Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!utgpu!water!watmath!clyde!rutgers!seismo!mimsy!aplcen!osiris!phil
From: phil@osiris.UUCP
Newsgroups: sci.electronics,comp.graphics
Subject: Re: Mixing computer and video signals
Message-ID: <1171@osiris.UUCP>
Date: Mon, 15-Jun-87 10:15:23 EDT
Article-I.D.: osiris.1171
Posted: Mon Jun 15 10:15:23 1987
Date-Received: Thu, 18-Jun-87 01:07:04 EDT
References: <173@mv06.ecf.toronto.edu> <6583@shemp.UCLA.EDU>
Organization: Johns Hopkins Hospital
Lines: 22
Keywords: video overlaying, keying
Xref: utgpu sci.electronics:727 comp.graphics:730
Summary: chroma keying

In article <6583@shemp.UCLA.EDU>, ex499mih@CS.UCLA.EDU writes:
>  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).

Wow, what a novel concept!  :-)  Suppose IBM will try to patent it?

But seriously.. this sounds just like a digital form of chroma keying.  (I
even seem to remember someone suggesting chroma keying as a solution to the
original poster's problem.)

Question for all you serious broadcast people out there: I imagine the
original chroma key hardware was analog.  Is it still done that way, or is
it done digitally now?  Seems to me that with these new-fangled digital
video controllers stuff like this would be getting much simpler, if not
cheaper...

...!decvax!decuac!\                                               Phil Kos
  ...!seismo!mimsy!aplcen!osiris!phil           The Johns Hopkins Hospital
...!allegra!/                                                Baltimore, MD