Green Screen

The Screen is Green

Thought I’d do an article about chromakey (often called green screen or blue screen). Just to clear a few things up and, to be honest, remind myself I need to improve.

The subject of chromakeying has come up a few times since I shot this, not least from my own cousin who wants me to teach his son how to use green screens.

Why green?

I was recently on one of those Zoom conferences (have you heard of Zoom?) with a company I occasionally work for who run media training courses with videographers and journalists.

One of the journalists asked if her living room wall, which was lime green, could work as a ‘green screen’. She was told it wasn’t a ‘proper’ green screen colour but it may work. She was also told green was ‘fashionable’, then blue became more popular for a while.

Whilst there may be an element of truth in that, the colour doesn’t actually matter. We’re talking digital here. Every colour, and every shade of that colour is given a specific reference – a set of digits – in a computer code. The higher the bit rate, the greater the range of colours and shades can be accommodated (maybe I should do a piece of bit rates at some point).

The software simply (I say ‘simply’ as though it’s a natural as breathing) removes a specific code, or range of codes, from the picture. That code can represent green, red, black, orange…you get the picture (or at least, the bit of the picture that hasn’t been removed).

Fashion doesn’t really come into it, nor does the correct shade of green (although some very specific shades are recommended). The journalist’s lime green wall would have worked fine. The reason green and/or blue was chosen was simply because it is the furthest colour from our skin tones.

It don't matter if you're black or white

When I say ‘skin tones’, I don’t mean skin colour. As remarkable as it may seem, we all have a similar colour when it comes to digital video, a reddish colour. If you think about that, it sort of makes sense. Your skin may be black, brown, white (not that any skin is actually white) or yellow, but it is very, very thin. Immediately behind that thin covering of skin are teeny little veins of blood. There is, in fact, a specific ‘Skin Tone Line’, a narrow range on the Vector Scope (one of the main scopes used to colour correct, or colour grade, your video signal). That line is between the red and yellow points on the scope.

Complimentary colour of red is green.

Complimentary colour of yellow is blue.

So it stands to reason that, if you don’t want to eliminate half of your subjects face when you remove the specific digital code, you make sure that specific code is the furthest away from the red/yellow that makes up our faces.

The weather girl in the blue dress

You’ve all seen someone reading the weather, knowing full well they can’t actually see the map on the wall behind them, wearing a medium blue dress. Of course, they do this by using a green screen; the blue stays, the green goes.

Likewise, when the weatherman in in a green jacket, they will use a blue screen. The first time I ever did chromakey, I shot some pictures of my son. I made sure he was wearing reds or yellows. I then gave him a green blow up guitar to ‘play’ (forgetting the screen was green). When I removed the green, half his stomach disappeared* in a guitar shape.

(*It didn’t really disappear. No children were harmed in the making of that film).

Flat is best

I was working in Cyprus for that same media training company. We had a chromakey ‘set up’ that consisted of a really cheap bit of green cloth taped to the wall.

The cloth was creased and the lighting was variable – partly lit from the window, partly from a couple of tungsten spotlights. What that does is increase the range of colours and shades you have to remove.

If you think about it, the less code you need to remove, the better. In an ideal world, you want to remove just ONE line of code (for arguments sake, let’s say this is 11010001). If your green background is very flat (let’s say it has been painted onto a very smooth wall) and is consistently lit (so you don’t have shadow on the edges with bright daylight on one side and a small patch of tungsten on the other side) you should, in theory, ONLY need to instruct the computer programme to remove 11010001. Easy.

But that is hard to achieve. The creases in our Cypriot green rag produced shadows, moving the colours to the darker shades of green and even toward black. The daylight gives a bluer colour and the tungsten a yellow colour. The combination means you need to remove a fairly wide range of code, increasing the likelihood of including some of the shades in the presenter’s jacket, or shirt, or hair, etc.

So the trick is, as far as possible, to light the green screen as consistently as you can. Try to get the same amount of brightness in the middle as at the edges. It isn’t easy to do but it will make the chromakeying much easier.

On reflection

Another problem is the reflection you get back from a green (or blue) screen. It is fine to light the screen well but if you have a rim of green light around the face of the presenter, that will be taken out too.

So the answer is to bring the ‘talent’ as far away from the screen as possible. That is fine on a huge budget production (Harry Potter can afford it) but not so easy when you’re working in a small space or with a small green screen. And if your actor is standing on a green floor, it is impossible not to get shadows. So lighting there is critical.

In an ideal world, you place the green screen as far behind the talent as you can. you light the green screen separately – making sure the lighting is flat and consistent. Your aim is to minimise the range of code you need to take out of the image. You then light the actor or presenter according to the replacement background (so if they are in a scene where the sun is setting on their right, don’t light them from the left). You make sure the lights on the talent do not spill onto the green screen, and neither does their shadow.

Did I get it right?

Of course I didn’t.

The ideal is very hard to achieve so we all have to compromise. I had a teeny green screen so I couldn’t move left or right too far or my arm would have gone out of frame. I couldn’t move far away from the screen and, whilst I lit the screen separately, I would often get some of that light on me.

Shutter speed

But we all learn. I wish (and I have learned this from the project) that I had shot at a faster shutter speed.

The shutter speed in video is closely related to the frame rate you are using. I was shooting on the PAL standard, 25 Frames Per Second (fps). You want your frame rate and your shutter speed either to match or to be divisible, so you have options: shoot at 1/25 of a second (which I did), or 1/50 second or even 1/100 of a second.

The shutter speed is exactly the same as in photography. The slower the rate, the more blur you get. The crisper shots are at the faster shutter speeds. But because video is a moving image, shooting a fast shutter speed can give a jerky look so, generally, I prefer the slower shutter speed.

However, that means that when you move quickly across the screen, there is no clear definition. The background and the subject (the hand, for example) blur together as the slower exposure dictates. Therefore, when my hand moves away from my body (most notable in the Butcher’s Shop scene when I say, ‘Were you jogging together?’) it is impossible to get a clear key.

I tried to correct this by painting each individual frame for a second or two (anything up to 50 or 60 frames) but it is far from ideal.

So, if you want a crisp alpha channel (the name for the ‘blank’ bit of the picture once you have removed the key colour), you need to keep green spillage away from the subject, cut out as little a colour range as possible and make sure your shutter speed is fast enough to differentiate between subject and green screen without being so fast it causes the frames to stutter.

If you can do all of that, you’re laughing. 

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