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From: ex499mih@CS.UCLA.EDU
Newsgroups: sci.electronics,comp.graphics
Subject: Re: Mixing computer and video signals
Message-ID: <6583@shemp.UCLA.EDU>
Date: Thu, 11-Jun-87 22:41:25 EDT
Article-I.D.: shemp.6583
Posted: Thu Jun 11 22:41:25 1987
Date-Received: Sat, 20-Jun-87 13:01:29 EDT
References: <173@mv06.ecf.toronto.edu>
Sender: root@CS.UCLA.EDU
Reply-To: ex499mih@CS.UCLA.EDU (Michael Hartsough)
Organization: UCLA Computer Science Department
Lines: 18
Keywords: video overlaying, keying
Xref: mnetor sci.electronics:819 comp.graphics:789


IBM has recently released a piece of hardware which it calls "Infowindow".

Its purpose is as a controller for level III interactive videodisc courses.

One of its primary functions is to "mix" computer graphics and ntsc video
on the same display. This is accomplished by using the EGA resolution mode
which most closely matches the resolution of the ntsc signal (I'm no video
expert, so please excuse my vagueness). Special hardware stops the EGA at
the beginning of each screen refresh in order to synch it with the ntsc
scan. The User (program) selects one graphics color which the hardware will
consider to be "transparent" to video. As the EGA is "read" during refresh,
any pel which is to have the same value as the transparent color is given 
the ntsc signal at that time (i.e. EGA goes to the display until transparent
color, if = transparent color then NTSC).

That's about it I guess.