As described earlier, the Facets animation system provides a program controlled means of changing object attributes over time. In this section we’ll provide a more in depth look at the various parts of the animation system, which can be divided into the following topics:
The animation system defines a number of animation classes that do all of the actual work needed to animate facet values over time. These classes are at the core of the animation system and perform such tasks as:
An animation consists of modifying the value of an object facet over time, starting with an initial begin value and ending at a final end value. The entire set of values that are sequentially assignable to an object facet during an animation cycle is referred to as the animation’s path. The animation system provides a number of path classes that compute the set of path values given the animation’s begin value, end value and current time.
There is more than one path class because there are many ways to produce a set of intermediate values from any given pair of begin and end values. In most cases, how the intermediate values are calculated depends upon the type of the begin and end values. A path for a pair of floating point values is probably very different than the path for a pair of strings.
The animation system provides a useful collections of path classes that handle many of the most common animatable value types, such as:
You can also create your own custom path classes to handle additional data types and algorithms for computing intermediate values.
As just described, a path defines the set of intermediate values that can be assigned to an animated facet as it traverses from its begin to end values. A tweener object, on the other hand, manages the flow of time during a facet’s animation.
Think of a movie. A movie consists of a series of individual stills, or frames, which are shown rapidly and sequentially to achieve the effect of continuous motion. Using our animation terminology, we can think of the collection of individual still images as forming our animation path. We start by showing the first still image, then the second, and so on until we have shown the last image, at which point the animation is complete.
Now think of some of the common special effects such as slow mo’ and fast forward we are so used to seeing when watching a movie. In essence, these special effects are actually affecting the flow of time as we sequence through the individual images forming the movie. A slow motion effect reduces the rate of flow of time for a certain period, making the length of time each individual image is shown longer than it would normally be (metaphorically speaking, since the technical details of doing this are actually quite different). It’s still the same set of images (or path values) being shown, but at a different rate of flow of time, since some images are shown for longer periods of time than others.
In essence, tweener objects allow us to add time-related special effects to an animation by varying the rate of flow of time from the beginning of the animation to the end of the animation. The same set of path values are still being used, but the rate at which the animation moves from one value to the next can vary over time.
Now, an astute reader might wonder why a path object couldn’t perform the job of a tweener as well, handling both the task of computing the intermediate values and managing the time at which they are used. And the answer is, of course, that it could, but the resulting system would not be as flexible or easy to use.
Consider our movie example again. Image a scene where the protagonist is jumping onto the roof of a moving train. For dramatic purposes we want to show the jump and landing in slow motion. Now we could try to film the sequence in slow motion by creating a system where the actor or stunt man is in some kind of a motion control harness that moves him or her very slowly onto the roof of a barely crawling train. This might work, but the results would probably be very unsatisafactory and expensive to produce. The more likely scenario is that the stunt man just jumps onto the moving train with cameras rolling and they add the slow motion effect in post. Much simpler and more cost effective.
By separating the path from the rate of flow of time, we simplify the process of creating paths and open up more possibilities for creatively using them in animations through the application of different types of tweeners.
The animation system includes a number of the most common types of tweeners:
You can of course create your own tweener classes to handle any custom animation requirements you might have.
Although Facets animation can be performed using just animation, path and tweener objects, the core HasFacets class defines a small set of helper methods that make it easier to create and manage animations for attributes on any Facets-based object. The helper methods allow you to:
With these introductory descriptions in mind, you can either continue on and read through all of the animation topics in order, or jump to a section of particular interest using the following links:
