CompuServe Thread

Hi All, Since I've seen some questions here regarding Bi-Cubic Patches (from B-Splines) and Catmull-Rom Splines as they relate to the forthcoming Yost Skin IPAS routine, I thought I would jump in and post this little blurb regarding that subject FWIW: You can generally classify spline curves into 2 categories: INTERPOLATING – Curve which passes through it's user-defined or program- generated control points. APPROXIMATING – Curve which does not pass through it's conrol points. The B-Cubic Patch (derived from B-Splines) is an APPROXIMATING spline type, while the Catmull-Rom type Gary recently mentioned is an INTERPOLATING type. Both types are useful and have different characteristics and considerations when employing them in geometry definitions. When using uniform B-Splines, usually (not always) you define the points for a control hull or lattice, and the curve generated only *approaches* the defined control points. By doubling or tripling a control point, you can force the curve to be pulled closer to the control point. As you can see in the SKINNING.gif example posted here, the cross-sections are somewhat angular in nature (the hull), but the resulting mesh is quite smooth – but will not represent the *actual* shape as defined with the cross-sections due to the approximating nature of the spline. On the other hand, when using the Catmull-Rom type, one can create the cross-sections pretty much as an *accurate* description of the desired skinned shape, as the curve will pass through the control points. The C-R spline exploits a parabolic blending technique, when extended, enables the concept of tension to be introduced which controls the tangent of the curve as it passes through the control points. I do not know if this concept of "weighted" adjustable control points will be employed or is even within the scope of the IPAS routine though. Examples of B-Spline implementations can be found in AutoCad & other CAD programs on the PC, as well as Topas, Real 3D and other 3D programs, if you have seen/used any of those. Examples of the Catmull-Rom type can be seen in Playmation (I believe, or certainly something similar). I hope this has been helpful to some whom have inquired, and if I have mis-stated something, especially regarding the IPAS implementations, I'm sure someone will correct me. Regards, Bill