Chromakeyer is basically a device which replaces everything of one particular colour with parts of another picture. This technique is known to most people, although normally they refer to it as "green screening" or something similar. Keying (there are other varieties, such as luma-keying or green screen keying) is used in a huge variety of films and television programmes to achieve all sorts of effects.

Almost every time you see a close up of Harrison Ford dangling off a cliff in Indiana Jones, it is chromakeyed. All that stuff in The Matrix was chromakeyed (although they had a lot of other stuff going on too). Pretty much every scene in Star Wars episode one was keyed together (excluding the ones that didn't exist outside a computer in the first place), and then Jar-jar Binks was keyed on top of that (unfortunately).

These days, keying is everywhere. Fortunately that includes the transparency menu in Adobe Premiere (and somewhere in Final Cut too) so pretty much everyone can do it. But just because you have Premiere does not mean you can produce high quality effects. You have to choose the right colour of screen (green or blue is normal, red is used now and again) so that it is different from the colours worn by the people in front of the screen. Then you have to light the screen well. Finally, you must set up the chromakeyer well so that all of the screen is removed cleanly. Don't forget that you need a good background of course.

Making a chromakey screen

The first thing to bear in mind when making a chromakey screen is what colour you need. If you are filming a person wearing blue clothes and use a blue screen, they will tend to disappear. Also, it is best to avoid a red screen as this can mess with people's skin (particularly if they have bad sunburn). Now you need to decide how big your screen needs to be. Remember that things look smaller as they get further away and, as you need a good distance between the actors and the screen, the screen needs to be quite big. Now you need to find a huge sheet of suitably coloured paper (probably rolled on a cardboard tube for easy storage). In the GUST office we have a green screen which is about eight feet high and ten wide. This is good for shooting people from the waist up; knees at a push.

A brilliant alternative to a chromakey screen (although hideously expensive) is to find (or build) a nice room with curved walls and paint it fluorescent blue (or green...) This helps deal with the lighting problems you will have later. It also uses up your overdraft and gives your house an interesting discussion point.

Lighting a chromakey screen

As noted above, lighting is quite problematic for chromakeying, but absolutely vital if it is to work well. You need to make sure the screen is lit evenly across it and that the actors are not casting shadows on the screen. To ensure this, the actors must be a good distance in front of the screen (ten feet works quite well), and they must be lit with separate lights from the screen. A screen like GUST's can be lit reasonably well with two lights (one from each side) a good distance from the screen (to avoid getting a bright spot in the centre of the lit area).

Setting up the chromakeyer

A bit of trial and error is involved in this. The keyer in Premiere (found in clip>video>transparency) has a helpful feature which allows you to see the picture in black and white matts. The white area is the area keyed out and the black is the part of the picture which is retained. Looking at this it is easy to see any spots which are wrongly keyed and adjust the settings correspondingly.

The settings in the chromakeyer are:

• Key Colour ¨C This sets the colour which is to be keyed out and should be set to the colour of the screen.
• Similarity ¨C Set at 0%, a colour has to be exactly the same as the selected key colour to be keyed, while for higher settings mean that similar colours will disappear. At 100%, a broad range of colours will be keyed out, often including things you want to keep.
• Blend ¨C Areas around the keyed area will fade between the two pictures rather than giving a harsh border
• Threshold/cutoff ¨C These are used along the same lines as similarity, adding greater flexibility to what is and is not keyed out. Experiment to see what works.

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