a picture of britain

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katangy
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Joined: Apr 6 2002

I watched the latest edition of the programme on Sunday, perhaps someone can explain how the shots of the foreground at normal speed were matched with the cloudscape moving at high speed. There was one particular shot of the Land Rover being driven through a Scottish glen at normal speed while the sky was moving fast above. Is it done using masks in after effects and joining 2 shots taken with the camera locked off or is there another way?

Katang

Robin Davies-Ro...
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Joined: Feb 21 2005

The Land-Rover could well have been driven quite slowly in that instance (I saw the programme as well - really excellent!) so that when the video was speeded up for the clouds, the vehicle would appear to be travelling normally.

Robin

johnpr98
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Joined: Aug 20 1999

I think the fast sky F/X is done in 'post'

It's has featured in a lot of progs recently

johnpr98
 
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RichardB
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Joined: Aug 27 1999

Replacing sky is fairly straightforward by using a luma matte- same principle as chromakey but using the difference in brightness, which of course is considerable when referencing the sky with any other object.

Because track matting is now so easy it's possible to get a good effect even with a moving camera.

Speeding up slow objects (i.e. achieving the result 'in-camera') is a less desirable way of getting the effect as you tend to get 'quiver' either in the object or the f/g or b/g; it also takes a l-o-o-ong time to get a good cloud effect and even then you often don't get the cloud effect you want: it's not dramatic enough, or you have to tweak it in post and then you're back to a luma matte: may as well replace the whole thing.

Still, I start work with a Top Gear cameraman on Monday, I'll ask him how they normally go about it and post back.

Mad_mardy
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Joined: Oct 19 2000

well i fi were doing it i would replace the sky.
if the camera is locked off it would be a doddle
if it was moving you would need background sky plates to match the original camera move.
in fact for a Pan you could film the normal landrover shot on a tripod then after you could create a slow pan exposing for the sky to use for the background

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tom hardwick
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Joined: Apr 8 1999

I remember doing it on Super-8 film in the 80s. I masked off the top of the frame and filmed that at 24fps. Then I rewound the film and turned the mask 180 degrees (it was simply a half blacked out UV filter) and filmed the sky at one frame every 10 seconds, hiding the join in the horizon's tree line. I used a combination of aperture and focal length to dissolve the two scenes together at the join. Ergo: the sky moves 2500 times faster than the walking people.

tom.