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SVG SMIL

• By default, all svg_smil animations interpolate linearly, which can feel unnatural or robotic.
• This becomes noticeable when animations are slowed down.

Customize Timing with Bézier Curves

• SMIL allows you to define custom easing functions using Bézier curves, similar to CSS's cubic-bezier(...).
• Unlike CSS, there are no preset keywords like ease-in-out—you must explicitly define the curve.

Attributes Required for Custom Timing

Attribute	Purpose
calcMode="spline"	Enables spline-based interpolation (non-linear timing)
keyTimes="0; 1"	Defines animation progress points (0 = start, 1 = end)
keySplines="..."	Defines the Bézier curve for each segment of the animation

All three must be present for custom timing to take effect.

Spline

• In this context, a spline is simply a Bézier curve used to shape the speed of the animation.
• Common easing curves (like ease-in-out) are represented using control points like 0.42 0 0.58 1.

Advanced: Multi-Keyframe Control

• You can animate multiple keyframes using values and define a different easing curve for each segment.

<animate attributeName="cy" values="0; 100; 0" calcMode="spline" keyTimes="0; 0.5; 1" keySplines="0.5 1 0.89 1; 0.11 0 0.5 0" />

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References

• https://developer.mozilla.org/en-US/docs/Web/SVG/Reference/Attribute/fill