Still mask on animated object

[Fanny15A8]

Hi !

I'm new to AE and I'm just starting to train on more complex compositions. And I have already a problem that I can't resolve thanks to tutorials online.

I'd like to create a mask above an animated object (the bubble layer - 3) but which can allow to show the background (a mix of a still background and a paper texture video - 4 and 5).
I've tried to create a rectangular mask in the Bubble layer but the mask is animated the same way the Bubble form is, making it wobbly.
My problem is that the "grille"  (that would the same form as the mask) is a png so using it as mask doesn't work. And creating a simple jpeg image will not allow me to see the animation of the background 😕

I'm quite sure there is something to do with the trackmatte option but I can't figure it out. Would you be able to help me ? I'm really looking forward to learn.

Thanks a lot!

[davidarbor · Accepted solution]

You can create a simple rectangle Shape Layer with the Rectangle tool. Make sure no layers are selected, otherwise you'll draw a mask instead of a Shape Layer. You can draw this rectangle to mimic the grille layer. Next, place this matte above the animated bubble and set the Track Matte option of the bubble to "Alpha Inverted Matte [Layer Name]."

This should create a fixed matte that the bubble goes behind, as long as your rectangle matte has a solid fill and isn't transparent.

[Fanny15A8]

Thanks a lot !
It worked. And I've learned from my mistake : I was trying to apply the Track Matte on the rectangle and not on the Bubble layer.
A small step in learn how to deal with AE but still a step : thank you so much for your fast and clear answer !

[davidarbor]

Awesome, glad to hear it worked! And yup, that's a common "gotcha" for people who are new to track mattes. The trick is to look what happens to the layer above the one you just enabled the matte for. AE will shut that layer off since you typically don't want it visible. However, there are plenty of good reasons to want to use the layer above as both a matte and have it be visible; if that's the case, you just re-enable the visibility switch.