This surface is somewhat special,

in that the trim edge and top natural edge now flow outwards from the top left vertex with an anti tangent relationship (i.e. the opposite of tangent)

First, let’s take a look at the underlying control vertices.

Good. Now, let’s switch to another example

where the modeling was done in a slightly different way. Notice how the anti tangent situation has been avoided altogether by the use of an additional little patch, but that will have left you with a five sided gap that you now somehow would need to fill.

So… some genius, decided to put a power fill function to work, most likely done in a solid modeling package.

Now, surprisingly enough, the highlights on this ridiculous surface look perfect! really, it does!

This is a shot of good fortune, because very often it only manages to generate a surface with forced G2 at the boundaries and a very tumultuous interior to the surface. I can only imagine the solid modeling operator also used additional constraint curves to control the interior of the patch better.

Now, let’s get back to our previous example, with our anti tangent situation. Albeit just in the trim space of a nice clean 4 sided UV structure. By now we have added the fillets along the theoretical style edge.

By this stage in the model, this is now what’s left of that parent surface.

Now, let’s check the tangential continuity relationship

by selecting the top left most point generated by the top natural edge and the trim edge of the wheel arch, using the Curve Continuity function.

As we can see, it now has about 10 degree difference.

It’s still an acute intersection, but it’s perfectly geometrically and topologically legit.

Now let’s look at this model’s top wheel arch patch,

it’s a trimmed out five sided surface just the same as the previous model, but a quick glance at the control vertices and you’ll understand that we didn’t need any power fill to get to where we wanted to be. And this patch we still have under control, it’s a nice added bonus compared to power fills!

I hope this has been informative! Enjoy your Alias

Kevin De Smet

To Learn Alias Automotive to Industry Entry level has been made easier.

For student’s who have the objective of finding a CAD job in the near future, I still teach One to One students and all of these students get Automotive Modeling or Design/Modeling jobs, so it’s a great investment to make. The hourly rate is very high and there are vacancies in UK. France, Germany, Sweden, Spain, USA, Brazil etc. I can only take a maximum of 5-7 students at any one time. I teach all levels right up to Post Graduates. I have never had a Private student fail to get an Alias Automotive job (Companies include Ferrari, GM, Lotus, Opel, Audi Seat etc. ). If you wish to apply for a place, please email me with all your details.
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Regards

Graham Bullock B.Ed.(CNAA)Des.Tech.