xgamma(1) General Commands Manual xgamma(1)NAME
xgamma - Alter a monitor's gamma correction through the X server
SYNOPSIS
xgamma [-display display] [-screen screen] [-quiet] [-gamma f.f | [[-rgamma f.f] [-ggamma f.f] [-bgamma f.f]]]
DESCRIPTION
xgamma allows X users to query and alter the gamma correction of a monitor via the X video mode extension (XFree86-VidModeExtension).
Note that the xgamma utility is obsolete and deficient, xrandr should be used with drivers that support the XRandr extension.
If no value for the gamma correction is given via the -gamma or -rgamma/-ggamma/-bgamma options, xgamma prints the current gamma correction
of the display.
OPTIONS -display display
This argument allows you to specify the server to connect to; see X(7).
-screen screen
When multiple displays are configured as a single logical display, this option allows you to select the screen you wish to change.
-quiet Silence the normal output of xgamma
-help Print out the `Usage:' command syntax summary.
-gamma f.f
The gamma correction can either be defined as a single value, or separately for the red, green and blue components. This argument
specifies the gamma correction as a single value.
-rgamma f.f
This argument specifies the red component of the gamma correction.
-ggamma f.f
This argument specifies the green component of the gamma correction.
-bgamma f.f
This argument specifies the blue component of the gamma correction.
ENVIRONMENT
DISPLAY To get default host and display number.
BUGS
This client changes the internal values of the gamma correction for the Xserver. Whether or not these values are respected depends on the
video drivers.
The gamma values are passed to the Xserver with 3 decimal places of accuracy.
SEE ALSO xvidtune(1), xrandr(1)AUTHORS
Kaleb S. Keithley, X Consortium.
David Dawes, David Bateman
X Version 11 xgamma 1.0.5 xgamma(1)
Check Out this Related Man Page
pnmgamma(1) General Commands Manual pnmgamma(1)NAME
pnmgamma - perform gamma correction on a portable anymap
SYNOPSIS
pnmgamma [-ungamma] [-cieramp|-srgbramp] [value [pnmfile]]
pnmgamma [-ungamma] [-cieramp|-srgbramp] redgamma greengamma bluegamma [pnmfile]
DESCRIPTION
Performs gamma correction on pseudo-PNM images.
The PPM format specification specify that certain sample values in a file represent certain light intensities in an image. In particular,
they specify that the sample values are directly proportional to gamma-corrected intensity values. The gamma correction they specify is
CIE Rec. 709.
However, people sometimes work with approximations of PPM and PGM where the relationship between the image intensities and the sample val-
ues are something else. For example, the sample value might be directly proportional to the intensity with no gamma correction (often
called "linear intensity"). Or a different gamma transfer function may be used.
pnmgamma allows you to manipulate the transfer function, thus working with and/or creating pseudo-PPM files that are useful for various
things.
For example, if you feed a true PPM to pnmgamma -cieramp -ungamma, you get as output a file which is PPM in every respect except that the
sample values are directly proportional to the light intensities in the image. If you feed such a file to pnmgamma -cieramp, you get out a
true PPM.
The situation for PGM images is analogous. And pnmgamma treats PBM images as PGM images.
When you feed a linear PPM image to a display program that expects a true PPM, the display appears darker than it should, so pnmgamma has
the effect of lightening the image. When you feed a true PPM to a display program that expects linear sample values, and therefore does a
gamma correction of its own on them, the display appears lighter than it should, so pnmgamma with a gamma value less than one (the multi-
plicative inverse of whatever gamma value the display program uses) has the effect of darkening the image.
PARAMETERS
The only parameters are the specification of the input image file and the gamma values. Every gamma transfer function pnmgamma uses con-
tains an exponent, which is the gamma value, and you can choose that value.
Furthermore, you can choose different values for each of the three RGB components. If you specify only one gamma value, pnmgamma uses that
value for all three RGB components.
If you don't specify any gamma parameters, pnmgamma chooses a default. For the transfer functions defined by standards, the default is the
value defined by the standard. If you specify anything else, you will be varying from the standard. For the simple power function trans-
fer function, the default gamma is 1/.45.
OPTIONS -ungamma
Apply the inverse of the specified transfer function (i.e. go from gamma-corrected nonlinear intensities to linear intensities).
-cieramp
Use the CIE Rec. 709 gamma transfer function. Note that it is true CIE Rec. 709 only if you use the default gamma value (i.e. don't
specify any gamma parameters). This transfer function is a power function modified with a linear ramp near black.
If you specify neither -cieramp nor -srgbramp, the transfer function defaults to a simple power function.
-srgbramp
Use the Internation Electrotechnical Commission (IEC) SRGB gamma transfer function (as specified in the standard IEC 61966-2-1).
Note that it is true SRGB only if you use the default gamma value (i.e. don't specify any gamma parameters). This transfer function
is like the one selected by -cieramp, but with different constants in it.
Note that SRGB is often spelled "sRGB". In this document, we use standard English typography, though, which doesn't allow for that
kind of capitalization.
If you specify neither -cieramp nor -srgbramp, the transfer function defaults to a simple power function.
WHAT IS GAMMA ?
A good explanation of gamma is in Charles Poynton's GammaFAQ at <http://www.inforamp.net/~poynton/ColorFAQ.html> and ColorFAQ at
<http://www.inforamp.net/~poynton/GammaFAQ.html>
In brief: The simplest way to code an image is by using sample values that are directly proportional to the intensity of the color compo-
nents. But that wastes the sample space because the human eye can't discern differences between low-intensity colors as well as it can
between high-intensity colors. So instead, we pass the light intensity values through a transfer function that makes it so that changing a
sample value by 1 causes the same level of perceived color change anywhere in the sample range. We store those resulting values in the
image file. That transfer function is called the gamma transfer function and the transformation is called gamma correcting.
Virtually all image formats, either specified or de facto, use gamma-corrected values for their sample values.
What's really nice about gamma is that by coincidence, the inverse function that you have to do to convert the gamma-corrected values back
to real light intensities is done automatically by CRTs. You just apply a voltage to the CRT's electron gun that is proportional to the
gamma-corrected sample value, and the intensity of light that comes out of the screen is close to the intensity value you had before you
applied the gamma transfer function!
And when you consider that computer video devices usually want you to store in video memory a value proportional to the signal voltage you
want to go to the monitor, which the monitor turns into a proportional drive voltage on the electron gun, it is really convenient to work
with gamma-corrected sample values.
SEE ALSO pnm(5)AUTHOR
Copyright (C) 1991 by Bill Davidson and Jef Poskanzer.
11 June 2001 pnmgamma(1)