What is the difference between ALPHA and LUMA?

[Matt]

A luma matte makes a matte using the luminance of a layer. An alpha matte makes a matte using the alpha channel. The alpha channel defines the transparency of a layer and can be created in photoshop to isolate a certain part of your image.

“is there a way of picking a layer beneath it and saying I dont want it to affect that layer?”
No. Since “stencil luma” is a transfer mode, it effects all the layers beneath it. You can use
track mattes to matte individual layers.

I’d suggest you look towards the After Effects help file or consult Trish and Chris Meyer’s After Effects books. You’ll learn this stuff a lot quicker and more in-depth than asking pretty straightforward questions on the board.

[Eric Chard]

[Daniel Haskett] “what is the difference between luma and alpha? Just because when I do stencil luma on a drawing it works, as it takes out the drawing from the layer beneath, however when I do stencil alpha it does nothing. Just wondering what the difference is between the two. Also when you have a layer with stencil luma, it affects all layers below it, but is there a way of picking a layer beneath it and saying I dont want it to affect that layer? I read somewhere about nesting a composition, but I dont know what that is?”

All these questions are answered in the manual.