function dnurbs = nrbderiv(nurbs) % % NRBDERIV: Construct the first derivative representation of a % NURBS curve or surface. % % Calling Sequence: % % ders = nrbderiv(nrb); % % Parameters: % % nrb : NURBS data structure, see nrbmak. % % ders : A data structure that represents the first % derivatives of a NURBS curve or surface. % % Description: % % The derivatives of a B-Spline are themselves a B-Spline of lower degree, % giving an efficient means of evaluating multiple derivatives. However, % although the same approach can be applied to NURBS, the situation for % NURBS is a more complex. I have at present restricted the derivatives % to just the first. I don't claim that this implentation is % the best approach, but it will have to do for now. The function returns % a data struture that for a NURBS curve contains the first derivatives of % the B-Spline representation. Remember that a NURBS curve is represent by % a univariate B-Spline using the homogeneous coordinates. % The derivative data structure can be evaluated later with the function % nrbdeval. % % Examples: % % See the function nrbdeval for an example. % % See: % % nrbdeval % D.M. Spink % Copyright (c) 2000. if ~isstruct(nurbs) error('NURBS representation is not structure!'); end if ~strcmp(nurbs.form,'B-NURBS') error('Not a recognised NURBS representation'); end degree = nurbs.order - 1; if iscell(nurbs.knots) % NURBS structure represents a surface num1 = nurbs.number(1); num2 = nurbs.number(2); % taking derivatives along the u direction dknots = nurbs.knots; dcoefs = permute(nurbs.coefs,[1 3 2]); dcoefs = reshape(dcoefs,4*num2,num1); [dcoefs,dknots{1}] = bspderiv(degree(1),dcoefs,nurbs.knots{1}); dcoefs = permute(reshape(dcoefs,[4 num2 size(dcoefs,2)]),[1 3 2]); dnurbs{1} = nrbmak(dcoefs, dknots); % taking derivatives along the v direction dknots = nurbs.knots; dcoefs = reshape(nurbs.coefs,4*num1,num2); = bspderiv(degree(2),dcoefs,nurbs.knots{2}); dcoefs = reshape(dcoefs,[4 num1 size(dcoefs,2)]); dnurbs{2} = nrbmak(dcoefs, dknots); else % NURBS structure represents a curve [dcoefs,dknots] = bspderiv(degree,nurbs.coefs,nurbs.knots); dnurbs = nrbmak(dcoefs, dknots); end

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