A geometric construction for ensuring Cl continuity of adjacent Bézier patches

It is in many cases practical to compose a continuous suiface out of some low-degree Bézier swface patches having C I (first order parametric) continuity bet1veen the adjacent patches. The paper preselzts a relatively simple geometric construction of the position of the control points in the neighbourhood of the com mon boundary cun'e to ensure the required Cl cOlltinuity. The construction is based on the well-known criteria of the continu ous joints of the Bézier slllface patclzes and 011 a straightfonvard geOlnetric similarity.

Nowadays the freeform surfaces are already popular not just in industrial design but also among the computer-aided architectural design (CAAD) systems.For constructing freeform surfaces, the Bézier surfaces are preferred in many cases, due to their advantageous properties.The idea of a Bézier surface goes back to that of a Bézier curve.
The parametric vec tor of the Bézier curve with the given control points or knots will be: i=O Bi,Il(t) at the right side is called the blending function, defined as follows: Bi,ll(t) = (';)t i (1t)1l-i , where the binomial coefficient (Il) = ' 1 (Il_!') ,.Of course, the l l.II l .
above vector equation can also be written separately for the parametric functions of the coordinates x(t), y(t) and zet) of the R (t) vector by means of the scalar equations as follows: Bi,ll(t), i=O where Xi, yi, Zi are the corresponding coordinates of the Fi

knots.
With the increased number of knots, the equation describing the curve becomes more complicated, consisting of more terms, the degree of the curve is also increased (in case of II + 1 knots, the degree of polynomial describing the curve will be ll), therefore, it is of ten preferable to describe more complicated shapes by connecting several Bézier curves of lower degree together, while using the property of the curve that the sides of its control polygon at the end points are tangential to the curve at its end points.

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In case of Bézier curves and surfaces, it follows from the mathematical description with parameters that the simple geometric tangential continuity does not mean at the same time the continuity with respect to the parameters.In order to fulfil this latter requirement (that is, that the derivative with respect to t shall be continuous), the ratio of tangential sections ending at the common end point of the two sections shall be l; that is, they shall be of equal length.Hereinafter, the continuity will also mean continuity with respect to the parameters.the common control net.Two of them run along the common boundary curve ensuring the zero order continuity, while the other two edges going in side the control nets of the two originally separate patches, are in general not collinear, resulting in the break along the joint.
For the Cl continuity (which, as mentioned, also means the continuity with respect to the parameters t and u) it is not sufficient that, in case of joint surfaces, the edges going to a control point inside one or the other of the two patches from a control point on the common boundary lie along a straight line.Although is a necessary condition; for the required first order continuity, the ratios of the length s of these edge pairs shall also be \ I equal.\ The methods used in the case of freefrom curves can also be extended to surfaces.Thus, creating the Descartes product of two Bézier curves results in a Bézier surface.In mathematical terms, this results in the vec tor equation as follows: where the values of parameters t and u vary between O and 1, Fi,j are the knots associated with the bidirectional set of curves, and Bi.1l (t) and Bj.m (u) are separate blending functions associated with the parameters t and ll, respectively, as described above with the Bézier curves.
For the geometric interpretation, the diagram below provides assistance (Fig. 2).
As shown in the diagram, the surface is specified by means of a 4 x 4 control net (4 knots in each direction giving 16 knots).In case of surfaces, it especially holds that the increase in the number of their knots may render their handling very complicated; therefore, it is preferable to describe more complicated surfaces by joining several simpier surface patches together.The present paper aims at presenting a geometric approach suitable for this purpose.
However, the continuity criterion described in case of the Bézier curves is valid only with some addition.Two Bézier surfaces are of zero order continuity if the control points of the boundary curves of the neighbouring surface patches coin ci de; in fact, it follows from the above that, in such cases, the common boundary curves specified by the given control points are the same; but, although the two surfaces are joined together; there is obviously a break concerning the tangents along the joint.Namely, in a control point belonging to the common boundary curve of the two surface patches meet in general four edges of 6 I Per.Pol.Arch.c_ Fig. 3.The edge pairs of two continual adjacent Bézier patchses Thus, bJ/b2 = Cl/C2 = dl/d2 = eJ/e2 is also required for the C I continuity as specified above.
As opposed to curves, surface joints may be multi-directional: in fact, further surface elements can be joined in both lateral directions.In such cases, a continuous surface joint without a break line, that is, of tangential continuity would require that the above condition of Cl continuity is fulfilled in both directions.This, however, sets further restrictions in case of foursurface elements joined at their corner.
In the following, the joint of four Bézier-surfaces at a corner point is presented, with the condition that first order continuity is fulfilled.As shown in Fig. 4 below, let AF, B F, C F and D F the four surfaces be joined at the corner point S, and their corner knots lie along the lines G, b and c in one direction and p, q, and r in the other direction.On the other hand, the knot B of surface BF li es at the intersection oflines G and p; the common knot E of surfaces AF and BF at the intersection of G and q; the knot A of the surface AF at the intersection of G and r; the common knot F of surfaces BF and CF at the intersection of b and p; the common knot S of the foUf surfaces at the intersection of b and q; the common knot H of surfaces A F and D F at the inter-t parameters (Fo.; Fl..i F 2 .)F 3 .j ), and set of curves sections PI, P2, of line p; sections ql, ql ofline q and sections rI, r2 of line r, the ratio described above is fulfilled, that is: parameters (Fi.o Fu Fu Fu), and set of curves q, a parallel projection direction can always be found where the image points A', B' , C' and D' of A, B, C and D knots projected on that plane form a trapezoid, the elongated non-parallel sides of which and one of the lines joining the knots passing through the point S (q in this case) meet at the point P while the other line joining the knots that passes through the point S (b in this case) lies parallel to the parallel sides of the trapezoid (of course, in extreme case, the intersection P may be in infinit y; the projection will take the shape of a parallelogram).
/ (the projection of al, that is aD is to the length of section E A' (the projection of az, that is a;)) as bl is to bz.Considering that al/az = bI!bz, therefore ai/a; = aI!az also holds.This results in the similarity of triangles B B' E and AA' E, with the consequence that A A' is parallel to B B'.It can also be seen that AA' is parallel to DD', BB' to CC' and CC' is parallel to DD'; that is, the directions of projection are parallel to each other.
The above considerations are important in setting the positions of the knots necessary for joining the Bézier-surfaces; in fact, the theorem is also true in reverse; starting from any  trapezoid (or parallelogram in a special case), the knots of the Bézier-surfaces can be created by means of parallel projection of the corner points in an arbitrary direction -while preserving the proportions spec ifi ed above, which ensures the Cl continuity of the complex surface in both directions.
The pictures below  show the constraints by means of which further three surfaces can be joined to aspecific ini ti al AF Bézier-surface, with the Cl continuity preserved.
Of course, the proportion valid for the given direction of joint shall also be ensured in the case of other neighbouring lines joining the knots -marked with dotted lines in the last figure -of the joint connecting surfaces.

Fig. 1 .
Fig. 1.Arc of Ion voluta -defined by means of several Bézier curves joined tangentially

Fig. 4 .
Fig. 4. The edge pairs of fOUf C l continual adjacent Bézier patches It will be shown that, for the plane detenruned by lines b and

Fig. 5 .
Fig.5.The projection of ABCD knot~ is the A'B'CD' trapezoid

Fig. 6 .
Fig.6.Specific initial AF Bézier-surface with knots S. E, A. and H around the comer and with lines joining the knots a2.b2.qj.rj.

Fig. 7 .
Fig. 7. Marking out a point P on the line q passing along the qj line joining the knots and connecting it with the knot H.

Fig. 8 .Fig. 9 .
Fig. 8. Marking out the knots F and G neighbouring surfaces alon g the line b passing through the knots b2 and along the line q.