Motion curve in animation—defines accelerated or decelerated movement transitions. Enables non-linear keyframe interpolation.
In the 3D and motion graphics workflow, the spline, beyond keyframes, determines how a movement actually feels. Not simply from point A to point B — but with what acceleration, deceleration, overshoot. You set two keyframes, and the spline decides what happens in between. Linear interpolation means uniform, dead. A spline gives you organic, physically believable transitions.
On set or in post-production, you work with different spline types: Linear is the last resort — rarely used because it's unnatural. Bezier splines are the standard in every animation software (Maya, 3ds Max, Blender, Cinema 4D) — you get handles on each keyframe with which you directly model the curve shape. Catmull-Rom ensures the curve flows automatically through each point without you having to touch handles — faster, less control-freak work. TCB splines (Tension, Continuity, Bias) give you three parameters per keyframe to microscopically fine-tune how sharp or soft a change of direction is.
In practice: You set up a camera movement, a character rig, a particle dynamic. Immediately, you check the curve editor — not the keyframes themselves, but their spline shape. Flat at the start, then rising? That's light acceleration — useful for focus pulls or object reveals. Steep, then flat again? That's fast in, slow out — ideal for emphasize moves or follow action. No spline at all, just sharp points? Then you have angular, robotic movements — sometimes intended (glitch effect, mechanical systems), mostly not.
The trick: Ease-in and ease-out are not separate tools, but the consequence of how you shape your spline curve. Flat start and end areas of the curve = gentle starts and stops. Peaks and dips in the middle = accelerations, overshoots, bounces. You learn to read the curve topology like a pulse — and then you immediately know whether the animation feels right or not.