--- /dev/null
+function [Bu, Bv, N] = nrb_srf_basisfun_der__ (points, nrb);
+
+% __NRB_SRF_BASISFUN_DER__: Undocumented internal function
+%
+% Copyright (C) 2009 Carlo de Falco
+% This software comes with ABSOLUTELY NO WARRANTY; see the file
+% COPYING for details. This is free software, and you are welcome
+% to distribute it under the conditions laid out in COPYING.
+
+ m = size (nrb.coefs, 2) -1;
+ n = size (nrb.coefs, 3) -1;
+
+ p = nrb.order(1) -1;
+ q = nrb.order(2) -1;
+
+ u = points(1,:);
+ v = points(2,:);
+ npt = length(u);
+
+ U = nrb.knots{1};
+ V = nrb.knots{2};
+
+ w = squeeze(nrb.coefs(4,:,:));
+
+ spu = findspan (m, p, u, U);
+ spv = findspan (n, q, v, V);
+ N = nrbnumbasisfun (points, nrb);
+
+ NuIkuk = basisfun (spu, u, p, U);
+ NvJkvk = basisfun (spv, v, q, V);
+
+ NuIkukprime = basisfunder (spu, p, u, U, 1);
+ NuIkukprime = reshape (NuIkukprime(:,2,:), npt, []);
+
+ NvJkvkprime = basisfunder (spv, q, v, V, 1);
+ NvJkvkprime = reshape (NvJkvkprime(:,2,:), npt, []);
+
+ for k=1:npt
+ wIkaJkb(1:p+1, 1:q+1) = reshape (w(N(k, :)), p+1, q+1);
+
+ Num = (NuIkuk(k, :).' * NvJkvk(k, :)) .* wIkaJkb;
+ Num_du = (NuIkukprime(k, :).' * NvJkvk(k, :)) .* wIkaJkb;
+ Num_dv = (NuIkuk(k, :).' * NvJkvkprime(k, :)) .* wIkaJkb;
+ Denom = sum(sum(Num));
+ Denom_du = sum(sum(Num_du));
+ Denom_dv = sum(sum(Num_dv));
+
+ Bu(k, :) = reshape((Num_du/Denom - Denom_du.*Num/Denom.^2),1,[]);
+ Bv(k, :) = reshape((Num_dv/Denom - Denom_dv.*Num/Denom.^2),1,[]);
+ end
+
+end
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