:));
  end
end