Hi Hypersurfacer:

Thanks for providing such a perfect case-study in the use of "Primary," "Secondary," and "Tertiary" surfaces. Lets see what we can do.

Take a look at the attached sketch. I've marked all of my primary surfaces "P", secondaries "S", and Tertiaries "T".

1. Basically I'd build the primaries first, since they're what we call "driving geometry", or "slabs": they can extend beyond themselves, and they don't have to be tangent to anything in particular. Their geometry should be very clean and light-weight.There are no primaries here that should need to be any more complex than a conic-section surface (degree 2).

2. After the primaries, I'd put trim curves in place. I'd create these by intersecting primaries. I'd then trim the primaries to the resulting intersection curves (NOT to each other, for more robust history). Once the intersection curves are in place, I'd create the rail curves for the secondary surfaces. These are just bridge-curves between the intersection curves. These would probably be G2 in all likelihood.

3. Once all the curves are in place, I'd build the secondaries. These are mostly sweeps in this case, probably bi-rail sweeps, using the primary surface boundary curves as sections.

4. Finally, the tertiary surfaces are put into place.

The important change I made to your configuration was that I did not try to build anything to a "point": even things that appear to be "sharp" in a class-a surface never actually are, as "sharp" corners within a surface lead to bad geometry. Here I've got my tertiary surface going to a flat-edge in the corner, rather to a point. Make sense?

Hope that helps. Let me know if it doesn't!

Adam

Not a bad start, but I think there are still some important issues. I'm working up a video response.

Adam

Adamohern, thanks for that video’s.

I finely completed my model.  Check the surface quality (I think secondary surfaces still have to be improved). The Tertiary surface are created by using the method that you described in second video.

Any suggestion for further improvement?

OK !!

I will fillet the sharp edges (Any advice for that?) & complete the remaining back portion.

Till then see you!

Hi adamohern,

I saw your videos on class A surfacing and came to your videos about the use of Conics to lighten up the CAD data.

Since Conics are essentially 2 degree curves, how then do you achieve curvature continuity? Doesn't curvature continuity require 2 CVs being in line with each other. (That would mean having to use 3-5 degree curves. I'm fairly new to class a surfacing...

Hi Kinetic8:

You are correct: conic surfaces are by definition never curvature continuous, however that can be one of their advantages as degree 3 surfaces introduce the possibility of curvature reversals (aka "ogee"). The advantage of lighter-weight geometry is only one of the advantages, and probably not even the primary benefit of conics: the real advantage is their inability to invert.

I would also point out that even though conics are not mathematically G2, they can be the visual equivalent of G2 in many cases. If the geometry is clean and simple, the difference can be difficult or impossible to tell, and in many cases the conic will appear "cleaner" than the G2 surface anyway.

In short, you are correct. However don't fall into the trap of believing that as a class-a surfacer every single surface must be G5 continuous; that will only make your geometry heavy, difficult to control, and possibly even quite ugly. Surface "quality" and surface "degree" are not necessarily the same thing.

Adam