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TUTORIAL: Compositing: Setting up a greenscreen

Posted: Tue, 20th Apr 2004, 1:29pm

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Simon K Jones

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Chromanator Tutorials: Compositing

Setting up a greenscreen

The easiest way to composite is to use a greenscreen. Chromanator can automatically remove the green from the image, saving you the task of having to manually cut out your subject (perhaps an actor, or a miniature). A couple of clicks and you will have a perfectly keyed subject ready to be placed onto another clip.

At least, that's the theory. Unfortunately, the process is often made more complicated due to a poor greenscreen set-up. Chromanator has been designed to work with less-than-perfect footage, so you do not need to worry about getting ideal shots every time. Here are a couple of examples of poor greenscreens, both of which Chromanator can still deal with:

Note the uneven lighting on the left-hand picture. The bottom left of the greenscreen is shadowed, whilst the top right suffers from a major highlight problem and along the top are a couple of big creases. Chromanator's colour difference key can still remove most of the green without any difficulties, requiring only a simple garbage matte to remove the highlight.

The right-hand picture is more problematic. In many compositing packages it would be almost impossible to use the shot. However, Chromanator can key out the green, despite the heavy shadowing. An animated garbage matte deals with the scaffolding and the rest of the image.

Of course, the general rule is that good quality footage will make the compositing much easier. If you want to save yourself time and hassle in post-production, it is worth spending a little more time and money setting up your greenscreen properly. There are a few things to bear in mind:

  • When using miniDV, it is best to use a greenscreen (as opposed to a bluescreen or any other colour), because consumer video cameras tend to store more information for the colour green. This will aid the compositing process and result in a better key.
  • Your greenscreen can be made from cloth, paint or paper. Each have their benefits and problems. Cloth is excellent if the greenscreen needs to be moved regularly. Paint is ideal if you have a dedicated, permanent studio space. Paper is a good alternative if you do not have the liberty of painting the studio area, but can prove difficult to move if necessary.
  • No matter which media you choose to construct your greenscreen, you must ensure that it is of a uniform colour, is flat and untextured and has a matte finish.
  • Check that the greenscreen is not shiny or overly reflective.
  • Be careful to avoid creases, as these can cause problems.
  • Try to construct it in an area large enough to light the screen and your subject separately. This will enable you to light your subject dramatically, whilst lighting the greenscreen in a flat and uniform manner. It is important to not cast any shadows on the greenscreen itself (unless you deliberately want to, of course).

Once you have set up your studio and filmed your footage, it is time to start compositing things together…

Last edited Fri, 27th Aug 2004, 2:42pm; edited 1 times in total.

Posted: Thu, 27th May 2004, 2:42am

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Sensoria

Force: 30 | Joined: 27th May 2004 | Posts: 1

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After reading this tutorial, I think that many of you will like this.

Please forgive my english since I dont always have the exact words in mind.

The perfect screen should be totaly flat, no curves, no curtins deformation caused by hooks, no hot spots from lights and easy movable. This cost alot of money unless........

when I was an editor/compositor (1995), I used to work for a small non profit organisation that obviously did not had money... Shooting was done on the worst media ever! Hi-8... Always full of glitchs, drop outs and so on... At the time, there was no uncompress at affordable price and resolution was AVR75 (about 3:1)

Now... trying to pull a perfect key in those conditions sound like "your dreaming" and that's where the idea came from.

Go to your favorite renovation store and look for vinyl floor cover... A retailler will gladly sold you his 12 feet wide unable to sold ungly model that is dying in the storeroom for 2 years... It's scratched, stained, the sides are shiped and he will almost pay you so he can get rid of it.

For you, only one thing is important... how is the BACK side of the floor cover! The back side is spongeous, porus, extra soft and gray as a base color... The perfect surface material for a green screen.

I put my hands on a 12 feet wide by 30 feet long roll for less than $100 CA. Bought Chroma Paint (flat paint) for $60 and a 14 feet long 2 inch DIA steel pipe for support for I dont remember... Add to that super glue and "rivet" and your ready to build yourself a high quality screen.

You completely unroll it BACK side facing up and fix the pipe to the end that was in the most interior of the rolled floor cover. Not only it will have a natural inward curve but will be protected from dammage.

You carefully remove any dust and roll paint the back surface with a first light coat so it get absorb by the soft material and tainte it. Once dry, comfirm that no dust or others are stick on the surface and apply another coat. On mine, it took 3 coats but it may vary depending of the material used by the manufacturer of the floor cover.

The result is a very flat, non relfective surface that will give you very good results even with poor lighting techniques.

OK, mooving around the thing needs space and good arms but it's precisely the weight that is helping you.

When you install the screen, you can just hook up the pipe for short time periods or build a more permanent fixture so that you can roll the screen up around the pipe when you dont need it, preventing damage at the same time.

What happen when you use it and the benefits using mine that I describe earlier about the size.

When you unroll the screen, you have enought to not only cover the back of the scene but also create a floor that you can walk on and add objects to interact with. With 12 feet wide, you can do quite alot with objects and multiple characters.

The mass of the screen make it stable. It wont move in wind condition. It also create a soft curve preventing any right angle complications between the floor and the back wall and is of course the exact same surface texture and color.... You certainly see the benefits here dont you?

The last things is about lightning. Even if this screen solution work great with about any flood light. I never ever use a white light on a screen for keying. I put gels of the same color of the screen used. Blue gel for blue screen and green for green. This will prevent white spots and chroma variation alot! Instead of having a green surface where you can see different tones of green caused by the light source, you will have a sort of glowing green feelling narrowing the chroma values and it is way easier to work with the material after that in post.

hope you will find this usefull
Posted: Thu, 27th May 2004, 10:37am

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Simon K Jones

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Good stuff, Sensoria. Thanks for the tips!
Posted: Wed, 20th Oct 2004, 10:10am

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NoClue

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Here's one for you. Slayer has green eyes. If we film using a green screen then key it out, will this produce "holes" in her eyes for the background plate to show through? If so, what could be done to resolve this (apart from using a bluescreen instead)?
Posted: Wed, 20th Oct 2004, 10:13am

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Simon K Jones

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Just put a simple garbage matte around her eyes, then change it to an Opaque Matte. This will retain the specified area.
Posted: Wed, 20th Oct 2004, 10:17am

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NoClue

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But doesn't that mean I've got to track her eyes for the entire length of the shot? This could be quite painstaking for a long film.

Perhaps I should just poke her eyes out with a pointy stick! biggrin
Posted: Wed, 20th Oct 2004, 10:19am

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Simon K Jones

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Or give her sunglasses. smile

It shouldn't be difficult (although depends on the shot obviously) - just a loose oblong shape around both eyes, you shouldn't need to use many keyframes at all. It's not like you need to track precisely to her eyes - you just need to have them covered.

What kind of shot is it?
Posted: Wed, 20th Oct 2004, 10:22am

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NoClue

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Just hyperthetical at the moment. Thinking out loud as it were.
Posted: Fri, 7th Jan 2005, 12:02am

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LilCaesars

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I bought this m4 at shorty usa and you can move all of the mods around
Posted: Sat, 8th Jan 2005, 2:20am

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ahaines

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I found that Henry's (www.henrys.com) carries paper for Blue and Green screening.

Available in 53 and 107 inch widths at 36 feet long.
$50 and $75 respectively (Canadian funds).

Studio Blue #58
Tech Green #46