Visual QuickStart Guide [Electronic resources] : Final Cut Express HD for Mac OS X نسخه متنی

Compositing

Compositing can be any process that combines two or more image layers to create an effect. A compositing process can also include filters and motion that have been applied to modify individual layers or to modify the composited image.

Setting Composite mode

A Composite mode is an algorithm, a mathematical formula that controls the way the colors in a clip combine with the clip colors in underlying video layers.

All compositing happens from the top down; the highest-numbered track's image sits on the front plane and obscures the image on a track beneath it, unless the foreground image is made transparent. Each clip's opacity setting influences the final effect of the Composite mode.

For a complete list of Final Cut Express's Composite modes, refer to Chapter 27 of Apple's

Final Cut Express Help PDF.

To set the Composite mode for a clip:

Select the clip in the Timeline; then

do one of the following:

Figure 16.12 ).



Figure 16.12. Choose Modify > Composite Mode and then select a Composite mode from the submenu. You can also access Composite modes from a Timeline clip's shortcut menu.







• Control-click the clip in the Timeline and then select a Composite mode from the submenu.

Using alpha channels

An RGB image is composed of three 8-bit grayscale imagesthe red, green, and blue color channelsthat express color information as a gray scale: 256 brightness values (ranging from 0 for black to 255 for white), plus an alpha channel. An alpha channel is a fourth 8-bit channel that can be assigned to an RGB image to track which areas of the image are transparent.

Figures 16.13, 16.14, and

16.15 illustrate how an alpha channel can be manipulated to mask selected areas in an image.

Alpha channels are useful critters with a variety of applications in digital imaging. Final Cut Express video clips automatically have alpha channels assigned to them; it's the alpha channel that's being manipulated when you adjust the opacity of a clip or apply a key or matte filter.

• When you import an image that has an alpha channel, Final Cut Express interprets the clip's alpha channel and sets it to Straight, Black, or White (corresponding to the background setting for the clip at the time the clip was created).

• When you place a clip in a sequence, Final Cut Express automatically makes the alpha channel area transparent. You can override the automatic setting by modifying the alpha channel type.

• You can reverse an alpha channel if you want to invert the transparency of an image, or you can choose to ignore the clip's alpha channel altogether.

To view or modify a clip's alpha channel type: