Join Dave Schultze for an in-depth discussion in this video Comparing isocurve surfaces and mesh surfaces, part of Rhino 4 Essential Training.
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In this video, we'll define and review the visual terminology relating to NURBS surfaces, which includes the especially confusing term Mesh, because it is used to describe two totally different things. We'll also define an Isocurve, and where it is communicating to us. So, to get started, I've taken the NURBS surface form the earlier video, and built this lovely boat. Notice that we have curves on the surface, and these curves always cross at 90 degrees. These are the isocurves. They are used as a visual feedback, and are controlled by a Display setting.
So, I can turn on this sail form, and in the Properties dialog on the right side here, you just crank up the density. So, we get more isocurves on the surface, but is the exact same surface with no more complexity. To find its true complexity, you can go back down to the Density of the default, which is 1, so most objects are created and displayed. So, remember, just like in NURBS curves, the NURBS surface is infinitely smooth.
It can be said that there is an infinite number of these isocurves on there. We don't need that many to get to visual feedback; just a few are enough to do the job. Also, what we are seeing is a representation of the surface, such that the video card can show us this Shaded view. I want to contrast it with the Wireframe view here, right-click on the View port label. So, without shading, this is what all of our surfaces might look like, and it wouldn't be nearly as fun, or helpful to work this way.
So, we will be typically in Shaded view, and why I mention this is the software and video card are showing you a representation of the surface, but that's overkill. We don't need to see an infinitely smooth surface. So, we are actually going to see a Screen Mesh is what it's called, which is on approximation that will be good enough for us to be able to move around in space. That Screen Mesh is where the confusion kicks in, because there is another Mesh, which is more of a Geometrical Mesh, and this is an entity that comes in from typically other software or file formats when they've been converted.
Let's take a look at a Mesh surface. Here, you can see that difference, right off the bat. We have individual facets connected on their edges. Typically, we'll see a lot of rectangles or triangular polygons, and this surface is technically editable, although I probably wouldn't try. Let me show you exactly why I say that. I'm going to turn on the control points for both of these entities. Here is the NURB surface, and the Mesh. I'm going to use the control point, turn them on, or you can hit F10.
Let's start with a NURB surface. We'll grab a couple of these points here at the top, just going to lift them up. To move the control points, I'm going to use the Alt+any of the four arrow keys to move. Later on, I'll show you how to change that default setting, so you can use just those arrow keys. Maybe I even nudge them out to show some wind blowing, great! Now let's try to edit the Geometrical Mesh, grab a couple points there and do some similar movements. And check it out! We've got basically a big disaster.
That is not anything that you can probably use. And if you would have tried to smooth it, that would be a big waste of time. So, this Geometric Mesh is typically used for reference, when people have exported file formats, and you just want to model around them or rebuild then in a NURBS format. Let's also talk now about Isocurves, and their density. I mentioned that the surface is infinity smooth, and the isocurves don't always tell you about the complexity, but sometimes they do. So, we have an example here of a surface with a lot more isocurves.
The reason is we have generated them from curves that had a large number of control points. So, I'm going to go ahead and select the curve, turn those on. We'll turn everything off first. Select these two. Turn the control points on. And you can see along this edge it was obviously built from a curve that was extremely complicated, and unnecessarily so, because it has these points are so tightly spaced, it makes it fairly unusable, when this four points along the top edge does the job just fine.
So, the Isocurves show you the level of surface complexity, especially at their default setting of 1. That's always a good thing to check for when you first generate the surface. Ask yourself, 'Does it look too dense?' If so, always make sure that your surface is as simple as possible, which means, was it created from the fewest number of curves and control points? When dealing with NURBS and curves, simpler is always better.
- Understanding 3D terminology
- Viewing a 3D model in Rhino 4.0
- Manipulating objects with commands
- Creating curves, surfaces, and solids
- Applying transformations to 3D objects
- Creating unique shapes with Boolean operators
- Snapping to objects and planes
- Defining curve and surface degree
- Prototyping a 3D model