Returns the current speed of the Tween, relative to straight linear motion being 1. The 'units' for this measure of speed are in units per duration. Ie. In linear motion over 100 pixels for a duration of 1 second, speed() would return 1, meaning "100 pixels per second." If easing were applied and speed() returns 0.25 this translates to "25 pixels per second."

Speed is often useful in determining Squash and Stretch, a common animation technique to convey a soft body.

This is identical to the current slope of the easing shape function.

import megamu.shapetween.*;

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