ELECTRICCHALK

…or blue screening.

We only starting to use this technology at our place about 18 months ago even though the it has been around for a bit mainly due to issues about pedagogy and our own levels of confidence. It is, however a really useful and fun thing to use in teaching and learning. Seeing people students have filmed magically transported to far away times or places creates a real “ooh” moment and can enhance teaching about storytelling methods or appreciation of place or period.

This is a piece we put together with one of our partner schools to help them introduce Mary Shelley’s Frankenstein to Y10′s. The actor is the school’s head of drama and the images are all Flickr Creative Commons. The sound effects came from the Freesound Project and the music from the Audio Network Library (schools’ license).

Frankenstein Introduction from Chris Thomson on Vimeo.

Tips for getting a good “key”

I don’t claim to be any sort of expert and our first forays into it were pure excrement. The following is what I learnt from making mistakes and advice from other, wiser types.

Use green or blue as your background.
The reason these 2 colours are used is mainly because it is furthest away from natural skin tones (unless you are hungover!). This way, when you remove the green, say, from the clip you don’t make your face transparent. It is possible to key out any colour, though. Just make sure that the object you are filming isn’t the same colour as the back. I’ve been told also that green screen works best for people with dark hair and blue screen for blond but I’ve never had problems with either.

You don’t need to spend thousands on professional equipment.
The best results I have had for doing chromakey work were filmed against a wall we had recently painted a lurid green (see pic).

Green screen painted wall

So long as the surface you are filming against is smooth and uniformly coloured and lit you can get away with most things. If you are using fabric, make sure that it is kept in a way that minimizes creases. Hoover it occasionally and I never let students on it with their shoes on!

Find out how to set the white balance on the camera.
You may have noticed when using digital movie cameras that if you change from filming in natural light to indoors light the hue changes slightly. This is the automatic white balance kicking in. Briefly, white light comes in different “temperatures” depending on the lighht source. Ever noticed that old-style light bulbs seem to give a warner glow than their energy saving brethren? There you go. The camera guesses the type of light it’s filming in and tries to compensate to keep all the colours looking natural, only sometimes it doesn’t quite work. Setting the WB manually tells the camera what white actually looks like so it can adjust the colours properly. How to do it? Once you have set up the screen and the camera and sorted your lighting get someone to hold a clean piece of white paper on the spot they will stand when being filmed. Zoom the camera right in so all you see is the white page. Now set the manual WB. You may notice a distinct change in the colours when you zoom out.

It’s important to do this so that green or blue are truly that colour on the clip. Otherwise you may have problems with the keying later.

Lighting – You can get away with rudimentary lighting so long as you obey a few rules:

• The backdrop needs to be evenly lit. If one bit is brighter that another then it won’t key out evenly and you’ll be left with fuzzy bits on the screen in the wrong place (called “Artefacts” apparently)
• Get as much natural light as you can. I once set up a screen facing a huge glass wall in a gym and that seemed to get good results (despite lousy audio)
• If you can get a light to shine on the actors from behind. This helps to outline then well against the backdrop and makes the shot look more professional.That’s all we did for the embedded example.
• Make sure the actors stand at least 2 feet from the screen to avoid shadows on the backdrop. Shadows make keying really tricky as your screen in no longer evenly lit. Also, remove any reflecting jewellry, glasses, props. If they reflect the bacground colour they’ll go transparent.

Clothing.
Obviously, avoid wearing the same colour as the background unless you want to look like a floating head. I’ve also found that wearing black or white is a problem for some software as it goes a little transparent.

Movement
The easiest way to act against green screen is to not move at all. If you watch the clip above we use the positioning of the camera to create a bit of dynamism in the footage. Walking on and off the shot always look rubbish as it’s all done in profile and nobody’s face looks good in profile apart from Liv Tyler. When your students are storyboarding they need to think cinematically, not theatrically. In other words, think how it will look on the screen, not how it looks then and there in the room.

Film in a quite place with as little echo as you can manage.
Audio is one of the bits which a lot of people forget about but can make real a difference to the outcome. Imagine footage which looks like someone is walking in a forest but sounds like they’re in a toilet. You can always add reverb an’ that later.

Use decent software.
It’s worth spending a bit of cash on software if you are going to do it seriously. I’ve already stated a preference for Adobe Premiere Elements as you get the same level of control as you do for their pro editing software (just about). We’ve used Ulead’s Videostudio 10 before which gives you results really quickly but not reliably. iMovie and Moviemaker can manage it but require extra downloads (imovie and moviemaker) and the results aren’t that great.

To make the editing job easier try to crop the clip so that all your are left with is a small area round the actor.
This means you only have to worry about keying out the area immediately round the actor. Watch though that they don’t move out of the cropped area or they will look like they are disappearing into thin air.

Setting the key
Some editors have an eye dropper tool which let you click on the image of the clip to tell it what colour to remove. For best results, follow the tip above about cropping then click on an area near the actor. Make sure you play around with the “similarity” settings. Basically, the lower the similarity the narrower the variation of pixels that go transparent. Conversely, if you set it way high all pixels will become see through and noby wants that, do they?

And finally – don’t worry about it!
You’re not George Lucas so nobody will care if it’s all slightly squiffy and looks like dodgy 70′s Dr Who. The experience of the process is often enough for kids to get excited about. I spend quite a long time getting things just right but that’s because I’m anal, it doesn’t make me a better person to talk to in the pub.