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Hard Install
We now come to the permanent chromakey installations.
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| Using a flat wall for chromakeying is great for medium and close shots. |
If you are going to be doing a lot of full shots where action might be taking place (a light saber duel perhaps?) then a chromakey cyc wall will be ideal. This type of set usually has two walls and a floor that curve into each other. A lot of people will also refer to these as infinity walls because the smooth, seamless transition between floor and wall gives the illusion that it goes on forever.
If you do not have a large space to build a dedicated set, there are many production studios that have cyc walls of various sizes. This means you have to rent the facility but often they will throw in a lighting package to help light the backdrop. A large number of small and medium size production houses will have a chromakey cyc set as more and more shooting for compositing is being done.
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| Even a medium sized hard cyc wall will let you shoot full subjects for post. |
That should be enough background on the background to get us to the next step.
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The Shoot
If you are shooting in DV format, there are some "gotchas" and "be carefuls" you should be aware of. The first is that DV is a compressed format much like JPEG, and can result in blockiness of the image or increased aliasing along curves and diagonals. The 5:1 compression ratio can result in less than desirable artifacts that can be difficult to get rid of during the keying process. If I could discourage you from shooting in DV, I would suggest using an analog format (such as Betacam) where you would get much better results. But because most low budget film autuers usually shoot DV we’ll stick with that.
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| If you look closely at the right image, you will notice compression artifacts that can show up in digital video. |
Depending on the digital format, different compression schemes will be employed; 4:2:2 (D1), 4:1:1 (DV and DVC), 4:2:0 (variations of DV), and so on, with each number representing the sampling ratio of the RGB/YUV channel. The fewer samples per channel, the more video artifacts occur, thus preventing a clean key from being made. If you are shooting in DV format, the green information comes from the Luminance (Y) channel. This channel is often the least compressed resulting in a better key. So often you will find using a green screen beneficial on a DV shoot.
If you are using a cloth backdrop, make sure there are no wrinkles and the fabric is smooth. This can be a problem if the cloth screen has been folded up in its travel bag all night and hung moments before the shoot. A simple solution that I have discovered is to take one of those portable steam irons with you. These handy items will take the wrinkle out of the cloth in no time and can be used after the cloth is hung.
One thing that is often overlooked when doing a chromakey shoot is lighting. Lighting a green screen is much different than how you light your talent. The green screen should be evenly lit, with no shadows. Kino Flo lights can sometimes be the best lights as they spread the light directionally and evenly. If Kino Flos are out of your price range, and all of your broads are being used, then use your spots and bounce them off a reflective surface like foamcore. The key to lighting a green screen is not to over light the backdrop because you will end up with some serious spill issues on your talent.
Spill is undesirable light that has bounced off the background and onto the subject. You can often see this as a slight discoloration in the shadows of the subject and is a dead give away that keying is being done. A way to keep spill from happening is to make sure the subject is moved as far away from the background as possible. The further the better. Then light your subject naturally, making sure the subject light isn’t falling on the background as this can cause unevenness on the background. Make use of flags and barn doors to keep this from happening.
Source
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