In general, transforms are operations that alter the position, size, or orientation of an object. Such basic transforms as Move, Scale, and Rotate are essential to any modeling task, because you have to be able to adjust the position and orientation of the separate objects to make a scene.
Every program uses a different interface for controlling transforms, but many of them involve having some sort of gizmo appear inside or around a 3D object when it is selected. The gizmo usually includes a visual axis indicator representing the local axes of the object and its pivot point, and has some sort of handles you drag with the mouse to perform the selected transform.
Transforms may be affected by axis locks or axis constraints, which are controls in the software that enable you to turn off movement along the X, Y or Z-axis, or any combination of them. Axis constraints enable you to transform objects along the desired axis only, which prevents accidental movement in unwanted directions.
Because so many things affect axis orientation, what is the correct axis one minute might be wrong the next. So, if your object stubbornly refuses to move, rotate, or do any other transform in the desired direction, check to see whether axis constraints or some other types of locks are active.
The current coordinate system (view, world, local) in use can have a big impact on transforms. Transforms may also be affected by the pivot point location, because they use it as the center of the transform operation. The pivot point may be centered in the object or offset into the boonies somewhere. Usually an indicator of some kind shows you whether the program is set to use the object's pivot point or whether it is using one that the software sets in the center of the object.
It comes as no surprise that Move relocates objects, allowing you to place shapes and objects anywhere in the 3D universe. In most cases, this is done with the mouse, but many programs also provide a way to enter this data numerically for precise adjustments.
Move may be affected by the current coordinate system and axis constraints only, unless inverse kinematics (see Chapter 9, "Animation") are in effect. The pivot point setting has no effect on Move.
Rotate makes an object revolve around the selected axis. The tricky part about Rotate is making sure everything is set up so that the object revolves in the way you want. For example, do you want to rotate using the Screen, World, or Local axes? Which pivot point are you using, and is it centered in the object, or offset somewhere? The pivot point of an object is located at the junction of its local axes, similar to the way the origin point resides at the center of the three world axes. When you rotate the object, it revolves around this pivot point. Your 3D software will probably have some sort of controls for defining multiple pivot points and selecting which pivot point you want to use for a given Rotate operation.
Imagine you were making a simple model of the solar system. For the sphere representing Earth, you would want to set the pivot point at its center and rotate it around the Y (up/down) axis, so it spins around more or less like it should. (Ignore the fact that the Earth is tilted slightly off-axis for the moment.) In addition to spinning around like a top, the Earth also has to orbit around the Sun, which means you also need to have another pivot point in the center of the sphere representing old Sol. When you rotate around that center point, the Earth sphere will move in a broad circle around the Sun sphere.
When doing rotation or other transforms, make sure your software's visual axis indicator is turned on, if it has one. This will tell you where and which way the axes are located before you do the rotation. As you would expect, the axis you choose to rotate the object around has a dramatic effect on the results (see Figure 3.26).
FIGURE 3.26 Rotate revolves objects around the desired axis. (a) The base object. (b) Rotation around the X-axis. (c) Rotation around the Y-axis. (d) Rotation around the Z-axis.
Use Scale to adjust the overall size of an object. Like other transforms, the results of a scale operation may vary according to the coordinate system, axis constraint settings, and pivot point. For example, if the X-axis is the only one active, a scale stretches the object horizontally only. If all three axes are active, scale re-sizes the object in all directions (see Figure 3.27).
FIGURE 3.27 The Scale command re-sizes objects along the desired axes. (a) The base object. (b) Scaling the X-axis. (c) Scaling the Y-axis. (d) Scaling the Z-axis. (e) Scaling all axes simultaneously.
If the scale operation is set to use the object's non-centered pivot point, the scale will transform the object toward or away from that point. For example, if the pivot point is located on the left face of a cube, a scale operation will leave the left face in the same position while scaling all of the other faces away from it.
The transform command Mirror either reverses an object or copies a reversed version of it along the selected axis (see Figure 3.28). Some programs enable you to select multiple axes to mirror around as well. Mirror may be affected by the coordinate system and axis constraints.
FIGURE 3.28 Effects of mirror: (a) The object is selected, and a mirror axis defined. (b) If the user elects to Mirror-copy the object, a second reversed version is created.
In addition to making reversed copies of entire objects, Mirror can be a great help when dealing with complex symmetrical objects, like human faces. If you work with only the right or left half of the face when editing, you reduce the clutter of having all those extra vertices. When you want to check your work, Mirror-copy the face to create the missing side.
Align enables you to bring object surfaces flush with each other or center multiple objects along one or more axes. Align is great for getting objects lined up the way you want them without tedious zooming and repositioning. Align is also useful for quickly bringing an object into the appropriate area of a scene if it has been accidentally created or imported into some obscure corner of 3D space. Align may be affected by the axis constraints.
There are quite a few different Align settings, depending on your software. The basic ones are Align Center, Align Left, and Align Right, which do exactly what they say (see Figure 3.29). The alignment can take place on one or more axes, and some products enable you to align to any object, face, edge, or vertex to another.
FIGURE 3.29 Typical Align types: (a) The base objects are currently centered on the Z-axis, and the bottoms are aligned with each other. In the following examples, the small cube will be aligned to the larger one. (b) Align Center, on both the X- and Y-axis. (c) Align Left on the X-axis. (d) Align Left on the X-axis, plus Align Center on the Y-axis.