about Edges and Faces
In SketchUp, everything is made up of one of two kinds of elements: edges or faces. They’re the basic building blocks of every model you’ll ever make. It’s kind of like ancient times, when everything was made of just earth, air, fire, and water.
The drawing on the left in Figure 3-1 is a basic cube drawn in SketchUp; it’s composed of 12 edges and 6 faces. The model on the right is a lot more complex, but the geometry’s the same; it’s all just edges and faces.
FIGURE 3-1: SketchUp models are made from edges and faces.
Living on the edge
Edges are lines. You can use lots of tools to draw them, erase them, move them, hide them, and even stretch them. Here are some things you ought to know about SketchUp edges:
Edges are always straight. Not only is everything in your SketchUp model made up of edges, but also, all those edges are perfectly straight. Even arcs and circles are made of small straight-line segments, as shown in Figure 3-2.
Edges don’t have a thickness. This one’s a little tricky to get your head around. You never have to worry about how thick the edges in your model are because that’s not how SketchUp works. Depending on how you choose to display your model, your edges may look like they have different thicknesses, but your edges themselves don’t have a built-in thickness.
Just because you can’t see the edges doesn’t mean they’re not there. Edges can be hidden so that you can’t see them; doing so is a popular way to make certain forms. Take a look at Figure 3-3. On the left is a model that looks rounded. On the right, the hidden edges are visible as dashed lines. See how even surfaces that look smoothly curved are made of straight edges?
FIGURE 3-2: Even curved lines are made up of straight edges.
FIGURE 3-3: Even organic shapes and curvy forms are made up of straight edges.
Facing the facts about faces
Faces are surfaces. If you think of SketchUp models as being made of toothpicks and paper (which they kind of are), faces are basically the paper. Here’s what you need to know about faces:
You can’t have faces without edges. To have a face, you need to have at least three coplanar (on the same plane) edges that are connected. In other words, a face is defined by the edges that surround it, and those edges all have to be on the same flat plane. Because you need at least three straight lines to make a closed shape, faces must have at least three sides. There’s no limit to the number of sides a SketchUp face can have, though. Figure 3-4 shows how faces can disappear when you erase an edge that defines one or more faces. We started with the model on the left and deleted the edge that completed both the top face and one of the side faces. The result, shown on the right, is that both of those faces disappeared.
Faces are always flat. In SketchUp, even surfaces that look curved are made of multiple, flat faces. In the model shown in Figure 3-5, what look like organically shaped surfaces (on the left) are really just lots of smaller faces (on the right). Think of the many small mirrors that make up the surface of a nightclub disco ball. To make a bunch of flat faces look like one big, curvy surface, the edges between them are smoothed. You find out about smoothing edges in Chapter 6.
Just like edges, faces don’t have any thickness. If faces are a lot like pieces of paper, they’re infinitely thin pieces of paper; they don’t have any thickness. To make a thick surface (say, a 6-inch-thick wall), you need to use two faces side by side. In SketchUp, even a solid concrete wall will be hollow.
FIGURE 3-4: You need at least three coplanar edges to make a face.
FIGURE 3-5: All faces are flat, even the ones that make up larger, curvy surfaces.
Understanding the relationship between edges and faces
Now you know that models are made from edges and faces, you’re most of the way to understanding how SketchUp works. Here’s some information that should fill in the gaps:
Every time SketchUp can make a face, it will. There’s no such thing as a “Face tool” in this software; SketchUp just automatically makes a face every time you finish drawing a closed shape out of three or more coplanar edges. Figure 3-6 shows this in action: As soon as a line connects the last edge to the first one, SketchUp creates a face.FIGURE 3-6: SketchUp automatically makes a face whenever you create a closed loop of coplanar edges.
You can’t stop SketchUp from creating faces, but you can erase them if you want. If SketchUp creates a face you don’t want, just right-click the face and either choose Erase from the shortcut menu or tap the Delete key on your keyboard. That face is deleted, but the edges that defined it remain, as illustrated in Figure 3-7.
Retracing an edge re-creates a missing face. If you already have a closed loop of coplanar edges but no face (because you erased it, perhaps), you can redraw one of the edges to make a new face. Just use the Line tool to trace over one of the edge segments, and a face reappears, as shown in Figure 3-8.
Drawing an edge all the way across a face splits the face in two. When you use the Line tool to draw an edge from one side of a face to another, you cut that face in two. The same thing happens when you draw a closed loop of edges, such as a rectangle on a face: You end up with two faces, one “inside” the other. In Figure 3-9, we split a face in two with the Line tool and then extruded one face a little bit with the Push/Pull tool.FIGURE 3-7: You can delete a face without deleting the edges that define it.FIGURE 3-8: Just retrace
