www.pudn.com > level_set_methods_1.1.zip > der_ENO2_minus.m, change:2005-05-05,size:1250b

```function [data_x] = der_ENO2_minus(data, dx)
%
% Calculates the derivative (minus) using
% second order accurate ENO scheme
% takes 1-D data
% data: input data
% dx: grid resolution
% Note: before entering this function, data needs to be
% extended by 2 at the beginning and end (values don't matter)
%
% Author: Baris Sumengen  sumengen@ece.ucsb.edu
% http://vision.ece.ucsb.edu/~sumengen/
%

data_x = zeros(size(data));

% extrapolate the beginning and end points of data
data(2) = 2*data(3)-data(4);
data(1) = 2*data(2)-data(3);
data(end-1) = 2*data(end-2)-data(end-3);
data(end) = 2*data(end-1)-data(end-2);

%Generate the divided difference tables
%ignoring division by dx for efficiency
D1 = (data(2:end)-data(1:end-1));
% ignoring division by dx since this will cancel out
D2 = (D1(2:end)-D1(1:end-1))/2;
absD2 = abs(D2);

for i=1:(length(data)-4)
k = i-1;

Q1p = D1(k+2); %D1k_half;

if absD2(k+1) <= absD2(k+2) %|D2k| <= |D2kp1|
c = D2(k+1); %D2k;
else
c = D2(k+2); %D2kp1;
end

% ignoring multiplication by dx since this will also cancel out
Q2p = c*(2*(i-k)-1);

data_x(i+2) = Q1p+Q2p;
data_x(i+2) = data_x(i+2)/dx;
end

```