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low light + greenscreen


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#1 ben jones

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Posted 16 April 2006 - 09:06 AM

Hi all.

Im going to be shooting a short in May whereby two men float down a river in a boat. Its a night time scene and the background (including the boat) will be computer generated (gulp!).

Now I wont pretend that I have a wealth of experience with green/blue screen practice, but I have been reading the past discussions on similar matters but am still a little uncertain in my approach.

first of all, im shooting on a DVX100 in 24P mode. The scene it self will be lit by 'moonlight' with maybe a lantern as an additional practical source. The faces of the charactors (esspecially the one paddling) can naturally fall into the shadows.

My questions are as follows - is it ok to light the charactors as close to the final desired look as possible (slightly underlit I guess) and still shoot it against a green screen that may be a few stops brighter than the key (moonlight).

or

should I expose the charactors a little brighter than intended for keying purposes?

or

should I light the charactors as intended and expose the screen slightly under the key (moon)

Any suggestions would be much appreciated. as I mentioned before, I have looked through the older posts, but need a little more guidence. I appologise if im making anyone cover old ground!

regards, Ben.
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#2 Lyle Holmes

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Posted 17 April 2006 - 12:42 AM

I hope this helps. I just completed a DV project w/ bluescreen (blue because we had a lot of green in the shots). A couple of notes:

1) Post (assuming Flame or Smoke) may pull a luma key as well as chroma; screen should be lit at least 2 stops brighter than tthe rest of the scene. This will let post tweak both luma and chroma.

2) Greater the distance between subject (forground) and greenscreen the better. Make sure the screen is evenly lit w/ no shadows.

3) Avoid any narrow vertical objects in the foreground...such as oar handles. These become more difficult to key around.

I would light the set as close to finished picture as possible. Since you are shooting "night" you shouldn't have to worry about overlighting, but don't go too dark either. If you have the luxury of color correction, you should be able to adjust the picture to your desired final look if you don't lose all your detail info (in blacks and whites).

I would try to make the sceen the brightest object on the set. I don't know exactly how you intend to effect the moon (practical or sfx), so I'm not sure about lighting of the moon relative to the screen.

Hope this helps.

Lyle
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#3 Michael Nash

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Posted 17 April 2006 - 01:32 AM

You should be able to light your foreground to whatever level you like and still pull a clean matte if the green screen is evenly lit and properly exposed. The only problem can be if bounce or reflection from the green screen starts to become visible on your foreground subjects. The easiest way to avoid that is to create enough distance between the greenscreen and the foreground (although that doesn't help with reflections).

You may not want to underxpose the foreground too much though, since it will have to match a similarly dark background. You'll want enough detail present in the foreground layer that you can still tweak the composite for the best match.

I wouldn't recommend overexposing the green screen however. One reason is the bounce I mentioned, but also video cameras tend to lose saturation as you get into higher luminances. I don't know that I buy the notion of trying to get a good luma key and chroma key off the same screen -- if it's bright enough for good luminance separation you're sacrificing chroma, and vice versa. I've never heard of a blue or green screen OVERexposed for a better key, and I've been doing both colors for years. Rule of thumb is key level or darker, depending on the compositing system.

And it should go without saying that DV25 is a really crappy format for keying...
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#4 Bob Hayes

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Posted 17 April 2006 - 01:42 AM

The key is taken off the background so the foreground can be completely black. Shooting low key has some tricky elements. When the subject is very dark or unlit there is a danger of the green screen giving a green edge to the foreground. This is especially a problem with reflective surfaces. This is usually solved by giving a slight edge to the foreground. This technique, while effective, can often make a shot look phony.

I find it difficult to light a low key scene when looking at a glowing green background. Sometimes I will drape a black solid over the green screen and light the dark scene as if the background was black. Then drop the solid and shoot the green screen.

The DVX 100 may not have the resolution or color depth to pull a quality green screen.
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#5 ben jones

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Posted 17 April 2006 - 09:32 AM

Thanks guys. really helpfull.

Ive taken on board what you all have said - From the small experience I have had with chroma key practice (dv) I know that Final cut pro HD is terrible at pulling a good key. I think this time its being done in something like key light? Any how DV is what I have been given, so fingers crossed.

I planned to so something similar to the drape idea - I plan to just turn of the screen lights altogether.

thanks again,

regards Ben Jones.
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