function r = radius_trui (xy) % Return grain radius at location (x,y)
persistent F
if isempty (F)                  % Read in the trui image once
    A = double (imread('trui.png','PNG')); A = flipud (A(:,:,1));
    rf = @(s) 0.002+0.006*s+0.012*s.^8 ; % Conversion of brightness to radius
    F = rf(A/255);
end
ixy = round (255*xy) ;          % Given a location (x,y) ,evaluate
r = F(ixy(:,2)+256*ixy(:,1)+1); % the corresponding grain radius