Description: Apple II Video Overlay Card: Using Key Colors (11/96)
Header: Apple II Video Overlay Card: Using Key Colors (11/96)

 Article Created: 03 April 89
 Article Reviewed/Updated: 18 November 1996

 TOPIC -----------------------------------------------------------

 This article discusses how to use the Key Color selection of the Video
 Overlay Card.

 DISCUSSION ------------------------------------------------------

 Creating an overlay with the Apple II Video Overlay Card requires selection
 of a control color, called Key Color.

 The VideoMix software treats the Key Color differently from the other colors.
 All colors not designated as the Key Color are treated as a group and are
 called non-Key colors.

 For example, if the Key Color is Black, the video input signal will show
 through only where black is displayed on the screen. Where any other color is
 shown, the video will not show through and those areas will be showing the
 computer generated graphic. This would be the display, assuming VideoMix is
 set for 100% Key and 0% non-Key. By moving the sliders within VideoMix, these
 proportions can be changed to suit the user.

 There are two methods for selecting the Key Color, select from Screen, and
 select from the color palette.

 Selecting from the color palette allows selection of the Key Color from a
 palette of sixteen different colors. Use the mouse (Apple IIGS), or the arrow
 keys (Apple IIe), to select a color rectangle from the palette.

 The other method is to use the option Select From Screen. An "X" appears in
 the box, and a hand with a pointing finger replaces the arrow cursor on the
 screen. With the hand cursor in the computer graphics area of the screen,
 click on the color of your choice. This becomes the Key Color.


 Apple IIGS Note
 ---------------
 The Apple IIGS can display graphics in one of two modes: 320 mode or 640
 mode. In 320 mode, the computer can display sixteen pure colors. In 640 mode,
 it can display only two true colors: black and white. All other colors are
 blended colors formed by a process known as dithering. Dithered colors are
 formed by displaying two colors in\adjacent pixels. Because the pixels are so
 small and close together, your eye perceives them as a single color. Light
 blue, for example, is formed by adjacent blue and white pixels. When a dithered 
	color is picked, the Key Color is the color of the pixel you
 happened to click on. If you picked the light blue color, the key color could
 be either blue or white.

 Using dithered colors for your Key Color can produce some interesting, and
 sometimes unexpected, translucence effects in your overlay. These effects,
 while attractive, may not be completely reproducible. To avoid this, it would
 better to have your computer in 320 mode when making overlays. If you wish to
 use 640 mode, it is best to use black or white as your Key Color.


 Article Change History:
 18 Nov 1996 - Reviewed for technical accuracy, revised formatting.
 Copyright 1989-96, Apple Computer, Inc.
 Keywords: