You’re all thinking of this too mathematically. While Ease & Whiz will get you 90% of what you want in most cases, controlling that sneaky slow ease in and snap at the end is a matter of keyframing magic, which really isn’t all that magical.
In the first 10 frames of the animation you have the shape or mask animate out about 10%, enough to iniate the motion. Move ahead 3 frames and animate about 90% of the rest of the animation, maybe 85 if you want more soft easing at the end. Then move ahead 10 more frames and add a keframe and finish the rest of the reveal. After you have your keyframes in place (all linear at this point), make the first and last key frame easy ease. Go to the graph editor and drag out the ease to 100% on the first and last keyframe. On the two keyframes right next to each other, pull the handles all the way in so there is no ramp (shouldn’t need to bother with the handles between the 2 keyframes, just between the first and 2nd, and 3rd and 4th). Here’s an example attached.
6986_keyframing.aep.zip
Jason Jantzen
vimeo.com/jasonj