Will Head
Once the preserve of blockbuster movies, green screens are becoming an increasingly popular tool. Here’s how to set up your own.
Green screen effects may have once been the preserve of big-budget Hollywood movies, but chroma key, as it’s also known, is increasingly cropping up in places you wouldn’t expect to see it.
Take a peek at Stargate Studios’ Virtual Backlot Reel 2009 (bit.ly/d4PnpE). In addition to scenes where you would expect it to be employed, such as explosions, spaceships and exploding spaceships, you can see that it’s even used for general outdoor shots in TV shows such as Ugly Betty.
While some of the more advanced techniques used still require blockbuster-shaped budgets, the basic principle is simple enough to execute with just a solid area of colour to shoot against, and a powerful enough Mac to process the video afterwards.
The basic principle behind chroma keying is to shoot your subject in front of a single-colour background and then remove that colour completely, so it’s transparent once you’ve imported the footage to your Mac. So rather than standing in a studio in London, the actor could be on a beach in the Caribbean or in front of the Eiffel Tower. Alternatively, you could place a presenter in front of a virtual set, which saves having to put a set together each time you shoot a video for a regular production.
Perhaps the most important thing about any work that involves chroma keying is to start with the best possible footage to work from. Since you’ll be digitally manipulating every single frame, the easier you can make the job for your Mac, the better the final results will be. That’s not to say you can’t tidy things up a little later, but you’ll have a much easier job if you have good original footage.
The first thing to consider is what colour to shoot against. You could choose any shade you like, but you need to ensure that the colour you want to remove doesn’t feature in any part of the foreground image you want to keep. If it does, you’ll end up with transparent areas in the part of the foreground shot, which will ruin the whole effect.
If you’re shooting people, that rules out red and leaves either blue or green (the purer the colour, the easier it is for your Mac to handle). You can choose either to shoot against, but obviously if you’re filming in front of blue, you can’t wear jeans and if you go for green, then it’s best to steer clear of khaki or any earth tones.
The area that you’re shooting against also needs to be one solid colour to get good results once it comes to cutting out everything on the Mac. Depending on the size of the subject you’re filming, you’ve got a few options available.
If you’re just shooting the head and shoulders, then you could get away with a portable collapsible screen, since there’s only a small area you need to fill. You can find suitable 1.5 x 2m screens for around the £100 mark, and they’re often reversible, with blue on one side and green on the other, so you have options when it comes to shooting.
If you need more of your subject in shot, or you’re filming more than one person, then you’ll need a bigger area to shoot against. A 3 x 6m fabric backdrop should be sufficient, but it’s best to check that it’s designed for chroma key work and not just as a regular background. A suitable material backdrop will set you back around £50, and you’ll need a stand, too, which you can pick up for around £80 if you don’t have one already.
A stand that’s 2.5 x 3m will provide plenty of space for your subjects. By draping the fabric on the floor and taping it in position to create a curve, you’ll also produce a seamless transition that will appear to be solid colour where the floor meets the screen. The main disadvantage of fabric screens is that they can often get creased or wrinkled while being stored, so the finish isn’t always completely flat.
If you have a large, smooth wall available, then you can buy specific chroma key paint and make your own permanent screen. Rosco Chroma Key paint comes in both blue and green, and a £60 tin will easily cover a couple of walls. If the plaster finish is smooth, then you won’t need to worry about uneven surfaces complicating your shot. The downside of painting a wall is covering the join where it meets the floor. For perfect results, you’ll need to create a curved ramp – also called an infinity cove – that blends the wall into the floor. Installing one is considerably more expensive than just painting a wall, however. If you’re just shooting from the waist up, though, then it’s not an issue. If you do need a person fully in shot, then the other option is to create a raised platform for them to stand on and paint that the same colour as the wall. Provided the platform is far enough away from the wall and lit correctly, the result should appear to be solid colour to the computer.
Once you’ve got your screen in place, the next thing to consider is lighting. In addition to lighting your subject as normal, you’ll also need to light the screen as evenly as possible. Differences in light levels will result in the colour varying in shade when the footage is imported into your edit program, so the more uniformly you can light it the better. More low-power lights are better than a few high-power ones. You also need to ensure that there’s sufficient light on the wall to avoid any shadows being cast by your subject, as that can also make things difficult once you’ve captured the shots to your Mac.
One way to help reduce this is to place your subject as far away from the screen as possible. Obviously, you’ll need to ensure that the screen still covers their outline, so how far away you can move the subject depends on how big your screen is (as well as the size of your studio). Ideally, you should be aiming for at least a couple of meters gap – more if the space will allow. Green light can be reflected back from the screen onto foreground elements if they’re too close. If any of this spills onto things you want to keep in the final shot, then cutting them out on your Mac will be tricky. The greater the distance between the screen and what you’re shooting, the less likely this is to happen.
Putting a decent amount of space between your subject and the screen also has another advantage – if you then shoot with a short depth of field, your performer will be in focus and the screen blurred slightly. This helps create the appearance of a solid area of colour and will remove some of the imperfections in the screen, which is especially useful if you’re using a fabric screen that’s creased.
When it comes to framing your subject, you don’t have to worry about safe areas or even too much about where they are in the shot. Since you’ll be cutting out the background, you can then place them anywhere on the frame during the edit. The main thing is to ensure that the background fully covers the area behind their outline. If they need to move during the shot, then rehearsing this first lets you double-check that they’ll be in front of the screen at all times. Even if they’re going to remain relatively static, it’s worth getting them to extend their arms to ensure they don’t go out of shot or off the background.
Finally, you’re ready to start filming, but even at this stage it’s worth shooting some quick test footage to check everything will work once you get the footage onto your Mac. If you can quickly test a shot while you’re in the studio, you’ll know that it works and not have to spend a lot of time later trying to fix mistakes or possibly even reshoot.
Of course, shooting your green screen footage is only half the challenge – there’s still plenty to do once you’ve got it onto your Mac, which is something we’ll be covering in a future column.
