Make it Go: Keying Video with Adobe Creative Suite (Green Screen)

You need to key something. You were pretty sure buying the Adobe Creative Suite meant that you had the capability, but nothing seems to work.  Everything you’ve tried gets you get one of two results:

  1. bits of your subject around the edges get keyed out (you don’t want that)
  2. bits of your green background around the edges of your subject are left in (you don’t want that either)

You sought help in various video edtiting and post production forums.  The advice you got probably centered on your lighting.  “You have to have perfect lighting, and a perfectly smooth green background with no variation in shading or color whatsoever,” they told you.

Don’t worry, you don’t need to reshoot (with a different camera or different lights or a different green screen).  You just need to use the right tools. You’ve probably been keying in Premiere with Chroma Key or Color Key.  Maybe someone told you to try keying in After Effects and, upon trying, you discovered that their keying plugins were just as useless.

Unfortunately, those methods are always going to result in an awkward and unusable key.  I don’t really understand why Adobe insists on including them in their programs; Keylight is the plugin you want.   They openly admit that their native plugins aren’t very good compared to Keylight.  Seriously:

Though the color keying effects built into After Effects can be useful for some purposes, you should try keying with Keylight before attempting to use these built-in keying effects. Some keying effects—such as the Color Key effect and the Luma Key effect—have been superseded by more modern effects like Keylight.

– Adobe Online Help Manual

Adobe includes Keylight with every purchase of their creative suite, but does not automatically install the plugin.  You’ll have to dig up your installation disc and find the ‘third party content’ either on its own disc or as its own option in the installation menu.  Tell the Adobe installation dialog to install only Keylight, reboot your machine, and you’ll be in business.

Open After Effects and begin a new compositon with parameters that match the footage you’d like to key.  Find Keylight in the effects window and drag it to the footage in the monitor tab or to the footage timeline in the timeline tab.  Select the effects tab to see Keylight’s options.

On the surface, Keylight works the same way any other keying plugin works: you tell it which color it is supposed to make transparent and it does it’s best to remove that color from your footage.  So first, tell Keylight which color you’d like to extract. Select the dropper tool and find and click on the green (or blue) area behind the subject.  Most of the background should immediately disappear, but the edges of the subject won’t be quite right (that’s normal).

Now select the ‘Status’ view.  Your footage will become a black and white representation of what is or is not transparent based on the key color you’ve selected.  The color white represents parts of the picture that will remain, black represents parts of the picture that will be taken out, and grey represents parts of the picture that will be partially transparent.  This is called an alpha layer.

Based on the status view above, you can tell that the footage won’t look good.  The gray in the background tells you that parts of your green screen will still be visible, while the grey in the foreground tells you that the subject will be partially transparent.  Your goal is a white subject and a black background.  Increasing the ‘Screen Gain’ will simulate a more saturated picture, and so bring more of the background into the keyable range.  A small increase, around 110, is best.  Too much and you will distort your subject.

Increase ‘Clip Black’ until black covers the entire background.  Too little and some of the background will remain; too much and it will remove some of your subject.   Decrease ‘Clip White’ until your status view shows a completely white subject.  The values you end up with will depend entirely on the nature of your footage.  The only guideline I can offer is that your clip black value can’t be greater than your clip white value (otherwise you will remove your subject and leave in your green screen).  You’ll need to adjust both until the border between white and black matches the border between your background and subject.  Your goal is a white subject against a black background, with a tiny amount of gray at edge.

These alpha layer adjustments are why Keylight works so well. Most keying plugins focus only on the key color and colors similar to it.  This is a problem because a plugin that uses this method will always remove bits of green on your subject near the edges of the key, and will always fail to remove bits of your background that aren’t similar enough to your specified color.  Keylight, on the other hand, only uses the color you’ve selected as a baseline to create an adjustable alpha layer.  By using the alpha layer, Keylight ignores these problem areas and predicts and edge based on similar and dissimilar colors.  Because it is only a prediction the alpha layer will leave in the bits of green that are obviously part of the subject, and remove the non-green bits that are obviously part of the background.

If you find the border between the white and black areas of the status screen are a little unnatural, adjust the ‘Screen Pre-blur’ to around 0.1 or more.  ‘Pre-blur’ will not blur your footage, but it will make keylight behave as though it was.  This will smooth out your key along the edges.

Some of the light reflected off the green screen will spill over onto the subject.  Keylight intelligently detects this spillage and replaces the green with a neutral gray (or whatever color you specify . . . but grey will work most of the time).  If this color replacement creates a dark shadow near the edge of the subject (and it likely will), change the ‘Replace Method’ to ‘Hard Colour’.

Switch the view back to ‘Final Result’ to check that you have a solid key with clean edges.

To be absolutely certain of your key, find your background and place it in a timeline below the footage.

Keylight is capable of a lot more, but as long as your footage is marginally clean you may not need those additional features.

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