Path: utzoo!mnetor!uunet!husc6!tut.cis.ohio-state.edu!mailrus!nrl-cmf!ames!eos!jbm
From: jbm@eos.UUCP (Jeffrey Mulligan)
Newsgroups: comp.graphics
Subject: Re: The 24-bit question.
Message-ID: <331@eos.UUCP>
Date: 14 Mar 88 23:56:53 GMT
References: <1528@tekcae.TEK.COM>
Organization: NASA Ames Research Center, California
Lines: 27

From article <1528@tekcae.TEK.COM>, by kurtk@tekcae.TEK.COM (Kurt Krueger):
> A warning on dither techniques:  If your output display is not "gamma corrected"
> you will have to include this correction in your rendering code.  A device that
> is not gamma corrected will not have equally perceived brightness changes
> across the available range (i.e. you ask for 2x change and you actually get
> 3x or 1.5x).

It can get worse that; since very few displays themselves are linear,
gamma correction must be done in software to compensate for the
physical nonlinearity.  This is fine.

A separate problem is the failure of spatial independence.
When this rears its ugly head, it can cause
a 50% density of black and white dots to be darker that
the (gamma corrected) mean of the white and black levels.
It should be emphasized that without gamma correction, the dithered
pattern would be brighter than the mid-level setting, since
the gamma nonlinearity is positively accelerating.

My theory about this is that it reflects inadequate video bandwidth;
i.e. the monitor can be modeled as a low pass filter followed by
a nonlinearity (the gamma function).  A simple way to measure this is
to put up alternating black and white lines, either horizontal or vertical.
The vertical line pattern can have a substantially lower mean luminance.

If anyone is interested, Lee Stone and myself are working on a manuscript
which contains a section on this topic.