How to achieve gradual acceleration w/ a CC Cylinder

Adobe After Effects Expressions

How to achieve gradual acceleration w/ a CC Cylinder

Posted by Preston Parish on November 10, 2009 at 11:18 pm

Hello,

I’m a novice but functional AE user, and I’m having trouble achieving a gradual acceleration while rotating a CC Cylinder background. The effect is to show an object reaching it’s apogee in the sky, and then falling to earth, gradually reaching it’s freefall speed.

I’m using a CC cylinder effect w/ a still of the sky, and rotating it for acceleration.

Is there an expression I can use that will give my cylinder that natural, gradual acceleration – from still to my desired maximum rate? I’m afraid I’ve spent way too much time trying to do this manually, so I really appreciate any helpful suggestions.

Thanks!
Preston

Preston Parish replied 16 years, 6 months ago 2 Members · 5 Replies
5 Replies

Xinlai Ni
November 10, 2009 at 11:40 pm

I didn’t quite understand the relationship between the rotation of the cylinder to the falling of the object, can you explain more?
But for uniform acceleration from still, the formula is
displacement = acceleration * time * time / 2
you can apply that to either angular displacement or positional displacement.

Xinlai Ni
Software Engineer, Google Inc.
Preston Parish
November 10, 2009 at 11:55 pm

hi Xinlai Ni – thanks so much for your response! It’s just what I needed. This is such a huge help.

Sorry for not clarifying what I mean. The cylinder scrolls the background behind the object, while the object only goes through small rotational changes to suggest tumbling through the air..

One more small question, though: what would I need to add to achieve a plateau to my speed? How would I specify an end to the acceleration, resulting in a constant rate? I see that adding a keyframe doesn’t necessarily stop the expression … perhaps I am missing something.

Preston Parish
Xinlai Ni
November 11, 2009 at 12:28 am

This should be easy, for example you want to animate the rotation property with uniform angular acceleration ‘a’ until time ‘t’, after which it stays with the constant speed. Just apply this expression to your rotation property:

if (time < t) { 0.5 * time * time * a; } else { 0.5 * t * t * a + a * t * (time - t) }

Xinlai Ni
Software Engineer, Google Inc.
Preston Parish
November 12, 2009 at 10:19 pm

There it is. Thank you so much, Xinlai – you’ve really helped me out here.

Cheers
Preston Parish
Preston Parish
November 12, 2009 at 10:57 pm

Ah.. one more small detail!

I’m trying to use the timeToStart property, so my rotation expression will begin about three seconds into the composition. I’m trying to look it up online, but I can’t seem to get the syntax right.

How can I add that to my expression?

Thanks!

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