I’m having trouble compositing my model over a background using the AlphaOver node. Here is a simple example I made to demonstrate. As you can see, a straight Render and Image combined with alphaover works fine.
Try after lens distortion add Convertot -> Set Alpha with image from Lens Convertor and Alpha from Your cube.
Hope it helps.
Edit: Of course it won’t work if You use for example “fit”. You can also use blur on render layer for alpha chanell to get more edges.
Hope it works.
This happens because the “Lens Distortion” node ignores the alpha map thus making further masking tedious. To fix it, simply run a identical “Lens Distortion” node on the alpha map of the image in question (the cube) and then as sftd suggests, use a “Set Alpha” node to combine the distorted image with the distorted alpha map. That way any distortion should work. Hope that helps.
Thanks for the help. That worked, but it caused a different issue, the world background color bleeds in around the edges…any ideas? Thanks!
You don’t have identical “Lens Distortion” nodes. The dispersion setting distorts the image somewhat as well; change both to 0.30 or 1.00, or just at least make them identical.
D’oh! Talk about careless mistakes…Thank you very much, works fine now.
Excellent! 
For what it’s worth, “the world background color” can be a real PITA … I always make it a point to carefully set it to solid-black zero.
sundialsvc4: True…I do the same.
Thanks for the help everyone!
FYI it’s way better to set the alpha method to Premul instead of Sky to “remove” the blue color, as premultiplied alpha is available for exactly that purpose reason is that premultiplied alpha will not save the sky color, while using a black color may affect the transition from the objects edges to the background to a shade of black/sky color.
