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Old February 15th, 2007, 01:49 PM   #1
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Green Screen questions

I have a shoot next week where we need to shoot a talking head green screen and also some documentary footage of a race.
We're shooting HVX to P2 cards.

I had advised the client to shoot 720 24PN or 30PN for all of it , but now I'm seeing posts that seems to suggest shooting the Green screen at 1080, which I have actually never done.

Shooting the doc material @ 1080 sounds like it will be very intensive on the P2 cards though.

1. My idea now is to shoot the doc stuff ( of a bike race) at 720/30PN or 24PN and then shoot the green screen at 1080 60i/30P or 24P (? I could also shoot it all at 1080 60i .)
Finally then downrez the green screen after it is composited so in the end all the HD is in 720 30P.
How hard is downrezzing?

2. How bad is 720 30PN for green screen, do you really need 1080?

3. What kind of 1080 will give me the best green screen -60 i or 30P?
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Old February 15th, 2007, 02:56 PM   #2
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What will your edit sequence be?
If 24p then shoot a test before hand in
1080 24pA and 720 24pN.
If 29.97 then 1080 30p or 720 30pN.....
I have done both and notice some improvement
shooting 1080. Of course it all depends on
how good your lighting is....
and your compositing skills in post.
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Old February 15th, 2007, 02:57 PM   #3
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I am not in a position to shoot any tests, and I don't do the compositing.
My lighting skills are excellent and I have alot of experience shooting green screen so its just the technical issues I'm concerned with.

I was thinking of 30P rather than 24P only because the doc footage will be of bike racers and perhaps it may seem smoother at 30P - also would intercut better with interlace if they need to do that.
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Old February 15th, 2007, 04:37 PM   #4
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Greenscreening is ideal at 1080, but it's more than doable at 720. You just have more to work with in 1080.

You can de-interlace the 1080 if you want, though - the camera shoots progressive and splits the frame, but you can do some magic and re-integrate the frames.

If I were in your shoes, though, I'd probably just shoot the whole thing at 720p/30p and cut it together. If you're good atlighting for greenscreen, it'll key out beautifully at 720. 1080 is just 'easier'. It's like photoshopping an image, you're gonna have an easier time with a 1280x720 image and making it smaller and hiding the imperfections than doing a 640x480 at the actual final size.
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Old February 15th, 2007, 11:31 PM   #5
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There is no "magic" to do to re-interlace 1080 frames as long as you shoot 24p mode. Any properly supportive NLE (FCP for example) will know what to do with the 1080i encoded 24p data and will provide the proper 2:3:3:2 pulldown automagically. No big deal. Er... Or at least if you tell it and its tools to do so, but it's not a big deal. No worse than working with 24p mode on the DVX100 if you've ever done that.

As for shooting for greenscreen with the HVX... I don't know that shooting in 1080 is really worth it if you have concerns over what you'll get. The resolution is hardly any better given the relatively low-res CCD block and res capabilities of the camera. But it can make a difference - if you properly light your subject and screen, you can get that extra bit of resolution and slightly cleaner/sharper edges with the added pixels. If you're predominantly shooting 720p for this project (assuming there's more to it than greenscreen work), then just stick with 720p. Otherwise, give the 1080 a try. A few tests would be ideal, but if that's not possible then just stick with shooting what you're most comfortable with. I've done enough green work at both 720p and 1080p with the HVX. Not a whole lot of difference between the two and like I said, if you light properly, you won't have any trouble pulling a good key. Whatever you do, don't shoot 1080i though... If you go to 1080, only shoot in 24PA mode.
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Old February 16th, 2007, 12:14 AM   #6
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Jeff,
Is what you are saying about re-interlacing 1080i/24P also true for 1080i /30P?

All we are shooting is a talking head, so if 720/30P is not going to be that much different than 1080i/30P I may try to go with it.


Also at one point you say 1080 24P then later 24PA. Which do you mean?
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Old February 16th, 2007, 11:51 PM   #7
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DVCPROHD @ 1080 is always encoded as 60i. When you shoot 1080p on the HVX, regardless of which mode you're actually shooting in, it gets encoded as DVCPROHD 100Mbps @ 60i. This is why in the record mode settings, all the 1080 modes are listed as 1080i/xxxx where the x's represent what it is actually recording over the 60i stream. For progressive modes, the true 24p or 30p progressive frames are still there, it just must be re-assembled by your NLE software. If you shoot at 30fps, it simply splits each frame into fields and encodes. At 24P there are two options, 24P or 24PA. If you're going to extract true 24P out of the stream it does not matter which one you shoot with, but be sure your editing software is aware and capable of hancling of the one you choose. The 24PA mode uses a 2:3:3:2 telecine sequence where as "standard" 24P is 2:3:2:3. The main advantage behind 24PA is that if you need to play back the stream without pulldown on a 1080i display (meaning you don't extract the true 24fps), it will have a bit smoother motion with less judder. If I'm shooting 24P over 1080i or over 480i I always use the 24PA mode.

When I used 24P and then later 24PA in the same post, I was just refering to 24fps in a general sense and then saying I recommended the 24PA mode.

If you're looking to do 30fps and it's a head shot - presumably you'll have to key around hair and other fine details. I would definitely give the 1080 30P mode a try.
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