Starting in the Editing workspace, place your background footage on V1 in the Timeline panel. Landscape shots containing large areas of sky provide a workable, blank canvas for your creativity.
Find a photo or video clip of an object that suits your alien landscape. Ideally, the object will be fully in view and isolated against a black or transparent background so you won’t have to mask it. Our sample uses the planet Jupiter. Place the footage on the track above the background video and adjust its Position, Scale, and Rotation values in the Motion section of the Effect Controls panel (Shift+5) until the added element fits into the scene.
Select the layer of the added object. In the Opacity section of the Effect Controls panel, choose a Blend Mode setting that merges it with the background. Your best choice may be Screen.
Now that you’ve established your background, consider adding an element in the foreground. Greenscreen footage works best for this. Place it on the top video track.
Apply the Ultra Key effect to the greenscreen footage. Click the Key Color eyedropper color picker and then sample the green in the clip to key it out, removing it from view. Holding Command/Control while sampling measures a larger color area and often provides better keying results. Depending on the evenness of the greenscreen in your footage, you may need to change Setting from Default to Aggressive to improve the keyed footage.
To check the quality of your key, change Output from Composite to Alpha Channel. Your keyed object should be solid white (opaque) and the background should be solid black (transparent). If you find any areas of gray, they will be translucent. Go back and change the Ultra Key settings to remove them.
Depending on what your object is and what you want it to do, you could keyframe its Motion controls (Position and Scale in our example) to make it appear to travel through your alien landscape.
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