"Universally Transparent" Object?

[polird]

This one may be a little difficult to explain, but here is my best attempt:
Not sure why it just posted, but:
I am working on a website for a school project. I have an old blend that i made with grass and a tree. i am trying to get an effect like the page is coming out of the grass. what i figured i could do was put a large plane through the grass to split it in half, then that plane would render as transparent to a png, and i could overlay the image over the page with the original image as the background. I am wondering is i could use a plane and make it transparent so that it doesn’t show the scene behind it, but renders as transparency. I was thinking maybe the chroma key node, but i am not too familiar with the node editor.

Thanks for any help!

And my project is due tomorrow

[arexma]

You are looking for z-mask and z-pass for rendering.
You can pretty much do what you want to achive.

Google for it, if you figure it out, cool, I am on my way to bed already got no time for long explainations =) but maybe another peep can take your hand

But z-mask is pretty self explainatory.

I made you a quick setup:
http://www.speedshare.org/download.php?id=E2472C0811

look at the render layer settings… if you ctrl+click one it gets the z-mask layer. I set the monkey to be the zmask but don´t render the layer.

hth