QUOTE(Click @ 12/30/06, 09:19 AM) [snapback]256036[/snapback]
I need to add a big moving shadow to a scene that shows a street and grass with shadows from trees across. If I just add a semitransparent solid black layer to the composition and let it glide over the street and grass, obviously the shadows from the trees will also become darker on the street and grass. This shouldn't be the case. I am looking for away to show the shadow only on areas where there are no shadows from trees, in essence filling up the sunny areas on the street and grass with shadow as the big shadow glides through the scene.
Is there any effective way to achieve this?
(Sorry for no photo of the scene, I did not find out how to upload a photo to the post)
I assume you're working with live action footage?
You could try to pull some sort of key from just the original shadows and use that as a matte for your new shadow. Try adding some effects, such as levels or brightness and contrast, and see if any of them can create a greater difference between the shadowed areas and the non shadowed areas than in your original. If you can get this to work, you could then use this in a separate comp as a luma matte for your new shadow, so that the new shadow only shows up in the areas that are bright in your luma source (or vice versa).
Or, there's always the hard way -- rotoscoping the original shadows by hand.
I doubt there's an "easy" way to do this though, but it's hard to say without seeing an image.