Videonics PowerScript PS4000S, Инструкция по эксплуатации

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Chapter 7 Advanced PowerScript Functions
142
Creating Transparency Channels
The PostScript page description language does not support transparency, but it is com-
monly used in video. PowerScript supports transparency by
extending the PostScript func-
tionality for any graphics and text it creates. PowerScript supports transparency in
imported EPS graphics using a proprietary technology called
CMYA . To use this advanced
technology you must prepare EPS graphics in such a way that transparency information
gets stored in the K channel of a CMYK EPS graphic.
EPS supports a variety of color models. RGB ( R ed, G reen, and B lue) is commonly used
for on-screen work. CMYK ( C yan, M agenta, Y ellow, and blac K ) is commonly used in
four-color process printing. Because cyan+yellow=black, the black channel
in a CMYK color model is redundant. It is required in printed material because inks
never mix to a perfect black. When a CMYK graphic appears on-screen, the K (black)
channel is not needed. PowerScript’s CMYA technology takes advantage of this by
reassigning the K channel so that it can be used for transparency (called an
alpha
channel). Because CMYA is not an industry-standard format, you need to prepare
your graphics to use the transparency channel.
Using Adobe
Illustrator or
Macromedia
FreeHand
When preparing CMYA files with Adobe Illustrator or Macromedia FreeHand (or most
other vector-based drawing applications), use custom colors that do not contain any
black. To darken an image, use additional cyan, magenta, and yellow rather than using
black.
Thereafter, apply only those colors in the K channel to the objects to create the transpar-
ency mask. The more black you use, the greater the transparency.
Using Adobe
Photoshop
Two different methods are available for creating CMYA files in Adobe Photoshop. Each
produces a slightly different color balance. Use the first method if you are starting out
with a CMYK file—use the second if you are starting from an RGB image.
(The following procedures were prepared based on Adobe Photoshop version 4.0. There is
little difference when using version 3.0 or later.)
To prepare the file based on the CMYK color model:
1
Convert the file to RGB mode—that is, display the Mode menu, point to the Mode sub-
menu item, then select RGB .
2
Display the File menu, point to the Color Settings sub-menu item, then choose Separa-
tion Setup .
3 When the dialog box appears, set the Black Generation option to None .
4 If necessary, adjust the image colors.
5
Display the Channels palette and select the K channel.
Note that the K channel is empty.
6
Paint black (or shades of gray) in the K channel where you want the image to be trans-
parent. Full black (100%) makes that part of the image completely transparent. A gray
shade of 50% makes that part of the image 50% opaque/transparent.
7 Save the image with an APS extension, such as BASEBALL.APS. Also be sure to observe
all of the recommendations under “Adobe Photoshop Considerations” on page 138.