When you need to composite soft-edged or motion-blurred 3D CG (which has been rendered over black) over a background, and you don't want matte lines, you will get great results if you use the 3D element's alpha channel as a holdback matte
Most 3D CG is rendered over black. Most 3D CG comes with an alpha channel that precisely fits the 3D foreground element. Here's how to composite that 3D CG over live action using the same image arithmetic that Shake's over node uses.
First negate the 3D element's alpha channel.
Next, multiply the inverted alpha channel times the background image. This causes the alpha channel to hold back portions of the background image, creating a soft hole into which the foreground element can be dropped.
Notice that the foreground is a soft element on a black background that already precisely fits the soft hole in the background.
Finally, simply add the foreground to the hole in the background. In Photoshop, you can find addition well hidden under Apply Image - or renamed as the blend mode Linear Dodge.
Hooray! No matte lines!
Here's a related page on matte lines and transparency mapping:
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