Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!shadooby!samsung!think!masscomp!peora!joel
From: joel@peora.ccur.com (Joel Upchurch)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Super Duper VGA boards
Message-ID: <4037@peora.ccur.com>
Date: 1 Dec 89 15:36:46 GMT
References: <8911250811.AA26716@cie.uoregon.edu> <10401@attctc.Dallas.TX.US> <3300@rti.UUCP>
Organization: Concurrent Computer Corp, Orlando FL
Lines: 46

In article <3300@rti.UUCP>, bcw@rti.UUCP (Bruce Wright) writes:
> This is certainly true for some applications, but if you want to do
> image processing it can be *VERY* useful to be able to play around 
> with a video lookup table.  For example, you can quickly and easily
> highlight a portion of the image that has a particular range of values,
> change the brightness mapping scale to provide better definition,
> and so forth.  It saves having to manipulate the image in your program
> and reload it onto the screen (saving both execution time and programming
> time, at least for large classes of display devices).

I think you are are quite correct when we are talking about fairly small
color maps. What I'm not so sure about is what is most efficent when the
size of the color map is on the same order of magnitude as the video
buffer, which it seems to me that it is when you start talking about
64k different colors on screen. It seems to me that at some point that
overhead of maintaining the color lookup table tends to overwhelm the
factors you mention. I could be wrong though, it is outside my area of
expertise.

> 
> Also, if the predefined colors aren't enough (for example, many images
> are processed with between 256 and 2048 shades of grey, which exceeds
> what you can get with 5 bits each for R,G,B = 32 shades of grey unless
> you get a monitor that can combine the signals from more than one of
> the color guns), you can always re-arrange the color map to get more
> of the types of colors you need and fewer of the ones you don't need.

You are quite right. 32 shades of gray isn't enough.  Of course, if they
are using some superset of normal VGA modes with 6 bit DACs, they only get
64, which isn't much of an improvement.  Even if they use 8 bits a piece
for RGB they only get 256 levels of gray, your lower limit.  It seems to
me that another mode is needed for working with gray-scale images, rather
than mapping it on to color modes, so you can get more bit of resolution
for the gray-scale modes.

This raises another question with me. How is the cost of a DAC effected
by the number of bits of input resolution? What is the cost differental
of 6 bit versus 8 bit versus n bit DACs? If you had a high resolution
gray scale mode, would it be cheaper to have a seperate DAC for it,
that set the R, G and B signals to the same levels, or use 3 high
resolution DACs all the time? Of course in real life, all these DACs
probably wind up on one chip, but which chip is cheaper to make?
-- 
Joel Upchurch/Concurrent Computer Corp/2486 Sand Lake Rd/Orlando, FL 32809
joel@peora.ccur.com {uiucuxc,hoptoad,petsd,ucf-cs}!peora!joel
Telephone: (407) 850-1040   Fax: (407) 857-0713