Adobe AFTER EFFECTS 7.0 - About Keying; About Masks

ADOBE AFTER EFFECTS 7.0
User Guide
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With straight (or unmatted)channels,transparencyinformationisonlystoredinthealphachannel,notinanyofthe
visiblecolorchannels.Withstraightchannels,theeffectsoftransparencyarenotvisibleuntiltheimageisdisplayed
in an application that supports straight channels.
With premultiplied (or matted) channels, transparency information is stored in the alpha channel and also in the
visible RGB channels, which are multiplied with a background color. The colors of semitransparent areas, such as
feathered edges, are shifted toward the background color in proportion to their degree of transparency.
Some software lets you specify the background color with which the channels are premultiplied; otherwise, the
background color is usually black or white.
Straight channels retain more accurate color information than premultiplied channels. Premultiplied channels are
compatible with a wider range of programs, such as Apple QuickTime Player. Often, the choice of whether to use
images with straight or premultiplied channels has been made for you when you receive the assets that you will be
editing and compositing. Fortunately, Adobe Premiere Pro and After Effects recognize both straight and premulti-
plied channels, so either type will produce satisfactory results for most projects.
About keying
Keying isdefiningtransparencybyaparticularcolorvalue(withacolorkeyorchromakey)orbrightnessvalue(with
a luminance key) in an image. When you key out a value, all pixels that have similar colors or luminance values
become transparent.
Keying makes it easy to replace a background of a consistent color or brightness with another image, which is
especially useful when working with objects too complex to mask easily. The technique of keying out a background
of a consistent color is often called bluescreening or greenscreening, although you do not have to use blue or green;
you can use any solid color for a background.
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.
About masks
A mask in After Effects is a path, or outline, that is used as a parameter to modify layer effects and properties. The
most common use of masks is the modification of a layer’s alpha channel.
Amaskconsistsofsegments and vertices. Segments are the lines or curves that connect vertices. Vertices define where
each segment of a path starts and ends.
A mask can be either an open or closed path. An open path has a beginning point that is not the same as its end point;
for example, a straight line is an open path. A closed path is continuous and has no beginning or end; for example, a
circle is a closed path. Closed-path masks can create transparent areas for a layer. Open paths cannot create trans-
parent areas for a layer but are useful as parameters for an effect; for example, creating a visible line or shape from
the mask using the Stroke effect.
A mask belongs to a specific layer. Each layer can contain multiple masks.
You can draw four types of masks:
Rectangular Arectangularmaskcanbesquare.Thistypeofmaskispreviewedandrenderedfasterthananyother
kind of drawn mask.
Elliptical An elliptical mask can be circular.
Bezier Create any shape of Bezier mask using the Pen tool.