Here is a video of the workflow.
Nobody has used this CLASU method maybe for two reasons:
It is not known any earlier.
It does not work which is why it does not exist.
The reasons for failure with Ferrari backwing maybe due to Rhino, not that CLASU method does not work.
What is wrong if Rhino says that seams are ok - on the good area - but then they are not ok in Alias? I would like to know this. Very confusing manual at least, I would say.
Hi Nathan,
My Ferrari F 40 example is a failure from the Class - A perspective but I was able to do a certainly continous seam manually with CLASU method with Autodesk Alias Student license. However it is only one seam yet, not a difficult crossjoint. It took a while to do it as curvature continous but it shows that my principle works at least in this case. I think Rhinoceros is a very difficult tool to make Class A but Alias is better from this point of view.
Method is indeed the same I used in my video. In Alias you have to draw with “CV snap” a linear curve, extend it and slide the CVs along this tangent ruler and thus find the continuity. I will later make a video of Alias approach. CLASU works so far but in Alias it is more tedious to do cos it is more accurate tool than Rhino. I will make later something more difficult with Alias to see if my method is useful.
In the image you can see the C check mark for full continuity, the tangent ruler CLASU lines and Zebra lines. This example is yet simple but promising start.
In my opinion, the attached 3d model uses a completely wrong modeling technique. Car body panels, and product design in general, require clean surfaces with logical patch layout that enables smooth transitioning. The rounded area where the rear wing meets the rear fender can never look good that way. That particular rounded surface must follow the same bent shape as in the real Ferrari F40, making a nearly 90 degree bent. Building a proper patch layout is essential for the overall surface quality.
You actually made it properly from the inner side of the rear wing, you just need to follow the same technique on the outside, too. 
But I recommend you to make a separate small blend surface just for the bent area, while the vertical one must be another surface.
Here is a similar area on another model:
And another similar transition:
Many thanks for the feedback. I prechuate it. The backwing of the Ferrari looks a bit diffent than your example models, but I got your point.
The actual issue was whether or not CLASU method works. Currently the evidence is poor - only one seam in Alias is really good - but it is encouraging start. This is the point here.
Anyway I will continue my attempts to fully proof the usability of my modeling technique, which can be used to make almost whatever geometries - right or wrong - with a continuity. If I am right.
Rhino is not “too inacurate”.
And the fact that Alias is better suited for class a modelling is not a questions of accuracy, but of specific tools to manipulate surfaces on the control point level.
Rhino shows “green” surface edges when the edge continuities you can define in the edge continuity panel are met. You simply failed to define adequate values.
as a total noob on this “Class - A” surfacing, are these values of analysis something that are documented somewhere, or are these those kind of values that change from company to company and from teacher to teacher.
Altamiro, everyone has an explanation for it. That is how broad a statement it is. You will find much usage on Autodesk and so on. Probably didn’t answer your question. —-Mark
So, I went to the tools of the moment to try to understand it better 
Google Bard:
Class-A surfacing is a term used in the 3D modeling and manufacturing industries to describe a surface that is perfectly smooth and free of any imperfections. It is the highest quality surface finish that can be achieved.
There are five key characteristics of a Class-A surface:
* **Continuity:** The surface should be smooth and continuous, with no bumps, ridges, or other irregularities.
* **Fairness:** The surface should be fair, meaning that it should flow smoothly from one point to another.
* **Accuracy:** The surface should be accurate to the design specifications.
* **Consistency:** The surface should be consistent, meaning that it should be the same quality throughout.
* **Completeness:** The surface should be complete, meaning that it should include all of the necessary details.
There are a number of ways to achieve a Class-A surface finish. Some of the most common methods include:
* **Hand-modeling:** This is the traditional method of achieving a Class-A surface finish. It involves using a variety of tools and techniques to shape and smooth the surface by hand.
* **Computer-aided design (CAD):** CAD software can be used to create a 3D model of the surface. The model can then be used to generate toolpaths for CNC machines or rapid prototyping machines.
* **Computer-aided manufacturing (CAM):** CAM software can be used to control CNC machines or rapid prototyping machines. These machines can then be used to create a physical model of the surface.
* **Finishing:** Once the surface has been created, it may need to be finished to achieve a Class-A surface finish. This may involve using a variety of tools and techniques, such as sanding, polishing, and buffing.
The specific method used to achieve a Class-A surface finish will vary depending on the application. However, all of the methods described above can be used to achieve a high-quality surface finish.
Here are some additional tips for achieving a Class-A surface finish:
* Use high-quality materials.
* Use the appropriate tools and techniques.
* Take your time and be patient.
* Practice makes perfect.
By following these tips, you can achieve a Class-A surface finish that meets your needs.
I hope this helps. Is there anything else I can help you with?
Chat GPT 3.5:
Class-A surfacing, also known as Class-A modeling or Class-A surfaces, refers to the creation of high-quality, aesthetically pleasing, and mathematically precise surfaces in 3D computer-aided design (CAD) models, particularly in the automotive and industrial design industries. Achieving Class-A surfacing involves several key characteristics and best practices:
1. **Curvature Continuity (G0, G1, G2):**
- **G0 (Positional continuity):** The surfaces must meet smoothly without any gaps or steps.
- **G1 (Tangential continuity):** Tangent vectors at the connection points are aligned, ensuring a smooth transition in surface direction.
- **G2 (Curvature continuity):** Curvature across the surface boundaries is continuous, providing a seamless visual appearance.
2. **Mathematical Precision:**
- Class-A surfaces require high mathematical precision to ensure that the surfaces are smooth and have no imperfections.
3. **Reflection Quality:**
- Surfaces should be designed to create consistent and high-quality reflections. This involves controlling the shape and curvature to avoid distortions or irregularities in the reflections.
4. **Global Surface Quality:**
- Consider the overall quality and continuity of surfaces across the entire model. This involves not only individual surface patches but also their interactions and transitions.
5. **Surface Continuity Across Transitions:**
- Pay attention to how surfaces transition into one another, ensuring smooth and gradual changes without sudden jumps or disruptions.
6. **Symmetry:**
- Achieve symmetry in the model to create a balanced and harmonious design.
7. **Surface Analysis and Evaluation:**
- Use analysis tools to evaluate surface quality, such as curvature analysis, zebra striping, and reflection analysis. This helps identify and address areas that may need improvement.
8. **Parametric Control:**
- Maintain parametric control over the surfaces, allowing for easy modifications while preserving the overall design intent.
9. **Complex Surface Creation:**
- Class-A surfaces often involve complex shapes and transitions, so the use of advanced surfacing tools and techniques is essential.
To achieve Class-A surfacing, designers typically use specialized CAD software like Alias, Rhino, CATIA, or Siemens NX. These tools provide advanced surfacing features and analysis tools to help designers meet the stringent requirements of Class-A surfacing. Additionally, extensive experience and expertise in surfacing techniques are crucial for achieving high-quality results. Continuous refinement and iteration are often necessary to meet the desired aesthetic and functional criteria.
GPT 4 Via Bing.com:
Class-A surfacing, also known as Class-A modeling or Class-A surfaces, refers to the creation of high-quality, aesthetically pleasing, and mathematically precise surfaces in 3D computer-aided design (CAD) models, particularly in the automotive and industrial design industries. Achieving Class-A surfacing involves several key characteristics and best practices:
1. **Curvature Continuity (G0, G1, G2):**
- **G0 (Positional continuity):** The surfaces must meet smoothly without any gaps or steps.
- **G1 (Tangential continuity):** Tangent vectors at the connection points are aligned, ensuring a smooth transition in surface direction.
- **G2 (Curvature continuity):** Curvature across the surface boundaries is continuous, providing a seamless visual appearance.
2. **Mathematical Precision:**
- Class-A surfaces require high mathematical precision to ensure that the surfaces are smooth and have no imperfections.
3. **Reflection Quality:**
- Surfaces should be designed to create consistent and high-quality reflections. This involves controlling the shape and curvature to avoid distortions or irregularities in the reflections.
4. **Global Surface Quality:**
- Consider the overall quality and continuity of surfaces across the entire model. This involves not only individual surface patches but also their interactions and transitions.
5. **Surface Continuity Across Transitions:**
- Pay attention to how surfaces transition into one another, ensuring smooth and gradual changes without sudden jumps or disruptions.
6. **Symmetry:**
- Achieve symmetry in the model to create a balanced and harmonious design.
7. **Surface Analysis and Evaluation:**
- Use analysis tools to evaluate surface quality, such as curvature analysis, zebra striping, and reflection analysis. This helps identify and address areas that may need improvement.
8. **Parametric Control:**
- Maintain parametric control over the surfaces, allowing for easy modifications while preserving the overall design intent.
9. **Complex Surface Creation:**
- Class-A surfaces often involve complex shapes and transitions, so the use of advanced surfacing tools and techniques is essential.
To achieve Class-A surfacing, designers typically use specialized CAD software like Alias, Rhino, CATIA, or Siemens NX. These tools provide advanced surfacing features and analysis tools to help designers meet the stringent requirements of Class-A surfacing. Additionally, extensive experience and expertise in surfacing techniques are crucial for achieving high-quality results. Continuous refinement and iteration are often necessary to meet the desired aesthetic and functional criteria.
And finally, You.com:
#### Key Characteristics of Class-A Surfacing
Class-A surfacing refers to the creation of high-quality, aesthetically pleasing surfaces, often used in automotive and consumer product design. The key characteristics of Class-A surfacing include:
1. **Curvature Continuity**: Achieving smooth transitions and curvature continuity between surfaces is crucial for Class-A surfacing.
2. **Surface Quality**: The main consideration for achieving Class-A quality surface data is the intention of the Golden....
3. **Precision and Tolerances**: Class-A surfaces require tight tolerances, often specified using Geometric Dimensioning and Tolerancing (GD&T) standards.
4. **Control Point Manipulation**: Precise control over surface control points and manipulation is essential for achieving Class-A surfaces.
5. **Surface Matching**: The ability to match surfaces seamlessly and accurately is a key characteristic of Class-A surfacing.
#### Achieving Class-A Surfacing
To achieve Class-A surfacing, designers and engineers can follow these steps:
1. **Use Specialized Software**: Utilize advanced 3D modeling and surfacing software such as Autodesk Alias, Fusion 360, or similar tools that offer specific features for Class-A surfacing.
2. **Understand Curvature Continuity**: Learn how to use the language of curvature continuity with confidence, as it is a fundamental aspect of Class-A surfacing.
3. **Master Surface Manipulation**: Gain expertise in manipulating surfaces with precision, including control point manipulation and surface matching.
4. **Adhere to Tight Tolerances**: Understand and apply GD&T standards to ensure the surfaces meet the required tolerances for Class-A quality.
5. **Continuous Learning**: Stay updated with the latest techniques and tools for Class-A surfacing to improve skills and capabilities.
By following these steps and utilizing the right tools and techniques, designers and engineers can achieve the high-quality surfaces characteristic of Class-A surfacing.
If you have further questions or need more details on any specific aspect, feel free to ask!
Given these definitions by the AI tools, I would summarize saying that:
Class-A Surfacing is the Art of combining continuity, surfacing and mathematics to create the best looking, most precise and most consistent surface possible, given the objectives of that project and/or client requests.
A common understanding of Class-A surfacing contains also the preference (if possible) of single span Bezier surfaces of minimum complexity and with well structured control points, to avoid unwanted surface inflections or in-surface continuity breaks (across spans)
But there are many more aspects to consider regarding class-A modelling.
To get a more comprehensive understanding, a good place to start imho are Aliasˋ Golden Rules
This is a good summary of what Class A surfacing should mean.
The term “Class A surfaces” was in use before computer software was used for surfacing.
Thank you David, I only wish that this would eliminate (illuminate) future semantics discussions on Class A surfaces. In my training it was expressed as the “quality” of the highlight on a surface, a visual qualification, nothing more, nothing less.
Thanks, this is a very clear example on how the AI is only taking the surface of a problem without really understand the problem itself.
This is what the majority of marketing manager want people to think, an AI in just enpowering this misconception.
In simple word Class surfaces refer to manufacturing problems, David explained perfectly.
A srfs are the one you see from outside.
B srfs the one you see opening a lid or a part
C srfs the one never seen from the users.
Because A class srfs would be visible they need to be pleasant and to achieve this in 3D modelling you need specific process and tools. But again what isn’t really expressed from the AI (nor from the marketing) are the parameters to evaluate “Quality”. And here is were the debate never stops.
Perfection doesn’t exists so how good should we make our things? How to measure the good ?
For example how much should be the curvature continuity or the “flowness” of a pair of surfaces to be considered good? 0 isn’t a number.
Every manufacturer has it’s own standards and this has nothing to do with modeling quality.
Not always. I have seen cars and consumer electronics with ugly transitions on the exterior body, which is a result of bad NURBS surfacing.
I had a similar “Samsung” vacuum-cleaner, but it was much worse, because its body was shiny and the reflections exposed everything wrong with the NURBS modeling of the body. Their designers and engineers even failed to achieve G1 tangency! My guess is that they have used Solidworks and the “Patch” tool to fill the multi-side hole.
It looks like Samsung had different surface quality standards for that product than you do.
I guess that someone forgot to click on the “Tangency” tickbox.
Or the “defects” you have identified were created when the molds were made and have nothing to do with math modeling of the surface. 
These shapes are a direct result of bad NURBS surfacing. Steel injection moulds are made with high precision CNC-machines and leave very little work to be done by manual sanding to smooth out the surface.
That would be a piss poor moldmaker then.
If the actual product is made by an injection mould that closely follows the 3d model provided by the design engineers, this only means that the latter were not good enough at NURBS modeling.
And the answer is that 99.9% of the people who buy those vacuum cleaners or what ever don’t care. And the designer(s) who originally made the CAD file maybe just don’t have the skills to do better, or weren’t given enough time or money to refine stuff. And management who OK’s the design and wants to get it into production as fast as possible for the least possible cost. Welcome to the real world of mass-produced consumer products.
