I've never done it but here are my thoughts: I would key out the green. You may then need to export out that keyed clip. I prefer the Animation codec set to millions +. Import the keyed clip and add a black color solid or something behind the clip so where the green was it is now black. Add a levels filter and adjust one of the bottom triangles so it makes the clip 100% white. Then create a gradient. If you make the gradient a horizontal liner gradient and the truck drives on and off the green screen in a straight line then you just need to make the gradient long enough to cover where the truck drives on and off. If you have like a radial gradient you will have to do a match move behavior or key frame it. Any ways, add an image mask so the white takes on the gradient. If you don't know how to do an image mask then check out my Creating a Hologram video tutorial. The image mask part is in the middle of the tutorial. Hope this helps and best of luck.
Thanks for your help. I got everything to work as you stated -- but it may not be the best solution for me given what I'm trying to do.
What I'm trying to do is to create a "depth Mask" for 3D conversion. It is basically a greyscale gradient version of the image that tells the 3D application hoe much depth to give different areas of the image.
So is there a way to somewhat-accurately make a gradient of the moving image? -- so the gradient moves with the image? (Hopefully it won't have to be placed frame by frame)