RGB Spectrum SuperView 3000 B

May9, 2002 SuperView3000 User Manual 49

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V

IDEO

L

EVELS AND

HROMA

K

EY

Ifyou have the optional BACKGROUND INPUT connector on your SuperView

unit, then in Autosync mode you have the option of using the Chroma Key

mode.In this Autosync display option, windows selectively replace the

backgroundcomputer graphics, based on the color of the graphics. The concept

ofchroma keyis discussedin greater detail in this chapter.

TheSu perView’schroma key serves two purposes. First, it allows the

boundariesof the video window to be user-defined. You can create windows of

arbitraryshape by “painting” a window of the desired shape in the key color.

Thesecond purpose is to overlay the video with the host computer’s graphics

andtext. The host computer graphics and text are generated on the computer

screenin the same area as the window. A key color for the background of the

hostcomputer graphics image is selected. This key color must be differentfrom

anyother color used in the computer graphics overlay. When the ChromaKey

isenabled, the window is displayed in the background of the host computer

image—i.e.,the graphic overlays block out the window image, and the window

imageappears where the key color is drawn.

Thekey color is defined by a range of values for each of the red, green, and blue

computersignals. The selected key defines whether the host computer’s display

signal is “transparent”, i.e., if the color from the computer falls within the

specifiedrange, the window signal is shown. For example, if a host computer

signalconsists of red lines on a white background, and white is chosen as the

key color, then the resultant image on the screen is of the window overlaid by

thered lines. Considering it anotherway, you could say that the window “shows

through” the white area.

TheSuperView’s chroma key circuitry works by examining the values of the

hostcomputer’s red, green, and blue signals at every point on the screen. If the

valueof each signal falls in the “acceptance range” for that signal, the host

computerimage is made transparent, and the windows are allowed to show

through.All three high-resolution signals (red, green, and blue) must lie

simultaneouslywithin their respective acceptance ranges for the host computer

imageto be transparent. For the windows to be made visible, the chroma key

circuitrymust indicate that the host signals are within the acceptance range, at

thes ame region as the window location.

Itis not possible to key on a single color value. This is because the precision of

thedigital representation of the color in the host computer’s frame buffer (or

colormap) is lost when it is converted into analog form in order to drive the

colormonitor. There is also variation in gain from frame buffer to frame buffer.

Evenif it were possible to specify the color exactly, slight amounts of noise,

videoovershoot and temperaturedrift would cause u nreliability.Therefore, you

mustspecify a range of colors on which tokey.