GreenScreen: Over lighting?

kentwolf wrote on 3/28/2006, 6:59 AM
I am working on a (home made) green screen setup that’s working pretty well.

The only issue is that around the shoulders of the subject, there is VERY SLIGHT green showing through. It’s not keying there as nice as I would like it. That is, I believe, the ONLY problematic area.

Could this be due to over lighting?

The green screen is approximately 6.5 Ft high, approximately 42” wide.

Top = Ceiling/rafters, Bottom = floor.

It is illuminated by 4 (four) 60 W light bulbs, emanating from the 4 corners, inward. Top left/right pointing toward the center of the green screen; bottom left/right, pointing the center of the green screen. It appears to be pretty evenly lit. There are not trails of light so as to be obvious that I have 4 lights, 1 at each corner. The lights are approximately less than a foot in front of the green screen.

There are 3 (three) 60 W light bulbs illuminating the subject, top left/right, pointing down toward the subject, one at bottom center, pointing up toward the subject.

There are a total of 7 (60W) lights plus ambient lighting. It certainly appears to be well lit.

Due to space constrictions, the subject is about 2 feet from the green screen, the camera is about 3 feet from the subject. There are no apparently no shadows on the green screen from the subject.

Questions:

1.) What are the symptoms of over-lighting?

2.) Could I possibly need more lighting? Go from 60 W to 100 W bulbs?

3.) Could my close proximity of camera, subject, and green screen be an issue?

4.) Is there a common cause of fringing at the shoulders of the subject?

I am really, really close to good execution of this, except for the issue mentioned.

If you have any other pointers would be most welcome.

Thanks!

Comments

JohnnyRoy wrote on 3/28/2006, 7:17 AM
> Due to space constrictions, the subject is about 2 feet from the green screen

There is your problem. You’re getting spill because the subject is too close. You’ve got to get the subject at least 5 feet from the screen to eliminate spill. What you should do is put a backlight on the subject to highlight the rim of their torso which is now filled with green. You might even want to add a red/purple gel to the light to neutralize the green spill. Also turning down the lights on the background will lessen the spill as well.

You really need a lot of room to get a good key but if you don’t have room, try and control the spill with your lights. If that doesn’t work, try a less reflective green background (i.e., darker green)

~jr
kentwolf wrote on 3/28/2006, 8:20 AM
>>...put a backlight on the subject...

Will do. I currently do not have that.

>>...turning down the lights on the background..

I thought the better it was lit, the better, overall. I did notice that I got a little better key without the lower lights, or so it appeared. I'll test it out.

>>...less reflective green background...

Actually, I would have to say that this is virtually 0 reflection; it is not reflective at all.

You certainly have given me something to go on.

Thank you very much for your help!
Yoyodyne wrote on 3/28/2006, 8:40 AM
Here are my tricks for greenscreen, your mileage may very...

1) try and have the screen as wrinkle free as possible. What has worked the best for me is go to a photo store and get one of those giant rolls of green paper - put it between to c stands and roll it out behind your subject.

2) get as much distance between your subject and the screen as possible.

3) backlight and if you can get a hold of it add a minus green gel (available from roscoe and other cine/camera stores) it should look pinkish and it adds the exact opposite color of green to the rim of your subject. Can really help kill the spill.

4) use your zebras to set the green ire value to around 60. I do this by setting the Zebras to read at 70 and then closing em' down a bit - it's not a perfect sience but you only have to get close. Then light your subject appropriately.

5) make sure your subject does not "go outside the green area". This is really important if you don't have a green screen that fills the frame - hence the love of the big sheet of paper! Watch hands and gestures because trying to fix that is a monster time killer.

6) shoot HD - oh man, chorma key in HD and HDV is so easy compared to DV.

7) get serious magic ultra. This thing is amazing, it will turn hours worth of work into a few minutes. Especially if you have very hard to key stuff going on!

Just my 02. - Hope this helps and good luck!
GregFlowers wrote on 3/28/2006, 8:45 AM
It just needs to be lit evenly. It does not need to be lit excessively. With the subject only two feet away, the green screen should have just enough light on it to wash the subject's shadows out. The green spill can often be harder to fix than shadows. A backlight can be very helpful. Also masking out extra parts of the green screen in Vegas and using the seconday color corrector to make the green more even and saturated can be helpful. You may also use a keying program that can remove spill and mabey add a "light wrap" to improve realism.
kentwolf wrote on 3/28/2006, 1:46 PM
>>..."light wrap" ..

I am not familiar with that.

What is it?

Thanks.
GregFlowers wrote on 3/28/2006, 2:42 PM
Some compositing programs, like the old Commotion Pro, come with advanced keying and compositing tools to seamlessly blend the foreground and background elements together. In real photography and video, background light, such as a sunset, carries over the edges of the foreground element. So in the case of a sunset, the orange sunlight actually falls onto the head and shoulders of the subject in front. Light wrap or equivalent is able to replicate this phenomenon to make the subject in front of the green screen more seamlessly blend into the background.

Commotion came with the Primatte Keyer, which was excellent for its time. It also came with Composite Wizard and Image Lounge which added Light Wrap, background color matching, real shadows, etc. Those plugins were invaluble at creating realistic composites.

DSE,
How does Primatte stack up against Serious Magic Ultra? Does Ultra come with any tools to help match the foreground and background elements?
Coursedesign wrote on 3/28/2006, 3:40 PM
Digital Film Tools Light Wrap is a very good lighwrap plug-in.

It needs After Effects though.