In the real world, objects never start or stop moving instantaneously―a car will gradually pick up speed and then slow down before coming to a halt.
The Slow In and Out principle replicates this natural acceleration and deceleration to match basic physics and create more engaging animations.
But how does an animator use this principle in practice? In this article, we give you five actionable tips to better implement smooth transitions. You'll discover how to manipulate ease curves, optimize secondary actions, and use audio cues to amplify your storytelling, motion trails, ghosting, and precise keyframe placement.
Why Slow In And Slow Out?
Transitions look smoother when the frames are gradually spaced closer together at the movement's start (slow in) and end (slow out).
The pacing of your transition is a great storytelling tool: a character who slowly raises their eyebrows before they widen quickly in shock uses this principle to underscore the surprise element.
1. Master Ease Curves in Animation Software
Ease curves, also known as motion graphs, represent how the speed of an animation changes over time.
By understanding and manipulating these curves, animators gain precise control over the acceleration and deceleration of their animated objects to make them look more natural.
This is particularly useful in creating motions that mirror real-world physics because objects rarely move at a constant speed: they accelerate and decelerate gradually, influenced by forces like gravity and inertia.
If you were to animate a bouncing ball without adjusting ease curves, the ball might move up and down at a constant rate, losing its sense of weight. But you can adjust bezier handles on the animation curve by using the graph editor within your animation software to create a gradual slowdown as the ball reaches its peak height, simulating the natural deceleration due to gravity. Similarly, by accelerating the curve as the ball descends, you can mimic the ball picking up speed as it falls back to the ground. These details make all the difference.
Different types of ease curves produce varying effects on animation. There are three main categories:
- Ease-in curves create a slow start and a faster finish, which is perfect for objects coming to life or launching from a standstill.
- Ease-out curves facilitate a fast start with a gradual end for simulating objects coming to a rest.
- Ease-in-out curves combine both principles for smooth starts and finishes.
You can then have slight variations of each category like sinusoidal, cubic, or quadratic curves for various effects. And of course you can use your DCC tool to create a custom curve manually:
2. Focus on Secondary Action
Secondary actions are smaller, complementary motions that add depth and nuance to the primary actions in a scene.
When applied to secondary actions, the slow-in-out principle ensures that these motions blend smoothly with the primary actions.
Let's take the example of a character waving. The primary action might be the arm moving back and forth, driven by the character's intention to greet someone. The secondary action could involve the subtle swaying of the character's ponytail, a gentle rise and fall of the shoulders, or the slight shift of weight from one foot to the other to signal excitement. These secondary actions should ideally begin with a subtle, gradual build-up (slow in), reaching a peak motion in harmony with the primary action, and then gently retreating (slow out).
Slight variations are sometimes necessary for stylistic or narrative purposes: if the animatic calls for a comedic effect, the secondary actions could exaggerate the ease or lack thereof, creating a more playful or exaggerated motion to fit the tone.
3. Synchronize with Audio Cues
Another effective way to improve the impact of your ease in and out transitions is by aligning them with audio cues.
These audio cues can underscore emotional beats, build anticipation, or provide comedic relief.
And when animations are synchronized with these audio cues, they can highlight and accentuate important movements and transitions within the animation, to make sure viewers feel the moment.
In a scene where a character is leaping off a cliff, you create a sense of propulsion by applying slow-out as the character pushes away from the edge. As the character soars through the air, the slow-in, when they begin to slow toward the apex of their jump, can be accentuated by a musical crescendo.
4. Use Motion Trails and Ghosting
Motion trails and ghosting offer animators a visual representation of the path of action to iteratively refine the easing in their animations.
Motion trails are particularly useful to visualize the flow of a moving object. They provide a line or curve that showcases the path through which an object travels to convey information about its velocity and trajectory.
Ghosting, on the other hand, creates a series of semi-transparent frames that represent an object's past or future positions so that animators can see multiple stages of motion simultaneously and get a comprehensive view of the action as it unfolds.
Animators can then evaluate and adjust the timing and spacing of an animation with great precision.
Consider the example of a sword swing animation. With motion trails, an animator can visualize the arc through which the sword travels to adjust the movement. Ghosting further enhances this process by allowing the animator to see the sword's position at various intervals.
5. Optimize Keyframe Placement
By carefully determining where to place keyframes, animators can maximize their control over the movements' start, middle, and end to improve the generated ease in and ease out transitions by in-betweening.
In a walk cycle, keyframes must be placed with precision to capture critical moments like the heel strike and toe-off phases of the step. The heel strike is the point where the heel meets the ground, and the toe-off is when the foot pushes away. By accurately defining these points with appropriately placed keyframes, animators can achieve precise control over easing the foot’s movement.
Strategic keyframe positions also allow animators to avoid unnecessary clutter and create smoother transitions.
Conclusion
The slow in and out principle is a given nowadays: every quality animation relies on smooth transitions, and easing curves are key in this aspect.
But mastering this principle is not easy. You need a good sense of timing and storytelling to match audio cues with secondary actions and keyframes. You also need to be comfortable with tools like motion trails, ghosting, and motion graphs.
When you work with a limited budget, smooth animations aren't often a priority, and you must do with what you have. A good animator can work around those constraints with efficient use of keyframes, tweening, and reusable assets, including animation cycles.
