Class A surfaces-

What is everyones understanding/knowledge of Class A surfaces? What classifies them as Class A surfaces?

I’m trying to describe Class A surfaces to a co-worker (non-designer) and I’ve used the old stand-by 'Class A surfaces are like a cars exterior/interior (body, dashboard, etc…) and Class B surfaces are like a cars brakes, hoses, frame, etc…

If you have to talk in technical terms, thats o.k.

Any info. is appreciated. Hopefully Class A surfaces aren’t too fast for BMW.

I thought there’s only class A and class C, with class A being the surface that you will see so it has to be perfect, and class C being on the other side where appearence isn’t importance.

From my understanding there is a bit of a dual definition of surface classes.

On the one hand you have appearane - is it visible / hidden etc.

When talking about CAD however Class A refers to a surface having “curvature continuity” with other surfaces adjacent to it. - Like on the exterior of a car. Most CAD packages handle tangent continuity through use of fillets however Class A surfaces are usually much more complex.

class a refers to complex surfaces and have continious curvature and usually defined tangency.

class b refers to complex shapes/surfaces with undefined tangency but they can have continious curvature.

class c refers to simple surfaces with normal tangency and curvature.

in catia, in generative shape design you can create a class c surface, then you can take it to free style: you get control points that shape the surface according to tangency and direction. this is more like class b.

then you can start “automotive class a” which gives you the class a control points you can manipulate in any direction and(they’re different from free style’s control points).
if you want to match two or more surfaces you can set tangencies between surfaces.

The classification you are referring to is a relative system of classification that is used in the automotive industry.

Class A- surfaces that are always visible (smooth painted hood, door panels etc)
Class B- surfaces that are sometimes visible (parts visible if you open the door or have to bend down to see)
Class C- surfaces that are never visible (back of stamped parts that are coverd or fastened to the guts)

So you see- different companies can have different standards for their external surfaces (class A) independent of surface continuity. BMW for example can have convoluted flame surfaces with only G0 continuity but it is still classified as class A. Surfaces that have crease and bone-lines are still considered class A -as are car seats and dash boards even though they are not shiny

a surface with a crease can be both class a and class b.

what about the curve degrees? like a 5 degree spline by poles?

I was under the impression class a meant something mathmatical the computer dose when it generates the geometry

A-Class surfacing is defined as these two major points
1 lightly created geometry and that the geometry is built correctly.
2 light reflections are reflecting in a desirable way. i.e. if g2 is required then the light reflects in a g2 way.

To meet qualifications of an A-class surface? The geometry often refers to the geometry that is facing outward. Also often opposed to b-side geometry. An A-class surface would not have texture and would be as shinny as possible like the body of an automobile. As in Alias the number of spans are items apart of definitions leading to g2 continuity.

model by Alfredo Santillan, this is the hood of the 350z created
in Wildfire using ISDX. Notice how the light reflections flow across
multiple surfaces to create continuity. Light reflections
are the key.

+Compare G0
positional conections. Not tangen but touching.

+Compare G1
simply tangent. The surfaces in this example are tangent, the curve plot is not tangent. This line can not be broken down to a more simple curve therefore it is not curvature continuous or G2. You can not take an integral at the intersection of the curve as it crosses to the other surface.

curvature continuous. When a curve plot is tangent. The equation of the curve plot can be broken down into a more simple curve by taking an integral. ie. calculus

five ways to determine g2 continuity.

  1. Evaluate with your eyes the light reflections
  2. Evaluate the math using Calculus
  3. Look at the curve plot in either Pro/E or Alias
  4. Spans or CV’s line up in Alias
  5. zebra stripes

Some people will use a Gaussian analysis incorrectly. In this tool G does refer to that specific mathematician however the standard Gaussian analysis refers to concave vs convex.

Below are examples of zebra stipes to better illustrate g0, g1, and g2 continuity respectively.

example if zebra stripes in Wildfire

example of zebra stripes in Alias Studio

Thank you design-engine for posting a great response to an often asked, but seldom answered question. Much appreciated!

I’ll be honest, I have no use for this level of surfacing in my current position, but as an avid user (and self-taught user) of Alias during Uni, I am still curious and have a question.

I understand positional continuity and tangent continuity, but what exactly is curvature continuity? I’ve even looked this up the last time I found an Alias reference book, but with no luck understanding it!

Notice the stripes on the ‘zebra stripes’ pictures. In the third option notice the stripes are tangent. Think of this sort of as looking at the surface only where the two surfaces connect under a microscope. The figurative microscope simply determines how the reflection will occur.

Use of the words The surfaces connect in the third option tangentially thus the connection is considered ‘curvature continuous’ or you could say the surface maintains a g2 continuity. G2 continuity can be used in small parts that maintain surface texture to some successes and is not only utilized for large parts that sit out in the omni sun. The curves that exist in Alias or Pro/E before the surface is applied can maintain g2 continuity and get some desired effects to accelerate the way light reflects off a surface or out of a surface. Can’t hurt to understand how light reflects off form because of how a human subconscious determines the size of form and conical g2 connection reflects light back to a users eyes differently thus changing the experience.

Example: Use these surface connections to make an interior seam more roomy because less light reflects back to the user. Interior of a cab.

Detection Example: Stealthy planes and missiles might use a g2 continuity so not to reflect back light to the detectors. We have administered these classes to several aerospace manufactuers in Europe and USA including NASA (if you could call them aerospace)

Auto Example: A Ford Taurus and a 1966 GTO are very close in size according to a tape measure. The GTO because of the sharp points peaks on the surface contour of the car reflects more light back to the user giving them more surface data to comprehend thus the car is larger. Note also the use of trim on a car can accentuate the details because the chrome guarantees light reflections. – refereeing to the ‘yo’ sketch on the sketching section. I bet the design by committee would have that detail pulled out of the jag sketch, ‘yo’? Ant comments? We have adminsiterd this workshop in both Alias and Wildfire to the likes of Triumph, Harley Davidson, Mac Trucks, Caterpillar, John Deere.

When aligning curves and surfaces you can also look at continuity from a CV perspective. If 2 curves simply touch at their end point CVs then it’s positional. If you have 4 CVs lined up in a row (the 2 touching, plus one more from each curve) then it’s tangential. And if you have 6 CVs in alignment (the 2 touching, plus 2 more from each curve) then it has curvature continuity. I think…

yep. but remember, ‘your surfaces are only as good as your curves’. That is cliche here because we say it so much. Don’t forget to establish the continuity at the surface level once the curve cv’s are aligned.