Cloth Green/Blue Screen: Front-Lit or Back-Lit?

Soniclight wrote on 7/22/2009, 2:37 PM
Before I get to this, got a preamble question:

--- It seems that green screen is far more used than blue in terms of filming human beings since skin hues are warm. Unless one is using cooler hues in lighting and/or other indoor or outdoor elements in the scene lean into blues and greens.

Which would you use for indoor, no-other-green-or-blue scene influences situations?

To Back-Light or Not to Back-Light

I'm thinking out loud here for I'm in the process of getting my chromakey set up going (figuring out what to buy, etc.). I'll eventually find out for myself if I decide to go with muslin or whatever works best and I've got another thread on that going.

I know that the standard way of lighting is front-lit.

However I'm thinking that if one is a pauper who can't afford a battery of fancy lights, maybe careful and even back-lighting may be a way to go. For example, Home Depot and such work lights and/or 4' fluorescent tubes and fixtures are cheap.

Still professing ignorance since in discovery and research mode on this, here are my current pro/con musings:

Advantages:

-- No subject shadows or conflict with other frontside lighting.

Disadvantages:

-- Standard chromakey muslin may not be thin enough to let enough light through. So one would have to get something thinner that would be as matte as realistically possible.

Got any thoughts on any or some of the above?
Thanks.

Comments

farss wrote on 7/22/2009, 4:32 PM
One significant issue with CK work is stopping spill i.e. getting the green light reflected off the screen on the subject. What you're proposing is putting the subject in front of a large green light source. I can see a problem here.

Other thing is you ideally want the screen no more than one stop below key. A piece of green muslim would have to block a lot of light so as you've noted you're going to need a lot of light behind it. Even more of a problem will be getting even light behind it. In reality I suspect you'll find you need twice as much depth in your studio. You need space behind the screen for your lighting plus you still need the same space in front of the screen.

In the end to answer all of your questions we need to know what your intent is. If you want pixel perfect results with a subject that's moving quickly that has translucent areas you need everything absolutely perfect, screen, lighting and camera.

On the other hand if it's a static shot of a talking head with slicked down hair anything will do. If it's only got to look OK on YouTube even more so.

You do realise for a static shot you might not even need a screen at all, in fact using a green screen can make life more difficult. A difference matte can be quite simple to use and is very easy to do in Vegas.

Bob.
Soniclight wrote on 7/22/2009, 6:26 PM
Hmmm...

Good points on some more disadvantages of back-lit route. As far as motion of subject or subjects, it will be more towards static but not still (slow motion-- think yoga, if you will). I may play around with back-lit for close-up stuff one day, but looks like I'd better stick to standard procedures.

As usual, your insights are detailed, thoughtful and most welcomed, Bob.
Tech Diver wrote on 7/23/2009, 7:09 AM
Here is an article on chromakey lighting that you might find useful:
http://westernreservepublicmedia.org/producer/vmchroma.htm

I like to light from the front because if need to remove anything additional from the scene (like a dangling wire) I can just apply some green masking tape (colored specifically for chromakeying).
Soniclight wrote on 7/23/2009, 7:58 AM
Tech Driver,

I've looked at quite a few articles and videos on chromakey, and this one has some good tips not found in others. -- especially the lighting the green screen from above instead of side. Thanks
Tollaksen wrote on 7/23/2009, 8:03 AM
I don't know what makes a $35.00 piece of cloth different from $300.00 "chroma") but if your just shooting for fun I have found that "squareperfect" (Seller) on ebay has a 10'x20' screen for $35.00(Stand (w/carry case) for $70.00). When lit properly works like a charm for me.
farss wrote on 7/23/2009, 8:21 AM
What appears to make the difference is the pigments / dyes used to get real chroma key green. One local guy had his studio painted and tried to save by having the paint mixed locally however that proved to be almost as costly as buying the imported paint made for the job.

From my tests for non critical keying almost anything can be made to work, including a blue blanket lit by the sun and blowing in the wind. It does take a bit of work and the odd garbage matte but if your time is your own and you're on a tight budget why not. The really expensive stuff is for studios where it's just got to work as well as it can be made to work.

Bob.
Tech Diver wrote on 7/23/2009, 9:51 AM
For me the biggest factor in choosing my colored/chromakey backdrops is how resistant they are to wrinkles. The last thing you want to do is show up at a shoot with an ironing board. I found that the cheap backdrops wrinkle far too easily. Also, make sure there is a pocket (or whatever it's called) sewn along the edge for a support rod. As Bob said, just about anything can be made to work with regard to color/keying with enough fiddling. However, the more time you spend fixing errors, the less profitable is the job.

Peter
Soniclight wrote on 7/23/2009, 10:05 AM
Thanks to all participants in this thread, it's been educational. That said, I'd like to return to my as of yet un-answered other question:

--- It seems that

One reason I'm asking this is that green is technically closer to human hues on the color wheel since there is yellow in green. The opposite of red is green, that of blue is orange, that of yellow is purple, etc.

But maybe this doesn't quite work the same way in RGB?
Whatever the case:

--- What would you use for indoor, no-other-green-or-blue scene influences situations?
kkolbo wrote on 7/23/2009, 10:18 AM


With modern software for Chroma Keying, the old needs of a glowing green screen is over. It actually causes more problems than it helps unless your talent is well lit and 25 feet from the screen. I like to place my chroma drop outside with the sun behind it letting reflection light it evenly. I use an opaque background cloth or painted wall. Then I light the talent with reflectors. For fast and easy chroma keying nothing works better.


Keith Kolbo
Coursedesign wrote on 7/23/2009, 11:17 AM
Most video cameras have less noise in the green channel, so a green screen can give you a good mask more easily.

For film, blue screen is the first choice because the blue emulsion gives the sharpest image.

If you're shooting a video interview with a blonde, blue screen usually works best (because the blond hair contains more green than blue).
Soniclight wrote on 7/23/2009, 6:14 PM
"If you're shooting a video interview with a blonde, blue screen usually works best (because the blond hair contains more green than blue)."

Hadn't thought about this. So I'll have to consider whether I want a blond model/actress when I get to shooting my little short film. So far, most that I know happen to be blondes.

Well, there's always the route of using wigs :)
john-beale wrote on 7/23/2009, 6:42 PM
One other consideration: if you are using a single-chip Bayer-pattern sensor, they usually have twice as many Green sensors as they have Red or Blue sensors, so logically you'd expect better resolution for green colors. Even if you have a 3-CCD camera, I would expect better green resolution in most codecs, unless you are recording 4:4:4 uncompressed format.

I can't say whether this theoretical technical advantage trumps any practical factors, though. One other possibly minor factor: if you are spending any significant time on-set, the blue screens are reportedly more psychologically comfortable to be immersed in, than green.
Soniclight wrote on 7/24/2009, 12:53 AM
"...One other consideration: if you are using a single-chip Bayer-pattern sensor, they usually have twice as many Green sensors as they have Red or Blue sensors, so logically you'd expect better resolution for green colors."

The only camera I have is a Canon HV30, in which... "the sensor incorporates on-chip noise reduction circuitry and a Bayer Placement RGB Primary Colour Filter for accurate and vivid colour reproduction" (from 2008 CES article).

I suppose that is what you are referring to.

Recording-wise, I doubt the HV30 records in 4-4-4 -- t'would be nice, though :)