Making Backgrounds Transparent in Fusion

I have a clip which I masked out and tracked. I would like to take that small portion which is visible and paste it on top of another clip, but I can't get rid of the white background. After looking for solutions for several hours online I still can't find it. It's a really easy effect but I just don't know how to do it in fusion. I tried adding a background node, all sorts of keyers, nothing works.
Here is a picture of the composit:

[img]<blockquote%20class="imgur-embed-pub"%20lang="en"%20data-id="a/8OqnDI9"%20data-context="false"%20><a%20href="//imgur.com/a/8OqnDI9"></a></blockquote><script%20async%20src="//s.imgur.com/min/embed.js"%20charset="utf-8"></script>[/img]

Just need to get rid of the white background.

Thanks in advance,

Dawid

Jump to around 2:18

Alternatively, doing a google search on the terms : fusion background transparent
Would have found you countless solutions in seconds, think you need to look at some of BlackMagic's excellent free training resources as this particular question is beyond trivial.

Hey there Dawid,

I see what you tried to do there. You want your polygon mask to follow the motion of the planar transform node. And it's as easy as it sounds. Just track the area you want to mask out with the planar tracker, create a planar transform node, then create a polygon mask around the object you wish to mask out and connect it to the media in node.
The mask will now move with the tracked movement.

I know it can be hard to work with the nodes in the beginning, but keep at it and you'll get there. And remember: there are no stupid questions

have a nice day,

Dawid