Clear blue sky as a blue screen?

[Filmkid2003]

Hey guys. I'm going to be shooting a time lapse scene in the city and one of the shots will have the camera pointed almost straight up (worm's eye view) and showing only skyscrapers and blue sky. My question is, since I live in Los Angeles and its very rare that we get nice puffy cumulus clouds that are great for time lapse, could I just shoot the skyscraper scene with a clear blue cloudless sky and then remove the blue sky in post and replace it with a time lapse sky of cumulus clouds? So, in other words, can I use a perfectly clear blue cloudless sky as a blue screen? Has anyone successfully done this yet? Thanks.

[Snazzy Flapper]

Sure. You can use a Luma key. Video Copilot.net uses that technique all the time. Search for sky replacement on his site.

[Jim Carswell]

There will be inconsistencies in the blue sky due to color variations due to atmospheric conditions. But, using multiple layers of keying should easily take care of this.

By the way, I thought the sky above Los Angeles was filled with smog. Where are yougoing to find a perfectly blue sky? ;-)

Jim

[APM-AR]

You should also check if there are any buildings which would reflect clouds usually.
If there are, make sure to add that reflection in post with the cloud footage you are using to replace the sky, or it might look a bit odd when the clouds are only in the sky, but not the reflection of it on the buildings.

[wgzn]

why not take a still and just cut an alpha matte in photoshop?

[dust'n the callipygous]

Robert rodriguez did it in planet terror. Bright blue sky turned black

[Filmkid2003]

Thanks for the replies everyone! Now that I know it will work I can start experimenting! To answer some of your questions:

Jim. Hahaha! You're right, most of the time there is a low lying layer (<--appreciate the alliteration, folks) of smog hovering over the beloved city of Los Angeles but, during the colder months, the skies are pretty clear! Question. Would using a circular polarizer filter help cut down on the inconsistencies in the blue sky?

Also, is it possible to film a skyscraper with mirror windows (that give off a very good reflection) and add realistic looking reflection of a cathedral in post? If so, would it be possible to do this if I'm tilting from the top of the skyscraper (from a worm's eye view) downward to street level? Or am I asking for too much on a small budget. I edit in Premiere Pro CS3 and will be getting After Effects.

WGZN: That's an interesting idea about taking a still. I'll be shooting time lapse with a Nikon D5000 DSLR so, that might be ab option! If I take a still pic and cut a matte in Photoshop, how do I make that one still pic run for say, 10 seconds, like a movie file? Thanks.

[wgzn]

how do I make that one still pic run for say, 10 seconds, like a movie file? Thanks.

depends on what software youre using. what id probably do is cut the matte in photoshop, take that into after effects give it a subtle scale or rotate over time (just to make it a bit more interesting) then export that as a 24bit animation compressed movie for use in FCP.

[El Director]

Robert used the sky for bluescreen trick in Planet Terror. Should work fine.