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From: sfeam <sfeam@users.sourceforge.net>
Newsgroups: comp.graphics.apps.gnuplot
Subject: Re: set palette cubehelix
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Date: Sat, 08 Oct 2011 10:48:38 -0700
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Ingo Thies wrote:

> Hi Ethan,
> 
> I have made a first test - looks good, well done!
> 
> I am only wondering why the gamma correction seems to have the
> opposite effect: gamma>1 makes dark regions appear brighter 

This is indeed the convention used by every program I know of. 
Here, for instance, is the relevant section from the ImageMagick
documentation:

  "The same color image displayed on two different workstations may
  look different due to differences in the display monitor. Use gamma
  correction to adjust for this color difference. Reasonable values
  extend from 0.8 to 2.3. Gamma less than 1.0 darkens the image and 
  gamma greater than 1.0 lightens it."

And here is what Photoshop says:

  "Increase the gamma value or drag the gray slider to the left to
  lighten the medium grays [...] Lower the gamma value to darken the
  medium grays."

If you have ImageMagick (or Gimp, Photoshop, etc) installed, you can see 
this very easily:

  display image.png
  display -gamma 2 image.png
  display -gamma 0.5 image.png

For that matter, you can see it in gnuplot itself using
   set palette gray gamma 1.0
   test palette
   set palette gray gamma 2.0
   test palette
etc

> while the same happens for gamma<1 in mine/Davids code. 

That is backwards from what everyone else does :-)

> According to http://en.wikipedia.org/wiki/Gamma_correction I would say
> that mine interpretation is correct (see the example photograph of the
> lady in the water). But the Wikipedia article might be wrong as well.

The Wikipedia article is very confusing as it does not, that I can see,
define the basic quantity V used in the fundamental equation given 
at the top of the article:

     Vout = Vin ** gamma

For any quantity V that increases with brightness (e.g. amplitude,
intensity, brightness) it is clear that gamma > 1 increases brightness.
And yes, it seems to me that the images on the Wikipedia page are 
mis-described.

I am not certain, but I suspect the source of confusion is that CRT
performance is also described using a gamma term, and this description
is in some sense the inverse of what you do to the image.  I.e., a
CRT with gamma = 2.0 looks dark.  To make the displayed image look
"correct" you have to brighten it with a gamma correction of 2.0
The Wikipedia article calls this an "inverse gamma correction",
which may well be the correct jargon in some fields.  But that
correction is what all programs I am familiar with apply as a 
"gamma correction".  Certainly it is what gnuplot has always done.

  cheers,

	Ethan


> Greetings,
> 
> Ingo