Vectorising circle drawing

[soren]

Hi All,
  I have a matrix 'cr' where each row contains the center and radius  
of a circle. Now, I want to draw these circles in a image. Currently,  
I do this by looping over each row, and then I draw the circle. This  
is however a bit slow, so I was wondering if I was missing some cool  
trick to vectorise this code:

  num_circles = rows (data);
  [XX, YY] = meshgrid (1:columns (I), 1:rows (I));
  for k = 1:num_circles
    C = (XX - cr (k, 2)).^2 + (YY - cr (k, 1)).^2 <= cr (k, 3).^2;
    mask (C) = 1;
  endfor

where 'I' is a matrix of the same size as the matrix 'mask'.

Any thoughts?

Søren