Hi....I wanted to know what are Class A surfaces...Are there other classes of surfaces also ? What is meant by curvature & tangent continuity..? What is G1,G2..?
Class A Surfaces
The definition depends on who you ask. I think the two basic opinions are that class of surface either refers to location or quality (or maybe both) For example:
LOCATION - all surfaces that a consumer normally sees can be considered class A surface.
The outside of a an automotive floor console would be class A, but the inside surfaces which normally include manufacturing flanges and attaching surfaces would be class B.
QUALITY - refers to surface topology. Position, tangency, and curvature across surface boundaries, and internal patch structure.
Some opinions are that position continuity is class C, tangency continuity is class B, and curvature continuity is class A. But I think that these are more appropriately defined as C0, C1, and C2 condition referring to the B-spline curve equation and its 1st derivative (tangency=C1) and its 2nd derivative (curvature=C2). So I think a class A surface can be discontinuous in curvature if that is the intention of the design (highlight reflection, or other reasons) and even discontinuous in tangency if the intention is a crease or sharp edge (but usually molding or stamping requires no sharp edges so Class A must be tangent continuous (C1)). Second Thought Hear is a further understanding of Class-A surfacing based on experiences with two automotive companies and whites goods Manufacturers. They independently have the same definition for the classification. The physical meaning: Class A refers to those surfaces, which are CURVATURE continuous to each other at their respective boundaries. Curvature continuity means that at each "point" of each surface along the common boundary has the same radius of curvature. This is different to surfaces having: Tangent continuity - which is directional continuity without radius continuity - like fillets. Point continuity - only touching without directional (tangent) or curvature equivalence. In fact, tangent and point continuity is the entire basis most industries (aerospace, shipbuilding, BIW etc.). For these applications, there is generally no need for curvature. By definition: Class A surface refers to those surfaces which are VISIBLE and abide to the physical meaning, in a product. This classification is primarily used in the automotive and increasingly in consumer goods (toothbrushes, PalmPC's, mobile phones, washing machines, toilet lids etc.). It is a requirement where aesthetics has a significant contribution. For this reason the exterior of automobiles are deemed Class-A. BIW is NOT Class-A. The exterior of you sexy toothbrush is Class-A, the interior with ribs and inserts etc. is NOT Class-A. The consequence: The consequence of these surfaces apart from visually and physically aesthetic shapes is the way they reflect the real world. What would one expect to see across the boundary of pairs of point continuity, tangent continuity and curvature continuity surfaces when reflecting a straight and dry tree stump in the desert? * Point Continuity (also known as G0 continuity) - will produce a reflection on one surface, then at the boundary disappear and re-appear at a location slightly different on the other surface. The same reflective phenomenon will show when there is a gap between the surfaces (the line markers on a road reflecting across the gap between the doors of a car). * Tangent Continuity (also known as G1 continuity) - will produce a reflection on one surface, then at the boundary have a kink and continue. Unlike Point continuity the reflection (repeat REFLECTION) is continuos but has a tangent discontinuity in it. In analogy, it is "like" a greater than symbol. * Curvature Continuity (also known as G2 continuity, Alias can do G3!) - this will produce the unbroken and smooth reflection across the boundary. Please do not believe me! This is the real physical world. Look at your cars rounded hood reflecting lines on the road or trees. Look at ripples of water that are not turbulent, reflection is everywhere but all blend into each other, as there is also curvature continuity everywhere. Still not convinced - For an analytical approach, you may simply prove this point using any rendering package (e.g. CATIA V4 VST), Neon textures in 4D Navigator or DMU Navigator (V5), using the traditional CURVE1+REFLECT or /ANADIA in V4 CATIA and of course the neon-tray dynamic reflect curve facility in V5. What about CATIA? Traditionally CATIA has been used to create the "engineering" side of most designs, rather then the exterior "aesthetic" shell (I.e. Class-A). These traditional yet awesome tools (like SURF2) are geared for this kind of engineering work. The best example being BIW in the automotive industry. Functions like SURF2 and FORMTOOL carve up even the most difficult inner panel structures into reality. This is why, historically, CATIA took an early strangle hold (amongst other reasons like a great capacity in all aspects of DMU and integration across disciplines). CATIA comes from the aerospace industry. The exterior of airplanes (whose panels buckle between frames and expand with every land-takeoff cycle) has very little "need" for curvature continuity and has 100% engineering factors driving its design (Aerodynamics and structures). That is, there is zero styling in the design of an aircraft body. The fact that airplanes looks good and "smooth" is by virtue of its operation (streamlined as possible), their general cleanliness and most importantly the distance that one generally views them. If one was to look carefully down the fuselage of an aircraft on the ground, there is nothing smooth about it! Having the capability to cater for these industries in an engineering and process capacity with existing function and not requiring the ability to create Class-A, has made CATIA the de-facto standard for the aerospace and automotive industries. As for Class-A, automotive manufacturers have utilized either or combinations of Alias and/or ICEM Surf (or others) to achieve these goals in a productive manner (remember the word productive). Alias has the ability cover the entire industrial design process from Sketches TO Surfaces on sketches TO Surface manipulation and build and further onto rendering and animation. In retrospect, CATIA V4 can create Class-A surfaces with (1) compromise (e.g. this deviation is OK, because it can be polished by the toolmaker) and (2) an idiosyncratic approach by the CATIA operator - i.e., it can be done but not as easily as with Alias or ICEM Surf. Historically, its been "difficult" of Dassault to create software in V4 to easily create Class-A surfaces due to the use of Bezier (polynomial) based mathematics. There is nothing against Bezier based surfaces though. They are excellent for creating the engineering surfaces we have all come to love (BIW etc.) utilizing intelligent use of multi-patch surface methodology. In fact, I doubt NURBS surfaces could do a better job. And without a doubt, V5, with its new architecture and use of Bezier and NURBS surfaces will go along way in being able to confidently and more importantly competently producing these Class-A surfaces for an ever growing aesthetic minded world. And what about V4 CATIA? CATIA V4 currently has the ability to create curvature continuous surfaces in two categories. Surfaces: a. Using SURF2 and SKIN (GSM) functions to sweep and loft as "long" a surface as possible. This will generally produce a curvature continuous surface with minimum deviation. b. Intelligent use of SPINES and LIMIT curves when using SURF2 and SKIN to closely match curvature across boundaries. c. Utilizing conic surfaces and conic curve approximations to mimic curvature conditions. d. For parts with large variations within its shape cause techniques a and b to struggle. For this reason, we may take three approaches. d1. Create "unstressed" surfaces to the point of struggle and fill in the blank with blend surfaces and curvature continuity. This is very much situation dependent. d2. Use ARC’s and PATCHES's - ARC's and PATCHES have the peculiar yet great ability to ? not go through all their constraints (good for the styling end of the design process) * the ability to deform a arc or patch to a point ? The ability to deform the boundary of a patch to an arc whilst maintaining the opposing continuity. ? Most importantly - the ability to reduce or increase degrees of arcs and patches to maximize or localize deformations. I have found these most useful. e. Utilize NURBSCRV and NURBSSRF when and arc or patch refuses to go close enough to the constraints of interest. Blends: These are a curious family of surfaces. One can utilize two functions within CATIA V4. The first is the ubiquitous BLENSURF functions, which allows a point/tangent/curvature continuos blend between any two curves on any part of any plane, FSUR, RSUR, surface, face or skin. OR automatically creating BI-rail curves along two surfaces at particular "radii" and placing a point/tangent/curvature continuous blend between them. Tensions and connectivity locations are also adjustable. Although it is a great tool, one issue with Blensurf is its inability to blend around a large angle. For instance, if one constructs two segment surfaces to each other at right angles with a gap between them and then placing a curvature continuos surface to connect them. The result is very surprising. The surface comes off one with curvature continuity, takes the shortest route to the other and then blends with curvature again. It is not the expected shape in the blend, when comparing it to the curves created using CURVE2+CONNECT with curvature from the isoparametric curves of each surface. The reason for this is that Blensurf creates purely mathematical curvature. For the correct shape, mathematical and isoparametric curvature is required. Guess what my friends, Dassault are already on the ball, this is possible using GSM's SKIN function blend and V5 GSD blends.
Class A is something visible: Like the hood of your car.
Class B is visible but not essential: Like the door aperture.
Class C is not visible, like the seat mounting under the carpet.
Our Understanding for Class A surfaces:
1. The fillets - Generally for Class A, the requirement is curvature continuous and
Uniform flow of flow lines from fillet to parent surface value of 0.005 or better
(Position 0.001mm and tangency to about 0.016 degrees)
2. The flow of the highlight lines - The lines should form a uniform family of lines.
Gradually widening or narrowing but in general never pinching in and out.
3. The control points should form a very ordered structure - again varying in
Angle from one Row to the next in a gradual manner (this will yield the good
Highlights required).
4. For a Class A model the fillet boundary should be edited and moved to form a
Gentle line - and then re-matched into the base surface.
5. Matched iso-params in U & V direction are also a good representation of class A.
6. The degree (order) of the Bezier fillets should generally be about 6 (also for arc
Radius direction) sometimes you may have to go higher.
7. We also take care of Draft angle, symmetry, gaps and matching of surfaces
Created with parent or reference surfaces.
8. Curvature cross-section needles across the part - we make sure the rate of
Change of curvature (or the flow of the capping line across the top of the part) is
Very gentle and well behaved.
Different companies define Class A differently. Generally Class A surfaces are curvature continuous (I've never seen C3 as a requirement for "class A," but I have seen it for stealthy aircraft), but there are varying tolerances allowed.
