How do I scale an object in only one direction?

Question

I am animating a volume meter, and have done the "tricky bit" of getting the volume value.

Now I want to apply the volume value to a mask , so that as the music moves, the mask is exposed in time to the beat.

I have tried two ways, the first by scaling the mask. However, I can't get it to scale in one direction only (i.e. it scales around a center point.) Even if I move the pan anchor it still doesn't pin the base of the shape.

The other way I tried was using a linear wipe. However, this was no good either, as the whole wipe effect is completed at 38% (i.e. if I move the slider the object vanishes totally at 38%, not 100% as I would expect.)

Is there a better way to do this?

Seems simple, but has me stumped.

Answer 1

1. Set the Anchor Point to the bottom of the asset by using the Anchor Point tool (Y)

   Tool icon in CC versions

   Tool icon in versions pre CC

   

2. Disable proportional scaling of that layer and set the desired keyframes

Demo in AE 22.0.0

Notes

• The shape also can be used as track matte, just set the layer below to Alpha.
• Consider to add a slider control
• As alternative you could use a trim path on a shape layer as track matte, see this answer: https://graphicdesign.stackexchange.com/questions/49643/how-to-use-after-effects-trim-path-with-an-illustrator-path/49975#49975