How to create a circle to "terminator" shape animation (and back again)

[PeterC8]

Really trying to work out how I would create the animation in the attached video!

It's a circle that stretches into a "terminator" (long rectangle with circle end) but doesn't warp or lose radius of initial circle... video explains it better!

Is this possible to create in Premiere Pro? If not in after effects? Animate?

Hoping to have it so that I can change how long it is per title, and how long it stays long for.

Seems like quite common in many videos, just can't figure out how to do it... 

Thanks!

See picture too:

[Phillip Harvey]

Highly possible that someone else would  have an approach that I don't know about, but doing this type of animation is super clunky to do in Premiere. I would do it in After Effects.

[PeterC8]

How could I do that?

[Peru Bob]

You could keyframe a mask over a solid background.

[PeterC8]

The trackmatte key doesn't seem to be working for me at the moment...

I had my long terminator shape as a png on one layer. On a new layer I drew a square over the section of the terminator i wante to show. Dragged the track matte key effect onto the terminator and selected v2 (the square i'd drawn) as the track but it didn't do what i've seen it should do in videos?

Is there a different way to use masks?

[Ann Bens]

two circles and and rectangular.

animate position on circle on the right

animate scale of rectangular.

If you have Ae then use Ae, much easier.

[PeterC8]

Thanks!

[Peru Bob]

Don't use track mattes. Use opacity masks.

That said, I like Ann's method better.