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Old March 1st, 2007, 11:25 AM   #1
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Green Screen Problem

I shot a green screen interview last week with the HVX and just got a call from the producer complaining that the shot was noisy and that made the key difficult to cut. The issue was the hair which on this guy was very frizzy.

I believe the lighting was fine. I have been doing this many years and have shot many keys.
The key was even at about 45 IRE and the backlights seemed right with a little warmth for separation.
Exposure was fine on the subject.

I shot the camera @ 1080i/30P with the Detail at 0 and Coring at 0 using HD Normal and CineMatrix probably at 0 though I'm not sure - maybe + 1.

Any ideas about this?

I thought about dropping detail a few points and bringing it up in post, but neither the client nor me wanted to experiment. Another thought was to perhaps make the green screen brighter.
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Old March 1st, 2007, 03:34 PM   #2
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Originally Posted by Leonard Levy View Post
I shot a green screen interview last week with the HVX and just got a call from the producer complaining that the shot was noisy and that made the key difficult to cut. The issue was the hair which on this guy was very frizzy.

I believe the lighting was fine. I have been doing this many years and have shot many keys.
The key was even at about 45 IRE and the backlights seemed right with a little warmth for separation.
Exposure was fine on the subject.

I shot the camera @ 1080i/30P with the Detail at 0 and Coring at 0 using HD Normal and CineMatrix probably at 0 though I'm not sure - maybe + 1.

Any ideas about this?

I thought about dropping detail a few points and bringing it up in post, but neither the client nor me wanted to experiment. Another thought was to perhaps make the green screen brighter.
What keying software are they using??
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Old March 1st, 2007, 05:03 PM   #3
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Maybe you should get in touch with the editor and find out what the issue is specifically. A 45 IRE Greenscreen should key out fairly easily and exhibit no noise, unless you used gain, so the "producer's" comments seem a little uninformed. Not to say there isn't an issue but noise should have very little to do with pulling a clean key.
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Old March 1st, 2007, 05:40 PM   #4
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I wonder if it was blond hair with lots of bounce from the green screen. Minimizing bounce helps. Keying blonds over a blue screen is also helpful.
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Old March 1st, 2007, 05:57 PM   #5
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no spill, no blond. Believe me please it is not a lighting or compositional problem.
Jim Arthurs suggested lowering detail to -3 and shooting abright key - he uses 65IRE.
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Old March 2nd, 2007, 02:54 AM   #6
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I used 60 IRE and was able to key frizzy light brown hair, even at standard definition.

As Mike mentioned, the keying software can make a big difference.
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Old March 2nd, 2007, 03:15 AM   #7
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I just got to thinking...

60 or 65 IRE? That's dang bright, considering

Y = 0.56G + 0.30R + 0.14

Which means the green screen is lit about 10 or 20% hotter than the foreground, if matte white is at 100 IRE. (Give or take, considering color temps and other variables.)

Or, looking at it the other way around, if everything is lit evenly and neutrally, and matte white is at 100 IRE, the green screen will be at 56 IRE.

If you leave headroom for glossy reflections in the foreground, the greenscreen will be a bit darker - or need even more light.

Am I wrong?
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Old March 2nd, 2007, 03:23 AM   #8
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The green screen is independent of the foreground.

The foreground can be bright or dark. Edge-lit or feature portrait lighting.

What matters is the edge where foreground meets the green screen. Is it clean? No unwanted artifacts? No halos? With the right software the edges don't have to be sharp because really good software can key smoke, wine glasses and even bubbles!
Best bet: Test before the task. Try out stuff. Have fun. See how far you can push it before it breaks. Then when it's time to do something important, you know the capabilities and limits.
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Old March 2nd, 2007, 12:44 PM   #9
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Originally Posted by Dean Sensui View Post
The green screen is independent of the foreground.
True. If you are keying a silhouette, you can really kick up the level of the screen.

But... Just thinking about

Y = 0.56G + 0.30R + 0.14B

If your screen is pure green, you can't have over 56 IRE as a legal color, unless you desaturate (add some Red or Blue to that Green). For instance, the only legal color at 100 IRE is white by definition. Sure, you can pump Green past 100%, but that's not a legal color, and risks clipping.

Of course, legal colors for a green screen don't really matter - if it all works for your signal flow and software. Who cares if it clips as long as you get a good key. The 120% green will be keyed out before it ever hits a broadcast tower or a projector anyway, so it's not as if you'll run into conformance problems.

Anyway 65 IRE green made my brain derail for a moment....
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Old March 2nd, 2007, 11:09 PM   #10
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Jon,

My theoretical engineering is pretty weak so if you can - please explain what you mean by
Y = 0.56G + 0.30R + 0.14B

& what you are talking about with respect to going over 100.

I get that Y = 0.56G + 0.30R + 0.14B adds up to 100 and gather that if the G is up at 60 then the total will be higher, but i don't know what it all means.

Sounds interesting though.

I usually put keys around 40- 45 but I have some distant memory of putting them higher at one time, but I can't remember why or when.

Jim Arthurs said he puts blue screens at 35 IRE and Greens at 65 IRE. How's that grab you?

Have you had experience pulling keys that use smoke or other semi transparent materials? I hear about this but only tried it once years ago and after being reassured by a technical "expert" on set that it would work, the results were disastrous.
My guess on that shoot was that the client cheaped out on his software in the end, (Rock Video- the money was going up his nose) but I always wondered whether the so called tech expert was also full of it.
That was the only bad experience I ever had with a key.

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Old March 3rd, 2007, 12:11 AM   #11
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Here's a rough test that was done with Keylight.

I had my stepdaughter hold the water bottle just to see if it would key the empty bottle. Not bad for a test. Since then I started using a different green fabric which gets better results.

Also, there's a "hole" where a green spot on the bottle's label keyed out.

http://imaginationhawaii.com/videos/...st_720_24h.mov
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Old March 3rd, 2007, 02:41 AM   #12
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Hi Lenny,

Let's say you have an analog camera with RGB outputs. On a perfect green screen R and B will be 0 and G will be 100% (or 700 mV in analog land).

Fine, but that's RGB. What about a YUV Component signal? That's where we use the Y = 0.56G + 0.30R + 0.14B equation. There are similar equations to figure U and V.

Anyway, the 100% G signal ends up creating a 56% Y (Luma) signal, or 56 IRE. U and V set the hue and saturation. In this case, we have 100% saturated green.

To get higher than 56 IRE from a pure green, we can crank the light/exposure/gain to output more than 700 mV, but at some point the thing will start to limit. In digital we will get a hard clip.

But an interesting thing happens in the YUV domain. We can make illegal colors, yet keep YU and V well within the legal limits.

Example: Set Y = 0; U = 100%; V = 0. With Y at 0, we have black. But with U at 100% we have super-saturated blue. It's illegal. You can't have super-blue black.

If we convert our super-blue black signal to RGB, for instance on a TV or projector, we would get a high Blue level, but NEGATIVE Red and Green. That would be less Green and Red than Black - it would have to suck the Green and Red out of your eyes to make up for the brightness of the Blue! Hence, it's illegal. TVs can't do that.

65 IRE Green isn't quite as bad. It means that the Green is brighter than 100%. That's likely to clip on an LCD or Plasma display. It might not on your CRT. (It might limit, or even pop a circuit!)

It's best practice to avoid illegal colors, since they could clip or be inaccurately produced down the chain.

However, for a chromakey you can probably get away with it, since the super-green will never be seen on a consumer's TV.
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Old March 3rd, 2007, 08:09 AM   #13
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Originally Posted by Dean Sensui View Post
Here's a rough test that was done with Keylight.
Dude, you need to clean your garage. ;)

Anyway, can I ask which green fabric you're using? I've been trying to find some comments/reviews of chromakey fabrics and can't find much. All the various manufacturers claim theirs is best, but I'm still looking.

Ideally, I'd like a 10x12 fabric... B&H has a couple options and so do some other dealers. I'm really eyeing the reversable green/blue fabrics as occasionally going blue can be useful.
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Old March 3rd, 2007, 11:39 AM   #14
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Dude, you need to clean your garage. ;)

Anyway, can I ask which green fabric you're using? I've been trying to find some comments/reviews of chromakey fabrics and can't find much. All the various manufacturers claim theirs is best, but I'm still looking.
My daughter and her husband moved in temporarily with us in our small house, so that's as clean as it gets! :-)

I'm now using a fabric from eefx.com. Wrinkles flatten out by themselves after a few minutes. And the fuzzy texture of the fabric means it lights up very evenly without hotspots. With just two Lowel Tota Lights I got the entire 10-foot width of the green screen consistently illuminated to within a quarter-stop at 60 IRE.

The fabric is also reinforced with nylon so it's tear-resistant. Pretty tough stuff.

Price is reasonable and the color provides an excellent key using After Effects' Keylight.

Here's a commercial spot we shot using this fabric:

Quicktime version:
http://imaginationhawaii.com/videos/noa_noa_pvo.mov

Original rough cut with demo audio; green screen:
http://imaginationhawaii.com/videos/noa_noa_green.mov
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Old March 4th, 2007, 04:39 PM   #15
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My daughter and her husband moved in temporarily with us in our small house, so that's as clean as it gets! :-)
Hehe. I was just teasin'... My garage is almost as bad and I have no excuse. :-O

Quote:
I'm now using a fabric from eefx.com. Wrinkles flatten out by themselves after a few minutes. And the fuzzy texture of the fabric means it lights up very evenly without hotspots. With just two Lowel Tota Lights I got the entire 10-foot width of the green screen consistently illuminated to within a quarter-stop at 60 IRE.
Thanks, I'll check 'em out.

Quote:
Here's a commercial spot we shot using this fabric:
Hey, you've posted this one before, I know I've seen it. It's nice to see the green footage though. Thanks for posting that. :) Anyway, nice commercial spot, well done.
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