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TUTORIAL: Compositing: Greenscreen keying

Posted: Tue, 20th Apr 2004, 3:15pm

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Simon K Jones

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Chromanator Tutorials: Compositing

Greenscreen keying

Once you have prepared your footage, it is time to use Chromanator. Greenscreening is the easiest technique for compositing – presuming the footage and screen itself is of good quality – and requires only a couple of tools:

  • Colour Difference Key – removes the majority of the green.
  • Spill Suppression – fixes the green 'fringing' around the edges of the key.

Higher layers obscure lower layers in Chromanator, so always make sure to put your greenscreen clip above the background clip on the timeline.

Colour Difference Key
This is a simple tool to use. It is designed specifically for green/bluescreening, with various optimisations that result in excellent keying results. It can be found in the Key -> Key toolset in the Object View.

For this example I'll use a simple shot of an actor against a greenscreen:

The first thing to do is choose the colour. This is done using the tool's drop-down menu. As mentioned in the previous section, greenscreens tend to be the most effective when working with miniDV:

The white point determines the amount of keying. At 255 there will be only a small change in the footage. As you lower the value, the chosen colour will be gradually removed. The required value will vary depending on the clip being used:

Keep lowering the white point until the green has disappeared. Any problematic areas – perhaps where a dark shadow or a bright light spoil the greenscreen – may have to be dealt with using the garbage matte system, which is covered in other tutorials.

The black point adjusts the sharpness of the edge of the key. The higher the value, the sharper the edge. The amount required is mostly down to trial and error and your own aesthetic tastes. Some shots may require a very sharp edge, whilst others might benefit from a softer one. Here you can see the difference between a soft and hard edge:

  • If the black point value is greater than the white point, the edge may begin to break up. It is generally wise to keep the black point lower than the white point value.

Spill suppression
Even the best greenscreen is likely to leave slight green fringing around the edges of the keyed subject. The spill suppression tool can be used to eradicate this fringing. It can be found in the Grade toolset.

This tool is extremely simple to use and just requires activation.

  • The screen colour is selected using the drop-down menu. If a greenscreen was used, be sure to set the spill suppression to green.
  • After the green fringing is removed the image itself may become discoloured. Turning on the extended function will automatically re-grade the image back to its normal colours.

Here are the three stages of spill suppression, and how they affect the edges:

1.2.

3.

  • 1 - the original keyed image, with the green fringing around the hood.
  • 2 - after spill suppression has been activated. Note the skin tones have become deeper than in the original.
  • 3 - the green fringing is still removed, whilst the skin tones have been automatically re-graded using the extended mode.

That is essentially all that you need to do when using good quality greenscreen footage.

Of course, filming does not always go according to plan. If you find problems keying your greenscreen footage, you may need to use a garbage matte, which we will examine in the next section.

Last edited Wed, 12th May 2004, 4:23pm; edited 1 times in total.

Posted: Wed, 21st Apr 2004, 4:05pm

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aaron 99

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I got the keying to work right but when I went back to project it said to Render it and when I did part of the person was messed up
Posted: Wed, 21st Apr 2004, 4:07pm

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Brettsta

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your settings are pal progressive?
Posted: Wed, 21st Apr 2004, 4:08pm

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Simon K Jones

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How do you mean by 'messed up', exactly?
Posted: Wed, 21st Apr 2004, 4:12pm

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aaron 99

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Well, sort of distorted. It is really blurry and part of is body is gone
Posted: Wed, 21st Apr 2004, 4:14pm

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Simon K Jones

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Could you show us a screenshot, so we have a better idea of what you mean?
Posted: Wed, 21st Apr 2004, 4:30pm

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aaron 99

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redface How do I do that? redface
Posted: Wed, 21st Apr 2004, 4:49pm

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Brettsta

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control print screen, bring it in photoshop or paint, take out what your showing us, host it somewhere. Yahoo breifcase is good for hosting so siogn up there
Posted: Wed, 21st Apr 2004, 4:58pm

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Simon K Jones

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If you can't work out hosting, just email it to support@fxhome.com.
Posted: Wed, 21st Apr 2004, 5:58pm

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Redhawksrymmer

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I made this fantastic greenscreen using me running in front of a greenscreen and then composited onto a star wars background. It's like I'm TRULY really there.
Posted: Mon, 9th Apr 2007, 4:34pm

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Melody

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Very new here...purchased visionlab and been playing with it for a few days now and wow..I love it but at same time I am very confused.
One part I am confused about is this tutorial.
Where is this key you are referring to (Colour difference key)
I am my video and green screen video on the timeline and I do not see this key anywhere. Here is a screen shot.
Thanks
Melody



I am trying to blend her in the background image
Posted: Mon, 9th Apr 2007, 4:37pm

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Simon K Jones

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Please register your product at FXhome.com to receive product support.
Posted: Mon, 9th Apr 2007, 5:43pm

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Melody

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Tarn wrote:

Please register your product at FXhome.com to receive product support.
Where do I find my serial number?
It tells me "All FXhome products require a special key to unlock their full functionality. This page will let you register your product with the FXhome website and allow you to retrieve the program key.

If you purchased your product via the FXhome.com online store then your product was automatically activated, and there is no need to fill in the form below."

Which I did purchase this online. The only thing I see is my programkey file.
Posted: Mon, 9th Apr 2007, 7:05pm

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January82000

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Help on this matter anyone?
Posted: Tue, 10th Apr 2007, 4:45am

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Axeman

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Well, I see you have your registration sorted already, so I guess you are good to go. Welcome to the community.
Posted: Tue, 10th Apr 2007, 8:01am

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Simon K Jones

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You're currently viewing the animation toolset. You need to click the 'Key' track on the timeline, then 'Matte'. Please check out the manual for more information, as well as this video tutorial:

http://fxhome.com/support/vidtutes/?t=16&f=%2Bcat%3A4&cat=4