Firstly, let me start by stating that I am fairly new to FCP / Motion so this may be a simple question for which I couldn't find any answers so decided to post in here.
I have a clip of water filling up the screen and its corresponding matte version.
What I am trying to do is "apply the matte to the video" for want of a better term, in the same way I'd apply a layer mask to an image.
I have managed to do what by changing the matte blend mode to "stencil luma" on the matte layer but the problem with that approach is that I always end up with a black background as you can see in the images below.
My end goal is to be able to apply the matte layer to the alpha channel of the water video, and then export the individual frames of the sequence so the background is transparent and I can add whatever I want to it.
I hope I am making sense but please let me know if you need any additional information and I'll do my best to add it here.