using layer below as alpha in after effects?

[chow choppe]

hi everyone

 

i have a quick and simple question for u guys

i have rendered an animation as tga sequence and i have rendered same set of images in black and white to be used as alpha for the first set of images. so that i can mask out certain areas

i dont know how i can tell my upper set of images to use the lower(black and white images) set of images as alpha/masks.

 

i am using after effects

i know its simple but have forgotten how to do it

 

thanks

[CRD]

Set the track matte to Luma with the name of your alpha's layer.