Path: utzoo!utgpu!news-server.csri.toronto.edu!bonnie.concordia.ca!uunet!cbmvax!daveh
From: daveh@cbmvax.commodore.com (Dave Haynie)
Newsgroups: comp.sys.amiga.advocacy
Subject: Re: Info wanted on new IBM vide card hacks.
Message-ID: <22525@cbmvax.commodore.com>
Date: 18 Jun 91 00:13:19 GMT
References: <1991Jun17.103306.29343@news.iastate.edu>
Reply-To: daveh@cbmvax.commodore.com (Dave Haynie)
Organization: Commodore, West Chester, PA
Lines: 38

In article <1991Jun17.103306.29343@news.iastate.edu> taab5@isuvax.iastate.edu writes:

>   While browsing through a recent issue of Computer Shopper, I came across
>an ad for something that looks interesting at the very least.  The ad is
>for an ordinary VGA card that comes equipped with something called the 
>"Edsun CEG DAC".  From the ad, this replacement DAC chip gives VGA cards 
>a resolution of 2048x1536 and 700,000+ colors "on a standard VGA monitor."

Well, you know what they say about believing what you read.  

This is essentially a RAMDAC with some special modes.  It's doing the same
kind of tricks HAM does, though not exactly the same way HAM does it.  All
modes trade a few color registers for special functions.  First of all, you 
start out at power up time, the device is in normal everyway VGA color lookup 
mode.  You can kick it into one mode that lets you reprogram color registers 
on the fly, a little bit as if you had a copper.  The chip finds a special
color value, and instead of going to the CLUT, it holds the color from the
last CLUT fetch.  After that value, the next 4 pixel values give you the new
CLUT register number and the color to put there.  So, if you can live with 
a color not changing for five pixels, you can reprogram one CLUT value.

Another mode (actually I think there are a couple of versions of this) lets 
you interpolate between color values.  You kick the display with another magic
value.  Instead of picking a CLUT entry, you're kicked into interpolation mode.
The next pixel value has your ending color (the starting color being the last
displayed, which is also displayed for the magic cookie pixel).  Then you get
to fetch new magic cookie pixels, which get expressed as a percentage of the
starting pixel versus the ending pixel.  

Basically, these new modes, like HAM, need a degree of precompilation and 
clever color allocations to be useful, but when done right, they produce images
that look better than the underlying bit depth would imply.  Any claims of
"2048x1536" pixels are, of course, lies.

-- 
Dave Haynie Commodore-Amiga (Amiga 3000) "The Crew That Never Rests"
   {uunet|pyramid|rutgers}!cbmvax!daveh      PLINK: hazy     BIX: hazy
	"This is my mistake.  Let me make it good." -R.E.M.