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In this section, we'll review the construction plane capabilities and logic inside Rhino. Note that Rhino refers to the Construction Plane as the CPlane for shorthand, but either term means the same thing. So, what is a CPlane and why do we use it? Well, without the use of the CPlane, you might not be able to model very much of anything, so that's how important it is. The CPlane is where all geometry is created by default, and that reason is important. Without a CPlane, Rhino would not know where you were clicking, because a point or object when viewed from the top- view might be at Z = 0, or Z = amillion.
So, let's take a look and build the few simple objects to demonstrate. So, just like the real-world, the Rhino interface is in 3D. We have three dimensions: X,Y and Z, and that is why we have three construction planes. These construction planes are marked by the grid on the screen. So, there is a CPlane for the Front Viewport, another for the Left or Right side and the third for the Top Viewport, which is also shared by the Perspective. So, remember, you can only see one CPlane at a time in each Viewport.
That's important, so the software knows where the objects will go. The CPlane is your default location for all functions like geometry creation, unless - here's the rule - you have a Snapping Override. For this first demo, I'm going to draw a freeform curve in the top view. We'll talk about curves in the 2D Geometry chapter. So, I'm going to select Freeform curve, and I'm just going to click some random points. So, you may want to check the Perspective View.
Note that it's perfectly flat and planar, and so that is the construction plane governing where all the points go, flat in the construction plane. Now, I'm going to make another freeform curve with one difference. I'm going to turn the Snaps on. These are covered in modeling aids, so I'm just going to turn on the End Snap. Draw another freeform curve on the Right view. So, note in Perspective that it is flat on the Right construction plane, except where it finds some geometry.
I'm going to go ahead and turn this Perspective View around a little bit, highlight the curve. And you can see how it's flat in some areas and then deformed in other. So, even if we drew this in the Right view, a side view, the curve may look exactly as needed; however, it's deforming in other directions as we've seen here. So, the best way to flatten that back out is to use another CPlane command located on the Transform menu, Project to CPlane.
Note that this command works best when the Viewport is highlighted, so in this case, we've got the Right Viewport highlighted, and we want to answer, Yes, delete the Input Objects. Now, note in the Perspective View, that line is now perfectly flat. So, this is a good solution to keep objects flat and planar if you want them, but then snap to other things forward and backwards. I'm going to go ahead and maximize the Perspective View here, and we're going to make some changes to the CPlane.
The starting positions of the CPlanes are just the file defaults. We can make changes to the CPlane at any time by right-clicking on the Viewport label, go to Set CPlane and we'll switch this view from where it is currently at the top to the Right view. We could now build geometry on that plane. Besides the three default positions of Top, Front and Side, we can also align to geometry in the scene. So, I'm going to go ahead and do that with this square here, floating at a funny angle.
Set CPlane, and I'm going to select To Object. So, notice how the construction plane flips over, and that will now allow me to draw a geometry at that same angle, as if it were flat. So, this square is probably a good example. I'm going to show you a bad example. Here's the circle. It doesn't really have an edge. So, if we were to use that for alignment, set the CPlane to the Object, then it goes off at some strange angle.
Probably the best object you can use for setting CPlanes is just a small plane in the scene. Here, I've got a piece of planar geometry. I'm going to set the CPlane to that, To Object. And now I can work up at that level. So, this is probably the best way to reset your CPlane just by having some geometry in the scene that you can visually see, and then it's perfectly aligned the way you want. So, the CPlane is one of the most fundamental modeling aids in any 3D software.
Rhino allows you to change a location and orientation of the CPlane for any view. But remember, you can only have one - no more, no less - CPlanes active per view. So, if the idea of moving or customizing the CPlane is a little too advanced for your comfort level, but it's always a good practice just to build objects centered on the origin. Then you can use one of the Transform commands to get it into its final position.
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