I have a screen that has an all-over photographic textured background. My intent is to have a static block of copy appear as a fade in. However, I want the fade to be in the form of a top to bottom vignette; if the background were flat colour, I'd create a box the same colour, graduate it to nothing, place it on top of the text and then either move or stretch it down the screen over many frames, so that as the fade to nothing moves down the screen, the text gradually appears from behind.
However, I can't work out how to create the same effect with the textured background in place. It must remain, or appear to remain static with everything else happening on all the layers above it. All I can achieve is to either make the layer move or distort. Help would be very much appreciated. Thank you.