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Re: sky replacement for a dolly shot?

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Ben HeusnerRe: sky replacement for a dolly shot?
by on Jul 29, 2006 at 4:12:04 pm

If you work with the 2D tracker then you don't have to worry about the Z-axis in After Effects.

When I talk about planes here, what I mean the distance plane from the camera. If we look at the moving image itself, any object at the same relative distance will move in the same proportion. So, for example, we have a road with a person standing very close to the camera, a road sign in the middle distance, and a building in the far distance. If the camera were to move slightly, either left or right, or up and down, then the person close up would appear to move more than the sign in the middle distance. The building in the far distance would appear to move very slightly. Thank you parallax.

When doing matchmoving like this, it's important that you track an object in the same relative plane, or when you put it in the composite, the object is going to behave very differently than you want. In the example above, if you wanted to put in another sign in the middle distance, then you would track the object in the middle distance. If you applied the movement of the person in the foreground then the movement will look off.

It's the same with the sky. The sky will have very little apparent movement relative to objects in the foreground or middle distance. So this is why you want to track an object in the farthest plane you can.

If you apply this tracker motion to a Null object, you can link this to your sky layer with a simple expression that can subtly change the values in the track, so that the sky appears to be in a different distance plane than the object you track. So, say you could only track the sign in the middle distance, you don't want the sky to follow that movement. Apply the tracker information to a Null and then use this expression on the Position of your replacement sky layer:

mul(thisComp.layer("Null 1").position, multiplier)

You can change the value "multiplier" to whatever you need. If you bring it down, the the sky will move less than the object you tracked (making it appear to be in a further distant plane). If you bring it up the sky layer will move more, so it will appear to be in a closer plane. The trick is to keep it subtle. This is a very unsubtle expression, so using numbers too far from 1 won't give you the effect you need, and the sky will look bizarre.

Wow, that's a bit longer than I'd planned. I hope this makes sense. :o)


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