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From: jru@etn-rad.UUCP (John Unekis)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Screen snap-shot for EGA request
Message-ID: <293@etn-rad.UUCP>
Date: Mon, 12-Oct-87 12:47:22 EDT
Article-I.D.: etn-rad.293
Posted: Mon Oct 12 12:47:22 1987
Date-Received: Wed, 14-Oct-87 05:36:39 EDT
References: <4010@ecsvax.UUCP>
Reply-To: jru@etn-rad.UUCP (John Unekis)
Organization: Eaton Inc. IMSD, Westlake Village, CA
Lines: 21

In article <4010@ecsvax.UUCP> ugogan@ecsvax.UUCP (Jim Gogan) writes:
>Does anyone know of a frame grabber (screen snap-shot)
>that takes full advantage of the EGA 256K graphics?
>Should do 512 squared frame with all 256 shades of each
>color.  Any sources for further investigation will also be apreciated.
....
Sorry to disappoint you, but it cant be done. If you are talking about a
frame grabber(video digitizer) that would simply use the EGA memory as a
buffer for images without trying to display them , it would sort of work,
except that the EGA memory is organized as 32Kbyte planes that overlay each
other in memory, and you must set a register to mask the one(s) that you 
are reading/writing to.
If you mean that you would like the EGA to display a frame that is 512x512
with eight bits per pixel it is completely impossible. The EGA uses four bits
per pixel, which are translated as an offset into a sixteen element lookup
table. The lookup table is 6 bits wide - giving you the net effect of selecting
16 out of a possible 64 colors at any given time. The output from the lookup 
table is physically sent to the monitor as 2 bits per color gun, so only four
shades of each color can be achieved. If you really need a better display try
the new VGA, or look for some EGA compatible products to be announced at
comdex.