How to create a core matte?

[Shadowplay]

I've just started using the IBK keyer in Nuke and have watched the tutorials on the Foundry site- in the my keys and in the video, the image comes out transparent when put in front of a background, and the tutor says to put a core matte in to make the fg image more opaque, but he doesn't explain how to create one, so any help with that would be much appreciated.


A quick second question- I'm having trouble figuring out how to do multiple keys on the same image, if anyone could point me in the right direction?

[Gravy]

Use any keyer then erode it in.

Use keymix to merge multiple keys or channel merge to combine.

Try using the search function I'm pretty sure both those questions have been answered before in way more detail than that.