bring them both into after effects. then select them both and drag them on the new comp button (film strip looking button at the bottom of the project window). choose single comp, no other options. in the timeline window, place the matte footage above the fill.
from the timeline, if needed bring up modes… there are a few ways to do this, one is from the little arrow in the upper right corner of the timeline, click and choose columns>modes. then, for the fill layer, from the trkmat pulldown in the maods panel choose luma matte from layer above.
that will apply the matte to the fill. now you can choose composition>make movie and click the output module, hwere it says lossless. from the top pulldown, where it says format, select targa sequence. then set the video output to rgb+alpha and millions+.
that’s the basics…
Kevin Camp
Designer – KCPQ, KMYQ & KRCW