An animation authored in Blend contains keyframes. The animation defines which element/property to affect and each keyframe defines a key time at which a value should be in effect. The process of animating the property values between successive key times is called interpolation.
Let’s say you’re translating an element from the top of a Grid to the bottom to represent a falling motion. With the element at the top of the screen you drop a keyframe at time 0. Then you move the playhead to time 1 and drag the element to the bottom of the screen. The interpolation between the two screen positions will be linear by default: the element will appear to fall at a steady rate.
If you want the motion to resemble a real-world falling object then you want the element’s speed to be slow at the beginning and gradually speed up towards the end. This kind of interpolation is known as ease out because the element’s position is easing out of its original value on its way to the new value. Similarly there exists an ease in type of interpolation which can either be used alone or combined with ease out. To apply full ease out to the animation you right-click the first keyframe and click Ease Out > 100%.
If you want more control over interpolation then we need to talk about key splines and you’ll need to tweak some XAML. If you followed along with the ease in walkthrough above then switch to XAML view and find the storyboard which contains your animations and key frames. There are two animations: one targeting TranslateTransform.X and one targeting TranslateTransform.Y. Take a look at the key frames in the Y animation. The second keyframe has a KeySpline attribute which is a string containing the coordinates of a pair of points – let’s call them x1,y1 and x2,y2 respectively. The secret to understanding and controlling keyframe interpolation lies with these coordinates and what they represent.
In the sense of the KeySpline attribute, a spline is a curve of value over time (drawn between implied coordinates of 0,0 and 1,1) whose shape is defined by two control points. The KeySpline attribute contains those control points. If you’ve drawn curves with Blend’s Pen tool then you may know such control points as tangents. When you omit the KeySpline attribute, its control points default to the coordinates 0,0 and 1,1. Here’s a visualization of the resulting key spline:
When the control points (shown in red) coincide with the start and end points of the curve, as they do by default, the curve is actually a straight line. This is why, unless you explicitly set ease in or ease out, Blend will give linear interpolation between keyframes. The unit square key spline is scaled to the actual value and time deltas of the keyframe which defines it. Here’s what the key spline looks like when ease out is set to 100%:
The first control point (x1,y1) has moved to 0.5,0. If you now set the ease in value of the second keyframe to 100%, the second control point (x2,y2) moves to 0.5,1 and the resulting visualization looks like this:
To be clear, the KeySpline values x1,y1,x2,y2 for keyframe n correspond to ease out for keyframe n-1 (first control point: x1,y1) and ease in for keyframe n (second control point: x2,y2) respectively. This means that, for a handoff animation, you can still define an ease out for the non-existent keyframe at time 0. When you first build the animation, drop a keyframe at time 0; then right-click it to set ease out; then delete it. The ease out value will be safely stored in the x1,y1 values of the second keyframe’s KeySpline.
The control point coordinates can be set to any pair of double values between 0 and 1. In this example the first control point is at 0.1,0.5 and the second control point is at 0.8,0.33:
Try this key spline out in your application. Replace the contents of the KeySpline attribute on the second keyframe with the control point coordinate string “0.1,0.5,0.8,0.33”. Build and run and see if the resulting motion matches the graph. Remember that the graph is showing an increase in the property which is being animated. In this case the property is TranslateTransform.Y and Y increases down the screen in WPF.
In fact, the control points for a given curve are identical whether the value is increasing or decreasing. But it’s easier to imagine them on a graph with the value axis flipped (so the origin moves to the top-left and positive y is downwards). Here’s an example where the value of keyframe n is less than the value of keyframe n-1 but y1 and y2 have the same positive values as the previous example:
You can use Blend’s Pen tool to draw graphs like the ones shown. If you want more complex value curves than the ones shown here then just draw more segments. Adjust the control points (tangents) so that the interpolation curve between each value is how you want it. Then imagine (or draw) the dotted unit square around each pair of values and use it to call off the control point coordinates in the range 0,0 to 1,1 and build a KeySpline string from them. It’s important that your control points don’t fall outside the dotted rectangle. For a value curve, control point coordinates outside the valid range mean either that the curve has a turning point (a keyframe value is missing) or that time is decreasing somewhere on the curve.
In the next post I’ll share a tool, based on Blend, that you can use to generate double animations from a set of value curves (drawn with the Pen tool) and parameter objects.