Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!nrl-cmf!ames!eos!jbm
From: jbm@eos.UUCP (Jeffrey Mulligan)
Newsgroups: comp.graphics
Subject: Re: ShowScan quality image bit rate
Message-ID: <2569@eos.UUCP>
Date: 9 Feb 89 18:10:07 GMT
References: <146@mlogic.UUCP>
Organization: NASA Ames Research Center, California
Lines: 51

From article <146@mlogic.UUCP>, by bennett@mlogic.UUCP (Bennett Leeds):
 
> 	Resolution should encompass more than just the number of pixels per
> screen dimension - it should include the range of colors (shades) displayed
> at each pixel.  Current TV technology has at least as long a way to go in
> this regard (if not longer) than it does in dimensional resolution.

Well, sure.  But the original posting included the bits used to encode
gray level (sorry for quoting out of context).
 
> 	For instance, turn your best computer monitor off.  What color is the
> screen?  Its a gray, hopefully a dark gray.  This is the blackest black your
> monitor can produce.  Now develop an unexposed roll of film, and play it
> through a projector in a conventional theatre.  I think you'll find this
                           ^^^^^^^^^^^^ ^^^^^^^
> black quite a bit darker.  You can apply similar tests at the top end,
> comparing the whites.  Again, the film white will be much brighter than the
> computer monitor's whitest white.

This is not a fair comparison.  In a "conventional theatre" presumably
the house lights are down.  If you power down even your worst computer
monitor and then turn off all the room lights it will be quite dark.
(Well, there might be a little long-term phosphor persistence,
but you'd have to dark-adapt to see it).  Similary, with no ambient light
your "white" will look white no matter what the absolute luminance
because your eye will adapt.  This is why television sets have a brightness
knob instead of just fixing it at the maximum:  if you like to watch TV
in a dark room, you will want to turn down the brightness.

A better comparison is the maximum display contrast.  With a reasonable
black level, this will be 100% regardless of the value of the white.

> The point is that the
> "dynamic range" of film is much greater than TV.

Is this true?  I'm not necessarily doubting it, it's just that the dynamic
range of [color] film is not all that large to begin with; I would guess
something slightly more than 1 log unit.  What are the units that are
used to describe this?  Gamma?  Anybody have any hard numbers?  Presumably
this is part of the NTSC standard.

> 	Back to the original discussion,

[impeccable comments about variable-resolution displays deleted]


-- 

	Jeff Mulligan (jbm@aurora.arc.nasa.gov)
	NASA/Ames Research Ctr., Mail Stop 239-3, Moffet Field CA, 94035
	(415) 694-6290