When I first started working with WPF and Silverlight, I was amazed at how simple it was to create animations, especially when using Expression Blend. I was always frustrated with one thing though... they just didn't look very professional. I could edit the speed of my animations, but the interpolation of the values in the animation was always linear. That is, if I had an animation that changed a button from size 1px to size 10 px, which lasted 10 seconds, it would cause the button to change at a rate of 1 px per second. This linear interpolation frustrated me. I wanted some of my animations to ease in, or ease out. For instance, when I Show a pop-out panel, I typically like to make the animation occur really fast at first, and then slow down towards the end. Like in the button example, I would like the button to grow to 8 px in the first 3 seconds, and grow from 8 px to 10 pix in the last 7 seconds. This tends to make the animations look more professional.
So... how do you do it?
First, create your animation in Blend. Once you have your animation (storyboard) created, you should see something like this in your Objects and Timeline panel. In this example, I have applied an animation to a grid named grdSelectDate which takes 1 second to complete. This is indicated by the little egg-shaped dot placed in the timeline next to the grdSelectDate grid.
If you click on this dot, it will change colors from white to gray. This now gives you the ability to use the KeySpline editor.
You can see the editor under the Properties tab. You can think of this editor like this. The X axis represents the value of the affected property. In my example, it would represent the size of my button (this is a made up example... the size would actually be two different properties.... height and width or a single scaleTransform property... you get the idea though!). When X = 0, the value is at it's original state, so the size of my button has not changed, when X = 1 (top of the scale), the value has now reached it's final value (10 px in my fictional example). The Y axis is a function of time. Y=0 is the start of your storyboard and Y=1 represents the end of the storyboard (10 seconds in my button example).
To edit these values, you can simply click and drag on the yellow dots or enter values in the textboxes below(not as easy...) to change the speed of easing in/out of your animations. In this example, the animation starts really quickly (indicated by the steep incline of the line) and then slows down towards the end (indicated by the very small change in the x value while the y value keeps changing).
Keep playing around with this graph to ease in and out and you will see your WPF and Silverlight applications kicking it up a notch in no time!