Keyframe Interpolation

Things can be large or small, slow or fast, serious, or humorous. They fly, fall and bounce. Real life objects have unique narratives and concepts of motion. Revealing their true character is the key to a powerful animation. In this tutorial, you will get to know how to refine animations by controlling keyframe interpolation, the process in which Fable fills in the values between keyframes.

We will start by diving into two core animation principles: timing and spacing. Timing is how long it takes for something to happen. It is a number of frames between two poses or two keyframes. In this example, we have a ball moving from left to right in one second or 24 frames.

Spacing is the action that happens in between keyframes. It is the distance that an object travels between one frame and the next. If this interval is the same between all of the frames, the object moves at a constant speed. The smaller the distance between each frame, the slower the animated object appears to move. The larger the distance, the faster an object appears to move.

Fable fills in the values between the two keyframes automatically. It also assigns quad interpolation to both points of the action. You can observe this process on a motion path, where every vertical line represents the ball's anchor point position at each frame. In this case, the ball slowly starts moving and accelerates and slows down towards the end.

Slow ins and slow outs soften the movement, making it feel more organic. Spacing between frames in Fable can be controlled by adjusting the curves on the value and the velocity graphs in the easing panel. The results will then be displayed on a motion path or the graph between the affected keyframes.

Here's how you can start adding different easings in Fable. Let's animate the ball moving from the left of the scene to the right.

When you have your keyframes set up, select the keyframe that represents the starting point of the action. The easing panel will now appear on the right side of the interface. The first graph displayed here is a value graph. It shows the progress of the value over time. The speed is defined by the steepness of the curve.

If it is flat in the beginning, that means that the action is slow. And when the curve becomes steep, that's where the object picks up speed and moves fast. Towards the end it slows down again.

You can change the graph type by pressing velocity. The velocity graph shows the speed over time. The higher the graph goes, the higher the speed. In the case of quad interpolation, the animation starts at zero and then picks up speed. The peak of the curve represents the point at which the action is happening at the highest speed. Then as the curve goes down, the object slows down until it reaches a complete stop.

The line between keyframes in the timeline is a direct reflection and a simplified version of the velocity curve.

Let's look at a quick overview of the interpolation presets in the easing panel and their representation on each of the curves. The list starts with a hold preset, which freezes the value of the animated property until it reaches the next keyframe and changes immediately. Linear keyframes create a uniform rate of change.

Action starts and ends with the same speed, and the spacing on a motion path is identical between each dot. This type of interpolation creates a mechanical and rigid appearance, therefore on most occasions it's best to be avoided.

Next we have several classical S shaped curves. Here, the rate of change gradually accelerates after the first keyframe, and then decelerates into the second keyframe. The further down you go in the preset list, the more drastic the relationship between the acceleration and deceleration becomes for each preset.

There is a back preset that creates anticipation for the action and overshoot, which means that for a few frames an object will go past this last pose and then ease back into it. The last two presets create bounce and elasticity. They function best when applied as ease out.

So, what do the ease-in, ease-out and ease-both options really mean. Let's take a look at our animation with the quad preset selected in the value graph to better understand what each of the options means. " Ease-in" sets our animation to accelerate gradually starting at the first keyframe and reaches the next keyframe with a linear speed.

"Ease-out" sets our animation to begin at full speed starting at the first keyframe and gradually decelerating to a smooth arrival at the next keyframe.

"Ease-both" sets the action to start slowly reaching the highest speed in the middle of the animation, and then decelerating before reaching the next keyframe.

You can take complete control of your animations by manipulating the curves manually. In the value graph, simply click and drag a handle to shape the curve. If you hold shift, the handle will snap to the points where the graph lines overlap. In the velocity graph section enable quick influence and adjust the numeric values for the in and out points separately or constrain them so that they share the same value.

If you have set the easings on one object and you're happy with its smooth and fluid transitions, you can copy and paste those settings onto other layers. Even if they have different animated properties like scale or rotation. By doing this, you're not only saving time, but also keeping the movement consistent throughout your entire animation.

And lastly, a word of advice is to always play your animations before jumping into Fable. Think of the emotions you would like to movements to communicate. Don't forget basic physics and explore the movements in the real world. Even the smallest of adjustments to the motion between keyframes can make all the difference.