/usr/share/octave/packages/nurbs-1.3.13/surfderivcpts.m is in octave-nurbs 1.3.13-4.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 | function pkl = surfderivcpts (n, p, U, m, q, V, P, d, r1, r2, s1, s2)
%
% SURFDERIVCPTS: Compute control points of n-th derivatives of a NURBS surface.
%
% usage: pkl = surfderivcpts (n, p, U, m, q, V, P, d)
%
% INPUT:
%
% n+1, m+1 = number of control points
% p, q = spline order
% U, V = knots
% P = control points
% d = derivative order
%
% OUTPUT:
%
% pkl (k+1, l+1, i+1, j+1) = i,jth control point
% of the surface differentiated k
% times in the u direction and l
% times in the v direction
%
% Adaptation of algorithm A3.7 from the NURBS book, pg114
%
% Copyright (C) 2009 Carlo de Falco
%
% This program is free software: you can redistribute it and/or modify
% it under the terms of the GNU General Public License as published by
% the Free Software Foundation, either version 3 of the License, or
% (at your option) any later version.
% This program is distributed in the hope that it will be useful,
% but WITHOUT ANY WARRANTY; without even the implied warranty of
% MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
% GNU General Public License for more details.
%
% You should have received a copy of the GNU General Public License
% along with this program. If not, see <http://www.gnu.org/licenses/>.
if (nargin <= 8)
r1 = 0; r2 = n;
s1 = 0; s2 = m;
end
r = r2-r1;
s = s2-s1;
du = min (d, p); dv = min (d, q);
for j=s1:s2
temp = curvederivcpts (n, p, U, P(:,j+1:end), du, r1, r2);
for k=0:du
for i=0:r-k
pkl (k+1, 1, i+1, j-s1+1) = temp (k+1, i+1);
end
end
end
for k=0:du
for i=0:r-k
dd = min (d-k, dv);
temp = curvederivcpts (m, q, V(s1+1:end), pkl(k+1, 1, i+1, :), ...
dd, 0, s);
for l=1:dd
for j=0:s-l
pkl (k+1, l+1, i+1, j+1) = temp (l+1, j+1);
end
end
end
end
end
%!test
%! coefs = cat(3,[0 0; 0 1],[1 1; 0 1]);
%! knots = {[0 0 1 1] [0 0 1 1]};
%! plane = nrbmak(coefs,knots);
%! pkl = surfderivcpts (plane.number(1)-1, plane.order(1)-1,...
%! plane.knots{1}, plane.number(2)-1,...
%! plane.order(2)-1, plane.knots{2}, ...
%! squeeze (plane.coefs(1,:,:)), 1);
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