Path: utzoo!utgpu!water!watmath!clyde!rutgers!ames!ucsd!sdcsvax!ucsdhub!hp-sdd!hplabs!hp-pcd!uoregon!omepd!mipos3!td2cad!cpocd2!howard
From: howard@cpocd2.UUCP (Howard A. Landman)
Newsgroups: comp.sys.mac
Subject: Re: 24-bit Video
Message-ID: <1118@cpocd2.UUCP>
Date: 16 Feb 88 19:09:27 GMT
References: <3634@batcomputer.tn.cornell.edu>
Reply-To: howard@cpocd2.UUCP (Howard A. Landman)
Organization: Intel Corp. ASIC Systems Organization, Chandler AZ
Lines: 41

In article <3634@batcomputer.tn.cornell.edu> fielding@geology.tn.cornell.edu writes:
>I asked the net a little while ago how the new 24-bit cards are working now,
>and all of the responses were that they looked good, but were slow.

>My experience is with special-purpose image processors that organize the
>video memory into 8-bit planes, and then have three separate LUT's one each
>for the red, green, and blue guns.  As Eric O. points out, it is very expensive
>to have one giant 24-bit LUT, but for anything I am interested in, the
>3 8-bit (actually I think ours are 12-bits internally) work perfectly.

No no no!  The main reason to have lookup tables is so you can have more
potential colors displayable than there are color codes available.  This makes
perfect sense with up to about 12 bit-planes.

But with 24 bits, YOU DON'T NEED A LOOK-UP TABLE!  You can actually just say
how much RGB you want, independently, for each pixel.  This is the ideal
solution for many image-processing applications, and saves endless horrible
kludges (like special programs to calculate what the best 256 colors are to
represent a given picture).

Now, there are SOME applications where you still want lookup.  An IC layout
system like Magic is one example: it is easier for the software to allocate
one bitplane per mask layer, and be able to do screen manipulations with
one bitplane active and the others write-protected.  But even in these
applications, you're losing a lot of power because of the separate LUTs.
Three 8-bit LUT's give you at most 768 available colors, but direct 24 bit
(no LUT) gives you 16M colors simultaneously displayable (if your display has
that many pixels!).

So, the hardware should really support both modes, but any given program
usually only needs one.

It's interesting to note that the X graphics library originally assumed that
LUTs were being used, but is now being written to handle direct 24-bit color
as well.  Hope we see X Windows under AUX soon ...

-- 
	Howard A. Landman
	{oliveb,hplabs}!intelca!mipos3!cpocd2!howard
	howard%cpocd2.intel.com@RELAY.CS.NET
	"Over the lights and through the fires, we went surging down the wires"