Specifications
37
A Digital Video Primer
Video compositing
Compositing is the process of combining two or more images to yield a resulting, enriched
image. Composites can be made with still or moving images. Compositing simply means playing
one clip on top of another.
e terms keying and matting, in video and lm production, refer to specic compositing techniques:
• Keying uses dierent types of transparency keys to nd pixels in an image that match a speci-
ed color or brightness and makes those pixels transparent or semitransparent. For example, if
you have a clip of a weatherman standing in front of a blue-screen background, you can key out
the blue using a blue-screen key, and replace it with a weather map.
• Matting uses a mask or matte to apply transparency or semitransparency to specied areas
of an image. By using keying or matting to apply transparency to portions of an image that is
layered on top of another image, portions of the lower image are revealed.
The Auto-trace feature in After Eects converts alpha channels into vector-based masks. This feature makes it
easy to use the edge of an object or any key you’ve created as a path. For example, you can use an alpha channel
from a green-screen shot to create an animated vector shape or use as the basis for text on a path.
Combining diverse types of media elements is one of the things for which Aer Eects is best known.
Aer Eects is the optimal program for layering media in motion because of its extensive transfer
mode support (just like in Photoshop), and its powerful masking capabilities, along with its wide
selection of keying methodologies.
Editing: In order to composite video clips, you rst edit and assemble them onto a timeline. Place
the clips to which you want to apply keys or mattes on superimpose tracks above the Video 1
track footage. Aer Eects includes tools and commands that streamline the process of con-
structing and rening compositions by turning time-consuming manual tasks into operations
that can be completed with a simple tool or command.
Masking: You can create, edit, and animate an unlimited number of masks on every layer in
Aer Eects. Draw paths to create transparencies or add new objects to an animation such as
stroked lines. Combine paths to make unusual shapes using operations such as Add, Subtract, and
Intersect. Rotate and scale masks, and apply opacity settings to make masks appear and disap-
pear over time. Lock masks to protect them from change. Extensive masking capabilities give
you extraordinary control:
• Edit masks in the Composition panel: Copy and paste masks into your compositions from Illustrator
and Photoshop, or create masks on the y by drawing them directly in the Aer Eects Composi-
tion window. is process saves time and can make it easier to adjust a mask precisely, relative to
other layers. You can also continue to create masks in the Layer panel.
• Assign mask colors: Assign colors to masks for easy identication.
• Feather the mask edge: Create and adjust the inner or outer feather of a mask by insetting or
outsetting the mask edge from the mask shape.
• Apply motion blur: By adding motion blur to masks, you create realistic-looking mask animations.
We applied the Glow and Stroke eects to the
mask created by the Auto-trace command.
Because the alpha channel is traced in each
frame, the mask animates smoothly.