I am looking for a technical defination for classes of surfaces i.e. what is A,B, C class surface?
I am looking for a technical defination I don't think there are "technical definitions". It's all about appearance (not technical) and surface structure, aesthetics and mathematics; things that are hard to "spec" in a meaningful manner. Search the web, though. There's tons of descriptions, definitions, discussions.... The latest one I've run across, and about as accurate as any (I suppose) is at: http://www.mcadcentral.com/proe/forum/forum_posts.asp?TID=25949&PN=1 Also look into the subject of geometric or curvature continuity, such as: http://news2.mcneel.com/scripts/dnewsweb.exe?cmd=article&group=rhino&item=7 3174&utag=
As Jeff says there is no real technical definition. Class A is used (mainly in automotive) to describe high quality surfaces which are visible on the final product. The surfaces have to "look good". Many people interpret this as meaning that the surfaces must be curvature continues to one another (light reflection lines run smoothly). Mathematically G2, where the radius of curvature of the end of one surface matches the radius of curvature on the start of the next. However some areas will only be good quality if they are G3, while on the other hand some good looking products have sharp edges (or at least hard blends). Class C I have come across being a short term for Concept surfaces. Surfaces created by a designer to define the main aesthetic characteristics of the product but not finished to define a closed (smooth) volume. Regards Steve
G2 & G3 or C2 & C3? It's a matter of the second partial derivatives IIRC. They require, in general, at least cubic polynomial representations (C2). Merely requiring a match in the "radius of curvature" is questionable, as is continuity of tangents, as both can be had with polynomials of degree 2.
Even a class A surface can be NURBS, but there are b-splines and c- splines, and corresponding b-spline surfaces and c-spline surfaces, which are completely different designations. The b- and c- splines just designate if the control points are on the spline itself or at the intersections of the tangent control frame. matt
