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Old August 5th, 2004, 02:50 PM   #1
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Green (or blue) screen advice

Hey guys--A new client would like me to shoot some green screen talking heads for them. Just a locked off medium shot of a person addressing the camera, standing in front of a greenscreen, that will later be encoded into Flash for integration into a digital training product they use. I have never shot green screen before and would love some advice. I am aware that the green screen should be evenly lit and to separate the subject as much as possible with lighting from the screen. But from there I am totally open to suggestions. Here is my gear:

DVX100A
Lowel lighting package
Avid Xpress Pro
After Effects
Sorenson Squeeze

I am wondering if I need any new software, and what sort of greenscreen backdrop you guys might recommend. What would be the best workflow given the tools I have and hte ultimate delivery? Thanks!

Peter
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Old August 5th, 2004, 03:36 PM   #2
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Hello,

Well I have always been told that Greenscreen is better for DV.. I have used After Effects (production bundle) for keying before..It works.. If there is any problem with lighting it can be tricky and hard to get a good key..I just upgraded to 6.5 which has a new keyer called Keyligh( I think thats it, dont quote me)it is supposed to work well with DV..but i have not tried it, as I purchased Ultra from Serious Magic, and it is superb for this type of work.. even if your lighting is not perfect..
One thing I can suggest is once you have the lighting setup and the camera locked down, shoot a few seconds of just the green/blue screen before the client is positioned.. and later on this can help, using a difference key..

good luck
Mike m.
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Old August 5th, 2004, 09:17 PM   #3
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thanks for the info mike.

peter
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Old August 6th, 2004, 04:39 AM   #4
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Keylight for After Effects will get you miraculous results. If your AE does not have it, check the Keylight website for the download.
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Old August 6th, 2004, 02:50 PM   #5
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I do a lot of chroma keying and use a product called Ultra from Serious magic (www.seriousmagic.com). It does wonders with chroma keying.

It's a few bucks but there's so much more to the product than just chroma keying. And personally I like (and have better luck with) green screens better than blue.
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Old August 6th, 2004, 04:26 PM   #6
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Thanks guys. Any recommendations on a green screen backdrop? I think we'd like to use a large fabric backdrop rather than the collapsible kind. THanks!

Peter
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Old August 6th, 2004, 04:38 PM   #7
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The green foam board found at places like Hobby Lobby and Michaels (sometimes) works just fine. It's cheap, portable and easy to use. It's got a flat to semi finish so it keeps green reflections to a minimum.
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Old August 6th, 2004, 07:03 PM   #8
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I'll cast my vote for AE's Key Light filter. I tested Key Light with a Star Wars toy over bright green construction paper and poor lighting with a 1CCD cam and STILL got a decent result. I've got some screen shots and a short MPEG2 clip:
http://www.philipwilliams.com/chromatutorial.aspx
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Old August 9th, 2004, 01:12 PM   #9
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Yep, Keylight is phat.

The cheapest greenscreen I ever used was a white bedsheet painted green with clothing-paint tablets. But you can propably also buy green cloth for a reasonable price. Not recommended for outdoor shooting.

The exact color really doesnt matter.
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Old August 9th, 2004, 05:06 PM   #10
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Thanks guys.

Peter
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Old August 11th, 2004, 08:00 AM   #11
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Here's a writeup on selecting blue vs. green.

This link demonstrates one option for building your own very large screen for only a few dollars (and no painting)

The exact color doesn't matter to be able to key something, but it can be optimized to improve the degree of detail you can extract from edges and areas where motion blur results in pixels that are a mix of foreground and backdrop color.

Most modern keyers are using a color difference calculation (rather than chroma) to production the matte. This uses the mathmatic difference between the key color channel and the non key channels to calculate a matte. (If your key color is Blue, the values in the Red and Green channels are compared, and the higher of the two is subtracted from blue to result in a number that will be used for Alpha)

What this all means is that you can optimize the color of your bluescreen by selecting a shade of blue that (to you camera) appears to have almost no brightness in the red and green channels AND has a very high value in the blue channel. For video, 25,25,210 would be fantastic, but more often, you end up with 40-ish, 40-ish, 170-ish.

Unless you deliberately practiced a lot, your eyes are not a very good judge of color channel values. A very bright green may look pretty good to you, but it actually has a lot of red (that was the case for the little greenscreen shown at the bottom of that second link)

The best thing you can do to optimize your color is to go to a paint store and pick up a bunch of those color sample cards with lots of shades of the same color, bring it to where you will be shooting, and using the same lighting and camera you plan to use for the shoot (don't forget to whitebalance to a good refenerce that you will have at the shoot), and get some video of the cards. Now load up a few frames into a program that will let you evaluate the RGB channel values: Media Studio Pro Video Paint, Photoshop, After Effects, etc.

Now you can identify the one shade of color that gets you as close as possible to 0,0,255 with YOUR equipment at YOUR location. Use the reference card of that color when shopping for cloth or other pre-colored materials, or simply take it back to the paint shop to have them mix up a can for you for under $10. (Make sure you get FLAT, not glossy, paint and pick up a can of primer too. A coat of primer often saves two coats or more of paint.)

Hope this helps.
Have fun.
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Old August 12th, 2004, 09:58 AM   #12
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Hey Nick--Thanks for that great explanation of keying. It will really help for the shoot. In regards to color balance, is it very important that I have only a single color temperature where I shoot? I imagine this to be the case. In my situation it is possible I will have some daylight coming through windows. I can gel my lights, or I could try to black out the windows, and then do a manual white balance to account for any mix. I guess my question is: is it a really good idea to have only one color temp (and is there a better one: daylight or tungsten) when doing green screen work? Thanks!

Peter
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Old August 12th, 2004, 01:49 PM   #13
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<<<-- Originally posted by Peter Richardson : ...I guess my question is: is it a really good idea to have only one color temp (and is there a better one: daylight or tungsten) when doing green screen work? -->>>
Consistency is the big thing you're after to keep the post production workflow moving. On that note, yes, it's a VERY good idea to ahve only one color temp on set and have your camera calibrated for that throughout the shoot. This will help avoid color variations from one shot to the next, which will in turn, make keying shot after shot much smoother than having to make many many adjustmenst to account for color drift from one shot to the next, or worse, during a particular shot.

As for which (daylight or tungsten) is better, I really have no idea. All my compositing shoots have been indoors thus far. Anytime there are windows, I try to black them out, usually just using that black flowerbed fabric you can get from garden stores or garden departments at the home improvement store. My gut feeling is that the overall color temp of the shoot is not critical so long as you calibrate your camera and maintain consistency thoughout the shoot, but again, I really don't have experience with both daylight and indoor shooting for compositing.

Hope this helps.
Have fun.
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Old August 12th, 2004, 03:30 PM   #14
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We just bought one of these:
http://www.bhphotovideo.com/bnh/controller/home?O=productlist&A=details&Q=&sku=302984&is=REG

Pretty cool. You'll need a way to support or hang it, but its portable
and a cheap solution. I have not yet put it to the test but I don't
predict any problems.
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