Here is a more detailed problem showing a common need to have a radius go to zero. Rhino is iffy with nurbs in this kind of construct. Here, the two upper edges should be tangent and the lower edge coincident…that requires the radius at the right to go to zero.
One consideration would be for the subd and resulting Nurbs surface to have tangency with an existing nurbs surface. I’m not quite sure what approach Rhino would take here but Cyborg3D (formerly PowerShape) has had a similar feature for quite a few years.
For the museums we are all using Rhino. The museums need to know the structure for plugging leaks, They need to create visitor access that satisfies fire marshals. They don’t want to cut an visitor opening through a bulkhead and find 11 inches of steel behind.
Since we are dealing with reality, we don’t have the luxury of simple models we can tweak by hand. We start from point clouds that define reference lines. To do the hull one needs to star with curves along the outer edges.
However, the data points are rounded to the nearest 1/16" (assuming no error). This produces steps rather than smooth curves. Thus one would ideally like a way to draw a curve through points within a tolerance.
The problem with CurveThroughPt is that if you use “interpolated” you get steps. If you use Control Points, the curve goes way out of tolerance at the sharper curves, such as at the bow and stern.
I have seen that video but four-sided shapes to not tend to create a good fit for moving to zero radius. Sweep2 to point gives the best shape (I’ll have to pull some specific examples). The cut away technique does work well in the general case of a triangular area.
A low density of data points. Because the points are at elevations and the hull moves inward here so the point density is lower. With Rhino primitives the hull tends to get deformed in this area without extra work. Fortunately, there are other data sets available for this hull.
This goes contrary to the standard advice of using as few points as possible. That’s find for a car hood but here we are dealing with a surface that is going to be 70-feet long. Single span curves here? Furgitabowtit.
The natural triangle.
The “right angle” where the radius of curvature gets
But it takes an odd patchwork of surfaces. In this case I was able to get a four-sided surface to fit where the radius goes to zero. However, I have found places were I have been completely unable to do so and have had to resort to 3-sided and work through the bad surface errors.
right, and that will be a complex nurbs model solution, for a complex modeling problem.
Subd is not going to be precise enough for what you are trying to model here.
you could do it in SubD, but not to the level of precision you are looking for.
you may be able to do this more “button click” with a product like xnurbs.
we are working on variable creasing for SubD, and when that becomes available shapes like this will be easier to create. However, it will still have the same limitations like subd does already where it does not do “exact” circles etc…
It would be nice to have a radius to zero with NURBS. I would presume such an animal is possible. For some reason Rhino likes to have surfaces to a point bend inwards towards the center of the curve rather than outward.