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Surprising as it may seem, there are only five basic geometric forms from which all families are composed. They are Extrusion, Blend, Sweep, Revolve and Swept Blend. In this chapter, we will create a simple example of each one of these forms with the goal of familiarizing ourselves with how each one behaves. Both Solid and Void Forms can be created giving us a total of 10 possibilities. Whether we take an additive strategy and build up our form from a series of smaller parts, or start with an overall form and carve away from it with voids, a so-called Subtractive strategy, is really a matter of personal preference.
Now, before we can begin looking at each of the forms we want to talk a little bit more about Reference Planes. If you followed the exercises in the last chapter, then you know that Reference Planes play an important role in allowing us to build our geometry. They provide the work plane upon which the sketches of each of the forms are created. Now each of the five forms that I just mentioned are all Sketch-based forms. So each of them in some way is going to interact with one or more work planes, which will typically be associated with Reference Planes.
So it's pretty important for us to understand how Reference Planes behave. Now what I have here on screen is just an empty template and I'm using the Furniture category. We're going to talk more about template files in another chapter, so for now we'll just stick with the Furniture Template that we used in the last chapter. There are two Reference Planes to start off on screen as we've already seen. The first one is the Reference Plane: Center Left/Right, and the second one is the Reference Plane: Center Front/Back. Now these two are created by default.
They do have the names on them; you can see the name listed here. They are pinned, and as we talked about in the last chapter, they are set to define the Origin. So we have one in this direction and one in this direction. Now, let me start off talking about the Defines Origin feature. If I create my own reference plane from here to here, select it, scroll down and check Defines Origin, what you're going to see is this one will no longer be defining the origin. So you can only have one Reference Plane in each direction defining the origin and obviously it takes two to really establish an independent point.
If you only do one, it's not quite enough. So at this point, the intersection between these two is the origin of this Family. If I come back and I select this one and I check this box again, now that's the origin point and this one will no longer have that. The next thing I would like to talk about is the direction of the Reference Plane. This is actually pretty important. What I mean by this is, I'm going to create a really simple extrusion in all three views. So I'm going to create one here in the Floor Plan, and that's just a simple box.
I'm going to click here in the Front view and I'll create a cylinder by just extruding a simple circle, and then I'll come over here to the Right view, and just so we can tell them apart, I'll do a Polygon. Now in all cases I accepted the defaults of 1 foot for the Depth, so they're all just one foot deep, but if we look at this thing in 3D what we see is, let's go ahead and Shade this.
What we see is that each one of these was drawn in a different direction, and you can sort of tell what's going on here. This one was drawn relative to the Center Front/Back Reference Plane, this one relative to the Floor, and then this one over here relative to the Center Left/Right Reference Plane. We saw this in the previous chapter as well. When you click into a view and make it active and if you do not go to the Home tab and set to Work Plane, then Revit will just assume a default work plane, and that's why in this case all three of these shapes ended up on a different work plane.
There's no way that you could work in an Elevation view and draw an extrusion that actually extruded up from the floor, because you're actually looking at the edge of that plane so you can't work on that plane. So what Revit does is it looks for the most logical plane that happens to be parallel to the view. Now we also saw that in previous movies that if we name this work plane, I'm just going to name it letter A, that I can come into a view that's actually looking at that work plane such as the Front view here. Go to the Home tab, Set the Work Plane, choose Reference Plane: A, and then draw a form and that form will extrude relative to that work plane.
Now if I switch over here into this view, here is Reference Plane A, here is the form, but now we see an interesting little thing that's happened here. Notice that this form that was drawn relative to the Center Front/Back Reference Plane has extruded to our left relative to this view and in this plane it's extruded to our right. Reference Planes have direction and this is a little bit of a challenging concept unfortunately. In general, it's predictable which direction the Reference Plane is going to go in for the ones that you create yourself, because you're going to click and place the two points that define the Reference Plane.
So if I start back here in Plan view and I go back to my Home tab, click on my Reference Plane tool, if I click my first point and drag to the right I'm going to get one behavior, if I drag to the left I'm going to get a different behavior. In other words, the first point to the second point determines the direction of that Reference Plane. If you imagined that there was a normal direction or a positive direction of that Reference Plane, from the start point to the endpoint, that direction would in this case point up.
So if you were standing at the start point looking toward the endpoint. So imagine that I'm a person standing right here and looking this way, then the positive direction will be on your left. Okay, and if I name this Reference Plane, I'll call it B, click into the Front view, go to my Home tab, click the Set button, I'm going to set that work plane active and then draw another Extrusion. Again I'll just do a simple cylinder; it doesn't really matter what shape.
You're going to see that that rule bears out that I just described to you. So again, imagine you were standing here on the left endpoint looking toward the right point; the extrusion went to the left. Now another interesting little behavior is that I can actually select this and if I decided that the direction is not what I wanted, I can actually grab this little endpoint and start to basically reverse the direction by pulling it the other way. So you notice how once I kind of reversed it on itself and got past the other endpoint here, the form that's on that Reference Plane actually flipped.
So the rule continues to bear out. Now I'm standing here looking toward this direction and the extrusion is still going to the left. Okay, so again just always imagine you're standing at the start point, looking toward the endpoint and the extrusion will go to the left. Let me reverse it again. See how it flipped around to the other side. Another way you can kind of look at this is the label tends to be at the second point; it tends to be at the end you're looking at.
That's the part that's not terribly consistent and I wish that it were. So there is a wonderful Revit blog called Revit OpEd, that's O-P-E-D for Revit Opinion and Editorial, and I have it open right now to an article that's actually a few years old, it goes back to 2006 actually, and it's called, Once Upon a Reference Plane. This is a nice little description of the behavior that we're witnessing right now. It talks about the start point of the Reference Plane, the endpoint of the Reference Plane, and what direction is positive or the normal direction and so forth, and there are even some illustrations down here, and Steve the author of this blog, refers to this as the Tail and this as the Head and this being the Positive Direction, and this is what we have sort of just born out.
And I'm telling you if you stand at the Tail and you look toward the Head, that the Positive Direction will be on your left. I'm just kind of saying it that way because you're not always going to be looking at these things in Plan. sometimes you're going to be looking at them in Elevation or other views. Anyway, this is worth a read. I would recommend that you go to this post. If you have access to the Exercise Files, I've provided a shortcut to this link directly, otherwise go to your favorite search engine and just put in Once Upon a Reference Plane and it will come up with this blogpost. That's definitely a good reference material for you to look at.
What Steve goes on to say at the end of that post is talk about a little anomaly here that's somewhat disturbing is that, if we stood here and looked here and we follow the rule that says that the label is at the end as we've seen with the ones that we drew, okay, there is the label at the end, here is the label at the end, positive direction, positive direction. Well, here's the label at the end. This should've extruded up rather than down because if I'm standing here and looking here, my left, it would've been up in this case and it didn't prove to be the case.
So some of the Family Templates that ship with the software, that default built-in Reference Planes don't really respect the rule. With all the testing I've been able to do is best I can tell is that's only really an issue with some of the built-in Reference Planes. So it's something I want you to be aware of, because as you get serious about creating forms here in the Family Editor, you have to be aware of this issue. The fact that I showed you here that you can actually reverse the Reference Plane is one way that you can correct that.
To reverse one of the built-in ones, you'd have to actually unpin it and flip it around this way, and then you see it flips over, and then I could sort of stretch it back, and then I could repin it. So it is possible to even reverse one of the built-in Reference Planes to kind of correct the problem, but it's something that I recommend that you do first before you build any serious geometry. You really don't want to be reversing the Reference Planes after you started creating geometry because things will move in funny ways. Now you at least have seen what we mean by a normal direction and a positive direction on Reference Planes, you know that that matters and that helps you control where the positive extrusion or depth is going to be, because otherwise without that you'd have to actually make this a negative, which is another opportunity that you have; you can actually use a negative depth.
So there are a few different ways to deal with the issue, but now you're at least aware of what the Reference Planes do in that behavior and with that we'll be able to move on to the next several movies here where we talk about each of the individual forms.
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