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From: jgk@osc.COM (Joe Keane)
Newsgroups: comp.arch,comp.graphics
Subject: Re: Resolution, etc.
Summary: Dithering doesn't help scanning quantization.
Message-ID: <4027@osc.COM>
Date: 22 Nov 90 00:27:12 GMT
References: <240@csinc.UUCP> <1990Nov15.052925.1265@imax.com> <2928@crdos1.crd.ge.COM> <1990Nov19.195042.19240@imax.com>
Reply-To: jgk@osc.COM (Joe Keane)
Followup-To: comp.arch
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Organization: Versant Object Technology, Menlo Park, CA
Lines: 48

In article <2928@crdos1.crd.ge.COM> davidsen@crdos1.crd.ge.com (bill davidsen)
writes:
>  I'm not convinced that you need 24 bits of color for the memory,
>either. Systems like the VGA which have a large palette and a limited
>number of selections work very well. If you look at the output of a 24
>bit color scanner scanning quality photographs, you rarely find an image
>which doesn't map into 256 colors nicely. Very rarely.

In article <1990Nov19.195042.19240@imax.com> dave@imax.com (Dave Martindale)
writes:
>I guess it depends on what you're doing.  This certainly isn't true when
>dealing with "photographic-quality" images.  When digitizing transparencies
>or negatives, 24 bits is clearly not enough - I can show you images with
>ugly banding artifacts due to quantization in the dark areas of the image.
>
>Even digitizing at 36 bits (12 bits/component) and then storing 8 bits
>of the logarithm of intensity is not enough in some circumstances.

This is true, but i think you're talking about a slightly different problem.
Quantization of display colors is a small problem, since you can use dithering
to get what you want.  Quantization in scanning is a more serious problem,
since once it's there you can't get rid of it.  One way to avoid the problem
is to add some noise to the sample values before they are quantized.  I'm not
happy about adding noise to my data, but it does work.

>I suspect we have different definitions of "acceptable".  Mine is
>"you can't see any artifacts due to the transfer from film to digital".
>Yours may be "it looks pretty good".  This may be adequate for
>most people dealing with images, but it certainly isn't good enough for
>everyone.

I'm sure most of you have been in stores and seen the various demo images they
use to show off computers.  The quality is so good they look almost like
photographs.  But if you look at the display specifications, they may use only
256 colors for the whole image.  How do they do it?  They certainly don't just
take a 256-color scanner and copy the output pixel by pixel.  They scan the
picture at high resolution, and then spend lots of CPU on a good dithering
algorithm.  They also carefully select the pallette to minimize the errors in
dithering.

>I currently use two monitors - 1600x1200x1 monochrome for editing, and
>1024x768x30 colour for image display.  This seems like a pretty good
>compromise for the moment - high resolution and fast drawing for text,
>while colour images appear much more slowly but with excellent quality.

This is a good combination.  I think that for a given technology, monochrome
monitors will always be sharper than color monitors.  Too bad you can't have
one monitor which switches between the two.