About Keying after effects

8/10/2019 About Keying after effects

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About keying: color keys, luminance keys, and difference

keys

Keyingis defining transparency by a particular color value or

luminance value in an image. When you key outa value, all pixels that

have colors or luminance values similar to that value become

transparent.

Keying makes it easy to replace a background, which is especially

useful when you work with objects too complex to mask easily. When

you place a keyed layer over another layer, the result forms a

composite, in which the background is visible wherever the keyed layer

is transparent.

You often see composites made with keying techniques in movies, for

example, when an actor appears to dangle from a helicopter or float in

outer space. To create this effect, the actor is filmed in an appropriate

position against a solid-color background screen. The background color

is then keyed out and the scene with the actor is composited over a new

background.

The technique of keying out a background of a consistent color is often

called bluescreeningorgreenscreening, although you dont have to use

a blue or green screen; you can use any solid color for a background.Red screens are often used for shooting non-human objects, such as

miniature models of cars and space ships. Magenta screens have been

used for keying work in some feature films renowned for their visual

effects. Other common terms for this kind of keying are color

keyingand chroma keying.

Difference keyingworks differently from color keying. Difference

keying defines transparency with respect to a particular baseline

background image. Instead of keying out a single-color screen, you can

key out an arbitrary background. To use difference keying, you musthave at least one frame that contains only the background; other frames

are compared to this frame, and the background pixels are made

transparent, leaving the foreground objects. Noise, grain, and other

subtle variations can make difference keying very difficult to use in

practice.


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Tips on color keying and compositingfrom experienced compositor,

Chris Zwar.

Chris & Trish Meyer sharestips & resources for color keyingwith

Keylight and other effects.

Note:

Keep in mind that generating a high-quality key can require the

application of multiple keying effects in sequence and careful

modification of their properties, especially if the footage was shot

without considering the requirements of the compositor.

Shooting and acquiring footage for keying

Keying footage that was not acquired correctly is much more difficult

than is keying footage that was acquired with keying in mind. For bestresults, use Adobe OnLocation to monitor color and lighting as you

acquire footage for color keying.

For tips on shooting footage so that color keying is easier and more

successful, see Jonas HummelstrandsGeneral Specialist website.

Alex Lindsay provides an article on theProVideo Coalition

websiteabout setting up and lighting a green background in preparation

for color keying work. In part two of his greenscreen primer on

theProVideo Coalition website,Alex Lindsay provides detailed

information about cameras, codecs, and capture systems to use when

acquiring footage for color keying.

Light your color screen uniformly, and keep it

free of wrinkles.

Start with the highest-quality materials you

can gather, such as film that you scan and

digitize.

Use uncompressed footage (or, at least, fileswith the least possible amount of

compression). Many compression algorithms,

especially the algorithms used in DV, HDV,

and Motion JPEG, discard subtle variations in

bluewhich may be necessary to create a

good key from a bluescreen. Use footage with
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the least color subsampling possiblefor

example, 4:2:2 rather than 4:2:0 or 4:1:1. (For

information about color subsampling, see

theWikipedia websiteand theAdobe

website.)

Robbie Carman and Richard Harrington provide an excerpt on

thePeachpit websitefrom their book Video Made On A Macthat

demonstrates how to plan, shoot, key, and composite a greenscreen

shot.

Tips for keying with After Effects

Noise and compression artifacts can cause

problems for keying, especially differencekeying. Often, applying a slight blur before

keying can reduce noise and compression

artifacts enough to improve keying results. For

example, blurring the blue channel for DV

footage can smooth out noise in a bluescreen.

The KeyerforDV animation preset available

through theAE Enhancers forumautomates

the process of blurring the blue and green

channels before keying DV footage.

Use a garbage matte to roughly outline your

subject so that you dont have to waste time

keying out parts of the background far from

the foreground subject. (SeeUse a garbage

matte.)

Use a hold-out matte to roughly protect areas

that are of a similar color to the background

from being keyed out. (SeeUse a hold-out

matte.)

To help you view transparency, temporarily

change the background color of the

composition, or include a background layer

behind the layer you are keying out. As you

apply the keying effect to the layer in the
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completely satisfied with the results, try

starting with or adding the Linear Color Key.

To key dark areas or shadows, use the Extract

Key on the Luminance channel.

To make a static background scene

transparent, use the Difference Matte Key.

Add the Simple Choker and other effects as

needed to refine the matte.

After you have used a key to create

transparency, use Matte effects to remove

traces of key color and create clean edges.

Blurring the alpha channel after keying cansoften the edges of the matte, which can

improve compositing results.

To the top

Use a garbage matte

Agarbage matte(orjunk matte) removes unneeded portions of the

scene, resulting in a rough area that contains only the subject that youwant to keep. When you are working with a poorly lit or uneven color

screen (for example, a bluescreen or greenscreen), sketching a garbage

matte around the subject can greatly reduce the amount of work that

you have to do in keying out the background. However, if you spend a

lot of time making a perfect garbage matte that exactly outlines the

subjectessentially rotoscopingyou lose the time-saving advantage

of keying.

1. Create a mask to roughly outline a subject.

2.

Apply one or more keying effects to mask out the remainder of

the background.

3.

Apply Matte effects as necessary to fine-tune the matte.
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3.

On the duplicate layer, create masks to mask out everything in

the image except the area that you want to preserve.

4.

Make sure that the copy (the hold-out matte) is positioned

directly on top of the keyed layer.

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About Keying after effects