Use Expression for Rectangular Mask Size

[updatesvc]

I am playing a round with music by animating different layers by the use of an expression .

thisComp.layer("Audio Amplitude").effect("Both Channels")("Slider")

I would like to use a Rectangular mask to hide/reveal a layer in a kind of  "horizontal music spectrum visualization".

Hence part of the layer will be hidden according to the beats.

I have used the expression in various other properties like opacity, but is it possible to do this for a rectangular mask for its width or length.

Thanks in advance.

[Mylenium · Accepted solution]

Masks cannot be manipulated using expressions. You have to apply it to effects like linear wipe or the scale of a solid and use that as matte. Bothe methods are simple enough and don't require any mask usage, if you realyl just want a rectangle.

Mylenium