3-9: Splitting a face with an edge and then extruding one of the new faces.
Drawing an edge that crosses another edge splits both edges where they touch. In this way, you can split simple edges you draw with the Line tool, as well as edges created when you draw shapes like rectangles and circles. Most of the time, this autoslicing is desirable, but if it’s not, you can always use groups and components to separate your geometry. Flip to the first part of Chapter 5 for more information.
Drawing in 3D on a 2D Screen
For computer programmers, drawing 3D objects on your screen is a difficult problem. You wouldn’t think it’d be such a big deal; after all, people have been drawing in perspective for a very long time. If Fillipo Brunelleschi could figure it out 500 years ago, why should your computer have problems?
The thing is, human perception of depth on paper is a trick of the eye. It isn’t really an optical illusion; it just looks like one. And of course, your computer doesn’t have eyes that enable it to interpret depth without thinking about it. You need to give your computer explicit instructions. In SketchUp, this means using drawing axes and inferences, as we explain in the sections that follow.
Contrary to popular belief, modeling in SketchUp doesn’t involve drawing in perspective and letting the software figure out what you mean. This turns out to be a very good thing, for two reasons:
Computers aren’t very good at figuring out what you’re trying to do. This has probably happened to you: You’re working away at your computer, and the software you’re using tries to “help” by guessing what you’re doing. Sometimes it works, but most of the time it doesn’t. The word processing software we all use is a good example. Eventually, the computer’s bad guesswork gets really annoying. Even if SketchUp could interpret your perspective drawings, you’d probably spend more time correcting its mistakes than actually building something.
Most people can’t draw in perspective anyway. Even if you’re one of the few folks who can, you know darn well that most people couldn’t draw an accurate 3D view of the inside of a room if their lives depended on it. Drawing just isn’t one of the things people are taught, unfortunately. So even if SketchUp did work by turning your 2D perspective drawings into 3D models (which it most certainly doesn’t), the vast majority of those who “can’t draw” couldn’t use it. And that would be a shame because building 3D models is a real kick.SketchUp defaults to displaying models in a perspective view because it’s commonly used for architectural designs. Real buildings are usually big enough that we can see the perspective effect. You can always switch to a parallel view (useful for 2D plans and other diagrams), which we cover in “Switching to a 2D view” in Chapter 4.
Giving instructions with the drawing axes
No, drawing axes are not used to turn your design into firewood. See the three colored lines that cross in the SketchUp modeling window? These are the drawing axes, and they’re the key to understanding how SketchUp works. Simply put, you use SketchUp’s drawing axes to figure out where you are and where you want to go in 3D space. When you’re working with the color axes, you need to keep three important things in mind:
The red, green, and blue drawing axes define 3D space in your model. If you were standing at the spot where all three axes meet — the axis origin — the blue axis would run vertically, passing through your feet and head. The red and green axes define the ground plane in SketchUp; you’d be standing on top of them. The axes are all at right angles to one another and extend to infinity in both directions from the origin. In most computer-aided design (CAD) software, the red, green, and blue axes are referred to as X, Y, and Z, respectively.
When you draw, move, or copy something parallel to one of the colored axes, you’re working in that color’s direction. Take a look at Figure 3-10. In the first image, we’re drawing a line parallel to the red axis, or drawing “in the red direction.” You know a line is parallel to the red axis because the line turns red and a little flag pops up to let you know. In the second image, we’re moving a box parallel to the blue (“up”) axis, or “moving in the blue direction.” The dotted blue line appears to tell you so.
The colored drawing axes help you tell SketchUp what you mean. In Figure 3-11, moving the cylinder in the blue direction and the green direction both involve moving the cursor up. The drawing axes help SketchUp know whether you want to move the cylinder up in space (above the ground) or back in space. When you work in SketchUp, you use the colored drawing axes all the time. They’re not just handy, but also what make SketchUp work. They make modeling in SketchUp quick, accurate, and relatively intuitive. As you model, all you have to do is make sure that you’re working in your intended color direction: Line up your geometry with the appropriate axis, and watch the visual cues that tell you what direction you’re working in.
Keeping an eye out for inferences
If you’ve spent any time fiddling with SketchUp, you’ve noticed all the little colored circles, squares, dotted lines, yellow screen tips, and other doodads that show up as you move your cursor around your modeling window. All this stuff is referred to collectively as SketchUp’s inference engine, and its sole purpose is to help you while you build models. Luckily, it does. Without inferences (the aforementioned doodads), SketchUp wouldn’t be very useful.
FIGURE 3-10: Visual cues tell you when you’re drawing or moving geometry parallel to a drawing axis.
FIGURE 3-11: The axes help you create 3D models on a 2D screen.
Point inferences
Generally, SketchUp’s inferences help you be more precise. Point inferences appear when you move your cursor over specific parts of your model. They look like little colored circles and squares, and if you pause for a second, a yellow label appears. The little green Endpoint inference, for example (which appears whenever your cursor hovers over the end of an edge), helps you accurately connect an edge that you’re drawing to the end of another edge in your model.
Figure 3-12 shows the point inferences that you use most often.
