Because we will compose our rendered UFO with the original footage we shot, we need to make sure that the lighting matches. Our original scene was lit mainly by the sun so let's set up a light to represent the sun in our 3D scene.
Create a free direct light and position it far above the UFO and scale it up so the cone of the light encompasses the entire scene. Aim it at the UFO so the light comes in at the same angle as the sun in the original scene.
If you select the light and go over to the 'Modify' tab, you should be able to change its settings to better match your sun. You may want to adjust the intensity, enable shadows and set the colour to something a little bit yellow.
How you set up your lights will depend on your actual scene so these are just rough guidelines. Remember, the more accurately you set up your virtual lights to match the original footage the better the 3D integration effect will work.
Let's do a test render of our scene!
That's not bad, but the underside of the UFO is entirely black. In the real world - assuming it contained aliens and UFOs - the sunlight bouncing off the ground would probably illuminate the UFO a little from the bottom. If you want to go all out, you could set up global illumination to simulate this effect, but if you're cheap like me you can fake it by setting up a simple omni light underneath the UFO
Create a simple omni light and position it below the UFO.
I've set my light to be very soft because I just want to brighten the underside of the UFO a little bit. Rendering the UFO again it should look a lot more realistic! Note that the bottom of the UFO is lit only a tiny bit so the shape of the UFO comes through.
Now it's time to render! But before we hit the render button, there's one more thing I want to cover: exporting individual layers separately.