Pyro Industries API-1394PCI Manual de usuario descargar pdf (Pagina 221)

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CHAPTER 6: IMPORTING, EXPORTING, AND DIGITAL CUT ■

Field dominance doesn’t play a big role in your determinations, unless your

source material has cuts within it, of course. For example, if we were to use a line cut

(line cut is a fancy term for a videotape that has been switched or mixed in a control

room) as our source, we would possibly encounter some field dominance issues. If the

switch from camera to camera occurred on field 2 instead of field 1, we would need

to delete an extra field.

Alpha Channel

When we work with images, the alpha channel determines which parts of the image are

seen and which are not. Alpha channels are used when we want to key an image over

another, or over a colored background.

For example, if we were to create a logo over a black background and wanted

to just show the logo, we would need to create an alpha channel that defined the black

background as transparent and the logo as opaque. Both Xpress Pro and Free DV sup-

port alpha channels in most file formats.

If you were to look at an alpha channel, you would see a grayscale image. Nor-

mally, the white pixels represent areas of the image that will be seen. Black pixels rep-

resent areas that will be masked or matted out. Gray pixels represent varied levels of

transparency of the image, with lighter pixels less transparent than darker pixels. Alpha

channels are represented in this manner for just about every graphic application that

has been created.

However, when evaluating alpha channels (such as the one shown in Figure 6.7),

Avid systems read them backward, with black pixels representing opacity and white

pixels being transparent. Apparently, this variation goes back to film mattes and how

they were originally created. Avid chose to go with the film industry standard. Keep in

mind that in the early days of nonlinear, the personal computer was relatively new. Not

all editors were computer operators, and not all editors used Photoshop.

Advantages of an Inverted Alpha

Although some find Avid’s inverted alpha to be a burr in their saddles,there is one distinct advan-

tage to this method.When the alpha channel is inverted,JPEG artifacts along the edge of the

matte fall outside of the keyed graphic,guaranteeing a clean matte,even at lower resolutions! So

there is a distinct advantage to preparing the matte in this way.

Note: Alpha channels have been called a lot of other names—mattes, holdouts,cookie cutters,and

hi-cons—but they are all the same and they all do the same thing:determine transparency.

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Pyro Industries API-1394PCI Manual de usuario Pagina 221