Animation Along a Path for Silverlight

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Introduction

Silverlight provides a good animation foundation, but lacks a convenient method to animate an object along an arbitrary geometric path. Windows Presentation Foundation, which is a superset of Silverlight, provides the animation classes, DoubleAnimationUsingPath and PointAnimationUsingPath. With these classes, it is easy to animate an object along a geometric path. This article provides the equivalent animation classes for Silverlight. I suspect future versions of Silverlight will provide this functionality, but until then, the code provided here will do the job.

Background

Currently, Silverlight does not provide animation classes for animating an object along an arbitrary geometric path, but Silverlight does provide key frame animation classes. With the key frame animation classes, you specify beginning and ending values, and then the animation class uses interpolation to calculate all the values in between. A collection of key frames can be assembled together to describe a motion path, but manually building these key frames for each motion path is tedious.

To programmatically generate the key frames describing the motion path, a method to ‘flatten’ the geometric path is needed. Flattening basically means the geometry is described by a series of line segments. The figure below shows an example of a flattened Bezier segment. Once we have the flattened path, then we can easily build the key frames for the storyboard.

About the Code

The heart of the code is contained in the ‘PathAnimation’ project. In this project, the abstract BaseAnimationUsingPath class defines a number of dependency properties to control the animation (see below). Notice that this class inherits form DependencyObject rather than a Silverlight animation class. This is because most of the animation classes in Silverlight are sealed, and not extendable like the WPF animation classes. This has implications for the usage of the DoubleAnimationUsingPath and PointAnimationUsingPath animation classes, like they cannot be added to a Storyboard! They can be added as a Resource or created in the code-behind (example provided in the demo app).

The other important project here is ‘PathGeometryHelper’. This project implements the path flattening routine. Charles Petzold developed a path flattening method in his article and gave permission to reuse the code. Charles’ method was written for WPF, and so it needed to be ported to Silverlight. In porting this code to Silverlight, more missing functionality was found. Namely, Silverlight lacks some methods in the Matrix class. Also, Silverlight has no Vector class. Equivalent functionality for these classes (and a few others) has been provided in the ‘MatrixMathHelpers’ project.

An important point to make about the flattening routine is the ‘Tolerance’ parameter. This parameter has been exposed all the way up to the path animation classes. Basically, this parameter controls the number of segments used to approximate the original geometry path. The tolerance value must be greater than zero. A smaller tolerance value will yield greater accuracy (i.e., more line segments), and a larger value will yield less accuracy (i.e., poorer approximation of the geometry path). It is not recommended to make the tolerance too small because this will cause more key frames to be added to the storyboard, slow down the animation, and use too much memory. Play with the demo app and see how adjusting the tolerance affects the accuracy, but be careful not to set the value too small!

Another important piece of code is contained in PathConverter.cs. Currently, Silverlight has no built-in functionality to convert the mini-language string into a PathGeometry, but thankfully, a converter has been provided and included here in PathConverter.cs. With this handy converter, we can feed the animation classes arbitrarily complicated geometries created with Blend or other tools.

Using the Code

Two classes have been provided to animate an object along a geometric path: DoubleAnimationUsingPath and PointAnimationUsingPath. Both share a number of common dependency properties. These properties are equivalent to the Silverlight provided animation properties, so they should be familiar:

BeginTime: The time when the animation should start
Duration: The duration/length of the animation
AutoReverse: Reverses the animation at end when true
FillBehavior: Holds the value at the end of the animation, or resets to the value before the animation started
RepeatBehavior: Number of times to repeat
PathGeometry: Specifies the geometry that the object will follow when animating
TargetProperty: The property to animate
Target: The name of the object to animate
Tolerance: Controls the accuracy of the path flattening; must be greater than zero; smaller values give more accuracy

In addition to the properties above, all of the standard animation methods: Begin, Stop, Pause, Resume, and Seek have been provided.

DoubleAnimationUsingPath will only animate object properties of type double (e.g., height, width, X, Y, etc.). This animation class has one additional dependency property: Source. The Source property specifies to use either the X or Y output value of the PathGeometry to drive the animation.

As discussed in the previous section, the animation classes provided here cannot be used in a Storyboard like other Silverlight animation classes. They can, though, be added as a Resource or created in the code-behind (see the demo app for an example).