alpha or opacity on layers


i have a question.
i own toon boom animate pro.
i am doing a christmas animation.
i have some psd file and layer that must appear from the top of the screen
to the end of the screen, and i need to set the alpha to 0 in the top and
arrive to full opacity at the background, how can i do it?

thank you

try this thread;action=display;threadid=2579