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Chroma Key or Hi-Key for white background

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Matt Slocum
Chroma Key or Hi-Key for white background
on Nov 26, 2008 at 5:11:00 pm

Hi all,
I'm working on a 30sec spot where a group of people gather together standing on a white background. We will have 50 or more people. The camera will be over 20 feet in the air looking down at the crowd. Feet will be seen on the wide shot. The desired look is to have everyone standing on flat white and casting a slight shadow in one direction.

http://www.miraclemultimedia.com/images/hi_group5.jpg

The current plan is to drape a large green cloth from the ceiling and across the floor. Key everyone out and place them on white, and add the shadow in post. I'm thinking about adding a second camera to the side to get the shadow perspective.

After doing some research online, I'm wondering if we should use a white cloth and try to get the effect live. One down side of this would be if we need to duplicate the group to make it look like an even larger crowd. A plus side would be that I wouldn't have to worry about green spill or shadows. It seems like I won't be able to get the white bright enough in contrast with the people.

Any thoughts?

~Matt


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Mark Suszko
Re: Chroma Key or Hi-Key for white background
on Nov 27, 2008 at 7:39:16 am

Never done a crowd that big, but I've done the white limbo both ways before, for full-body shots, as a chromakey green and using white photog's paper. Once on the same exact shoot: I did it a second time on white as insurance in case I failed with green. Green worked okay though.

If using real white, you can of course luma-key an even smoother or better white in post. Your potential problem that I see there is, there are much greater chances of problem spots happening in your large group with the white than with the green. More chances to get a flare off somebody's glasses, or a section of white shirt collar that will need masking off, etc. Plus, color-correcting the white with the people in it forces the people to go along for the ride, color-wise, and you may prefer the flexibility of having them color-adjustable separate from the final white.

The white, if you shoot real white is still not going to be "pure" as the white you create synthetically: you will get slight gradients in that white here and there, which I have found to be a hassle to deal with when layering and cropping. Myself, I'd rather work with clean alpha channels, no matter how they were derived, than to work with camera-grabbed white.

The lighting requirements are not significantly less for one than the other, so no great savings there.

If they walk on white, and it gets dirty, the key is harder to pull in luma IMO than if dirt gets on green.

Once you have the full-body keyed, you can create perfect shadows by just grabbing another copy of that matte, layering it on a lower track, and bending it and filling it with black and adjusting opacity and blur. For people that stand really still, I have also just painted-in a diffuse shadow pinned to their shoes and nobody was the wiser.

I think you'll have more clean-up and roto to do with camera-grabbed white, personally.

One last thought: you may not need to grab the whole crowd in one take: shooting individuals and small randomized groups may work just as well and be more affordable in terms of smaller space needs thus fewer lights, etc. Depends on the shot and script and your skills, but garbage mattes and cropping do miracles.


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grinner hester
Re: Chroma Key or Hi-Key for white background
on Nov 27, 2008 at 7:30:40 pm

well I'd chroma key em over white but if somebody actually shoots em over white, your' married to it. A luma key is your closest fix but you'll be maskin' enough to cuss the guy who waisted so much money shooting them over an unkeyable source.
Can they not be shot over a green screen so you can work with it?



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Bob Bonniol
Re: Chroma Key or Hi-Key for white background
on Nov 27, 2008 at 8:02:28 pm

Grinner,

Shouldn't you be tending to a turkey, or watching football, or something ? Go give thanks you big lunk...

Bob Bonniol


MODE Studios
http://www.modestudios.com
Contributing Editor, Entertainment Design Magazine
Art of the Edit Forum Leader
Live & Stage Event Forum Leader
HD Forum Leader


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grinner hester
Re: Chroma Key or Hi-Key for white background
on Nov 28, 2008 at 3:43:51 am

lol
ditto, dude.
happy turkey day from da hesters, man.



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Matt Slocum
Re: Chroma Key or Hi-Key for white background
on Dec 1, 2008 at 2:48:27 pm

Thanks,
I think I'll be sticking with Plan A, shooting over green. Just talked to my lighting guy, and we're going to use magenta rim lights to offset the spill.

Hope everyone had a good Thanksgiving and good turkey.

~Matt


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Mark Suszko
Re: Chroma Key or Hi-Key for white background
on Dec 1, 2008 at 7:26:01 pm

Don't over-do the magenta, and don't do it at all if specifically using Ultimatte. It screws up Ultimatte's special version of the process, and Ultimatte AdvantEdge has it's own artifical spill suppression built in.

If you have enough distance between the green and the talent, and didn;t over-light the screen, you shouldn't actually need any magenta, it's only needed where green reflection spills onto the talent. If you overdo it, you'll have the devil's own time trying to get rid of it later.

Been there, done that, got the t-shirt.


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Matt Slocum
Re: Chroma Key or Hi-Key for white background
on Dec 1, 2008 at 8:29:27 pm

It will probably be a very light 1/4 magenta or something. There will be several full body shots so the talent will come in direct contact with the green. I've read where someone else warned against using magenta with Ultimatte. I was just going to use Motion's and/or After Effects' built in keyers.

~Matt


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Tim Kolb
Re: Chroma Key or Hi-Key for white background
on Dec 2, 2008 at 2:32:47 am

[Matt Slocum] "I've read where someone else warned against using magenta with Ultimatte. I was just going to use Motion's and/or After Effects' built in keyers."

ANY keyer with spill suppression will have a tough time with counter-shaded rim lighting...and that's all of them.

This is an old wives tale that comes from the ancient times...

Plus, if the talent is making contact with the green screen, how will you rim them without spilling magenta onto the greenscreen?

The explanation is here:

http://forums.creativecow.net/readpost/162/866176





TimK,
Director, Consultant
Kolb Productions,


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Jeremy Garchow
Re: Chroma Key or Hi-Key for white background
on Dec 2, 2008 at 5:04:26 pm

I'm going to go off the map here and say go white. If you know for sure you want your background to go white, there's nothing easier to do than replace white with white. That flare off the glasses? Guess what, that's white. You can do a luma key and blur that matte and no none will ever know, and that's the point to creating a key, right? We have done a lot of shoots on all white and I luma key a lot to make the white consistent and get rid of any gradients or lighting holes on big wide shots. We have not, however done this with that many people. The easiest thing to do is find a studio with a big white sweep. The tough part is lighting for white which I find works best with daylight kinos, anything tungsten tends to warm up the white a bit. Also, with a group that big, the detail will be diminished (meaning there's no closeups) so pulling a lumakey on white will work in your favor.

I am not saying that I am right, I am just saying how I'd do it. That much green with that many people on a floor will be killer in the bad sense of the word.

You might want to think about adding a shadow later in post using the matte as a shape.

Mark makes a good point about the color correction, but I find it's easier to correct for something that's naturally lit, then it is to correct something that is minus green. Again, my opinion and you have some very solid advice here.

Also, I just looked at the picture again, are they holding white signs and waking in to frame? That might change my answer. :)

Jeremy


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Tim Kolb
Re: Chroma Key or Hi-Key for white background
on Dec 2, 2008 at 5:42:34 pm

I believe that I agree with Jeremy...if the signs are a concern that they'll blend with the background, possibly they should be a touch off-white and a matte finish.

Trying to contain green spill on a crowd that large seems an unnecessary chore...the white sweep's spill is simply practical fill...




TimK,
Director, Consultant
Kolb Productions,


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grinner hester
Re: Chroma Key or Hi-Key for white background
on Dec 2, 2008 at 6:38:44 pm

eewww dont let him do that. You can make the spil whatever color you want in post but you'll not be able to create a proper back light.
Harsh is yer buddy in this case. He has no background to try to match. He should just offer what pops the most.



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Matt Slocum
Re: Chroma Key or Hi-Key for white background
on Dec 3, 2008 at 8:50:23 pm

Thanks for all the input so far. This is what it looks like we're going to do.

We decided we wanted more people on the final shot, so we will be compositing the group with other shots with the same people wearing different clothes to make it look like a very large group. So we are going to shoot on a large greenscreen. We are going to put a slight (1/8 or less) magenta on their backs for rim light. The purpose is not spill correction, but to give a cleaner color separation on the edge. It will be slight enough to not be noticed by the eye, just the camera/keyer. Don't know if it will really make a difference, but it makes the gapher and producer feel better.

Yes, the people are going to be carrying gray cards with another color on the back.

~Matt


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Jeremy Garchow
Re: Chroma Key or Hi-Key for white background
on Dec 5, 2008 at 4:30:45 pm

[Matt Slocum] "We decided we wanted more people on the final shot, so we will be compositing the group with other shots with the same people wearing different clothes to make it look like a very large group."

Then green I guess it has to be. That makes your decision easier, but perhaps your work a bit more complicated. Either way, it's going to take some manipulating in post. You should consider getting an ioHD (if you use FCP) and doing on set captures to do small and light test keys/composites.


[Matt Slocum] "Yes, the people are going to be carrying gray cards with another color on the back."

Make sure they aren't picking up much green reflection, or put tracking points on the corners of the posters in case you need to track them out and replace them later.

Sounds like fun, man! Break a leg out there...

Jeremy


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