In the Set Extensions tutorial we learned a technique to add CG elements on top of a live action shot. But there are other effective ways to do this, include masking out parts and even using a green screen in the field.
In the above example, luminance keying and some masks were used to remove the sky and replace it with a destroyed city. Since our characters do not extend over the separating wall it is easy to remove the sky by masking the wall off and replacing it with our set extension.
This is a fake setup to demonstrate another possible way to remove the background. Obviously this would be even easier to remove than the white sky green keyer. Especially since it is pure green from the paint bucket in Photoshop. Of course in real life you want to add some track points as well so you can track the camera movement and I don’t mean in Photoshop.
In this example a fake green screen is painted on to demonstrate how using a small green screen in front of your actors will allow you to extend the background directly in front of your characters while maintaining the original dirt ground which makes your charcters look like they are really there. Once the green is removed a background plate of the original dirt lot can be placed back into the scene, then by using a subtraction mask and some feathering, you can blend the real foreground dirt with the background plate of the matte painting. Just be careful not to fade the mask beyond the end of the green screen or you will get a sharp edge where the green screen ends.
Another idea would be to park a static object like a car in front of the actors with a green screen behind it. Then you can key out the green and use a garbage matte to cut the rest of the scene out, and this way you wouldn’t have to blend the original footage with the background matte painting because the car would serve as a separator.
You can also extend the set with a camera movement and place some large building in the BG and simply pan upwards to the top of it… or even… Okay, that is enough for now.