How to Replace a Sky in Motion

In this installment of Motion Magic in Under 5 Minutes, Mark Spencer shows us a great trick for replacing a boring background (such as an empty sky) with a much more interesting one. On top of that, he demonstrates how to quickly make that background realistically track with any camera motion in the shot.

In today’s under 5, I’m going to show you how to replace the sky in a video clip.

Here I have a hand-held shot of a drone.

I’d like to replace the sky with something more dramatic. In my File Browser, I have a variety of sky shots to choose from.

First, I’ll create a new group and move it below the current group.

Then, I’ll add a couple of sky images and scale them up, holding down Option and Command as I drag a bounding box handle to scale proportionally around the anchor point.

If I disable the video clip, we can see each of the sky images. I’ll re-enable the video clip.

To reveal the sky images below, I’ll go to the Library, to Filters, to the Keying category, and I’ll add the Keyer filter to the video clip. The sky in the video clip is immediately keyed out, revealing the sky image below.

If we go to the Inspector, to the Filters tab, and select the Matte view, we can see that Motion created a very good key automatically. I’ll go back to the composite view.

However, if I play the clip, the sky doesn’t move with the camera movement.

To fix that, back in the Library, under Behaviors, in the Motion Tracking Category, I’ll apply the Match Move behavior to the Group containing the sky images. Then in the Inspector under Behaviors, I’ll drag the Video clip to the Source well. I’ll drag the Tracker over the top of the flagpole. Then I’ll click Analyze.

When the analysis is done, we can see the motion path of the sky image. I’ll deselect the Match Move behavior to improve playback performance and play the clip.

That looks great but the sky tracks a little too perfectly to the top of the flag pole so I’ll select the group of sky images, then in the Properties Inspector I’ll right-click on Position and add the Wriggle parameter behavior. I’ll set it to a low amount of about 10, apply mode of Add and subtract, lower the frequency and noisiness, and play that back.

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