Thursday, 28 February 2013

Join The Green Screen Scene...


Image from Sam Greenhalgh, under a CC
BY licence

“Greenscreening” is a something we get asked about a lot by students who use our Creative Media facilities. Known in the trade as “Chroma Key”, it is a visual effect that keys out a particular colour to create a mask, and is usually used to take one piece of video and superimpose it on to another.

It’s an effect that you’ll see every day on TV and in films – it is not only the method that is used to put actors into dangerous situations or sees them face virtual, CG opponents, but is also used in more mundane circumstances, putting weather forecasters in front of their maps, and newsreaders into virtual studios. If done well, you will never even notice it’s been used.

The good news is this effect is easy to create using the Creative Media facilities. The bad news is it’s quite difficult to do well… However if you follow some of the guidance below you can create something effective, although don’t expect professional quality results first time, and experiment a few times before doing it for real.

Recording your material

The hardest part is in recording your material to a good enough level where Chroma Key is possible.

You need to pick your background well, and for best results record a bright green or bright blue backdrop. The reason these colours are chosen is because they are the ones least likely to show up in skin tones, so you don’t end up with people’s faces and arms becoming invisible once the effect is applied.

Once the background is chosen you need to light it well. I’m going to presume you don’t have access to professional lights to do this properly, but the general rule is to light the backdrop as well and as evenly as possible. You need the colour to be as flat as you can get it, with no shadows. You need to make sure the shadow from the subject cannot be seen on the wall in the area that is being recorded – to help do this move the subject as far away from the wall as is possible.

Finally, make sure that the subject wears no clothes or accessories that are the same colour as the backdrop, as these will become see-through when the effect is applied. The same goes for the chair the subject is sitting in, or anything else in shot.

How to use Chroma Key on iMovie

You can use Chroma Key on iMovie, and while it can be quite effective to create something rough-and-ready, it does not have the capabilities to fine tune the effect. Saying that it can have surprisingly good results, especially if you’ve used a well-lit backdrop that’s a particularly vibrant green or blue, but don’t expect wonders.

  • First of all place your desired background video or image on to your Project.
  • Then you will need to ensure that you have Advanced Tools activated – to ensure you do, go to iMovie > iMovie Preferences, and select “Show Advanced Tools”.
  • Find your footage with your green or blue background in your event, select the part you want, and drag this on top of the background footage on your project.
  • Once you release this, a menu will pop up with some options – select “Blue Screen” or “Green Screen”, depending on your background colour.

This should now appear with the blue or green backgrounds removed. You do not have a lot of control over this to iron out mistakes such as shadows. You can try adjusting the blue gain or green gain in the clip’s Video Settings, which will make some difference, but this will also change the colours in the foreground subject.

How to use Chroma Key on Adobe Premiere

The Creative Media facilities also have Adobe Premiere installed, which is a professional editing package that can handle Chroma Key very well and with a lot of control. Be aware it cannot work miracles, though - your material will still need to have been recorded well in the first place.

The walkthrough below presumes some familiarity with Premiere.
  • Place your background clip on the timeline.
  • Place your foreground clip on the timeline, on a higher video layer than the background.
  • Go to the Effects Browser, and find “Colour Key”, in Video Effects > Keying. Drag this on to your foreground clip.
  • On this clip’s Effects Control, open the Colour Key tab and select the eyedropper next to the Key Colour. In the preview window, select the colour you want to key out of your background (i.e. blue or green).
  •  In the Effects Control, move the Colour Tolerance slider until the background has disappeared. It is at this point you will notice the problems with any shadows that appear – it may be that you have to find a compromise between shadows appearing in the background if you have any, and parts of the foreground subjects disappearing.
  • To further adjust your image, move the sliders on Edge Thin (to select the thickness of the edge between the two layers) and Edge Feather (to select the softness of the edge).
As you can see, using Premiere gives you fine control over the whole process. You can also use colours other than blue and green, but these are still the preferred colours for the above reasons.

One final method… Photobooth.

If the above techniques seem a bit daunting there is one quick way to get a similar effect, as long as you’re not expecting wonders from it!

PhotoBooth doesn’t use green or blue screens at all. Instead, it will take a picture of your current background using the computer’s webcam, and then once you are in shot work out from that what is foreground and background, and pastes you onto whatever image or video you select. It’s not the most accurate thing in the world, but if you want something quick and dirty then it may do you fine.

So that’s it!

So can you do Greenscreen effects using the Creative Media facilities? Yes! Is it easy? It’s not too hard, but to do it well requires time, patience and practice. But it’s a fun process, and well worth having a go with. If anyone fancies using the facilities to produce effects such as these give us a shout.

Pete

3 comments:

  1. I did a lot of chroma key when i worked on films and animation, its fun as hell. The one thing we noticed is modern dyes on cloths contain blue/green. You would be surprised how "green" a pair of grey trousers are.

    ReplyDelete
  2. I'm always amazed how many shots we see on Film & TV are chroma keyed, especially when they don't neccessarily contain lots of sfx. This video is fascinating:

    http://youtu.be/clnozSXyF4k

    ReplyDelete
  3. The green screen technology has gained much popularity – in TV, film, and in photography. One important thing in using green screen is the lightning technique to give the quality of the video.

    ReplyDelete

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