what is the roll-off factor and cutoff-slope of Butterworth filter?

Hi all,

I am doing this in digital domain... I have used the Butterwork lowpass
filter grabbed from Internet:

% X and Y matrices with ranges normalised to +/- 0.5
x = (ones(rows,1) * [1:cols] - (fix(cols/2)+1))/cols;
y = ([1:rows]' * ones(1,cols) - (fix(rows/2)+1))/rows;

radius = sqrt(x.^2 + y.^2);

f = 1 ./ (1.0 + (radius ./ cutoff).^(2*n));


Then "f" is the frequency domain representation of the filter. n is the
order of the filter; cutoff is the normolized cutoff frequency between [0,
0.5]...

Now I want to compute the roll-off factora nd cutoff-slope of this n-order
Butterworth filter...

How do I get those? Can anybody give me some pointers?

Thanks a lot