Objects can be animated in many ways. They can be animated as Objects, changing their position, orientation or size in time; they can be animated by deforming them; that is animating their vertices or control points; or they can be animated via very complex and flexible interaction with a special kind of object: the Armature.
In this chapter we will cover the first case, but the basics given here are actually vital for understanding the following chapters as well.
Three methods are normally used in animation software to make a 3D object move:
Key frames Complete positions are saved for units of time (frames). An animation is created by interpolating an object fluidly through the frames. The advantage of this method is that it allows you to work with clearly visualized units. The animator can work from one position to the next and can change previously created positions, or move them in time.
Motion Curves Curves can be drawn for each XYZ component for location, rotation, and size. These form the graphs for the movement, with time set out horizontally and the value set out vertically. The advantage of this method is that it gives you precise control over the results of the movement.
Path A curve is drawn in 3D space, and the Object is constrained to follow it according to a given time function of the position along the path.
The first two systems in Blender are completely integrated in a single one, the IPO (InterPOlation) system. Fundamentally, the IPO system consists of standard motion curves. A simple press of a button changes the IPO to a key system, without conversion, and with no change to the results. The user can work any way he chooses to with the keys, switching to motion curves and back again, in whatever way produces the best result or satisfies the user's preferences.
The IPO system also has relevant implication in Path animations.
The IPO block in Blender is universal. It makes no difference whether an object's movement is controlled or the material settings. Once you have learned to work with object IPOs, how you work with other IPOs will become obvious. Blender does distinguish between different types of IPOs. Blender concerns itself only with the type of blocks on which IPOs can work. The interface keeps track of it automatically.
Every type of IPO block has a fixed number of available channels. These each have a name (LocX, SizeZ, etc.) that indicates how they are applied. When you add an IPOCurve to a channel, animation begins immediately. At your discretion (and there are separate channels for this), a curve can be linked directly to a value (LocX...), or it can affect a variance of it (dLocX...). The latter enables you to move an object as per usual, with the Grabber, without disrupting the IPO. The actual location is then determined by IPOCurves relative to that location.
The Blender interface offers many options for copying IPOs, linking IPOs to more than one object (one IPO can animate multiple objects.), or deleting IPO links. The IPOWindow Reference section gives a detailed description of this. This chapter is restricted to the main options for application.