Pnmgamma User Manual(0) Pnmgamma User Manual(0)
NAME
pnmgamma - perform gamma correction on a PNM image
SYNOPSIS
pnmgamma [
-bt709ramp |
-srgbramp | ] [-ungamma] [gamma | redgamma greengamma bluegamma [pnm-
file]]
pnmgamma {
-bt709tolinear |
-lineartobt709 |
-bt709tosrgb |
-srgbtobt709 } [-gamma=float] [-rgamma=float] [-ggamma=float]
[-bgamma=float]
[pnmfile]
DESCRIPTION
This program is part of Netpbm(1).
Pnmgamma performs gamma correction on pseudo-PNM images.
The PPM format specification specifies 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 lumi-
nance as defined by ITU-R Recommendation BT.709.
However, people sometimes work with approximations of PPM and PGM where
the sample values represent intensity in different ways. For example,
the sample value might be directly proportional to radiance (often
called ’linear intensity’). Or a different gamma transfer function may
be used. The relationship between the sample values and radiance is
often called a gamma transfer function.
pnmgamma allows you to manipulate the gamma transfer function, thus
working with and/or creating pseudo-PPM files that are useful for vari-
ous things.
For example, if you feed a true PPM to pnmgamma -bt709tolinear , you
get as output a file which is PPM in every respect except that the sam-
ple values are radiances. If you feed such a file to pnmgamma -lin-
earto709, you get back a true PPM.
The situation for PGM images is analogous. And pnmgamma treats PBM
images as PGM images.
When you feed a radiance-proportional pseudo-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 radiance-proportional
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 multiplicative inverse of whatever gamma
value the display program uses) has the effect of darkening the image.
PARAMETERS
The form of the parameters depends on whether you’re using the old syn-
tax or the new syntax. With the old syntax, the parameters are a
mixture of gamma values and the input file name. With the new syntax,
the only parameter is the input file name and you specify gamma values
with option.
You use the old syntax if you specify -bt709ramp (or its synonym -cier-
amp) or -srgramp or if you don’t specify any transfer function at all
(and thus default to a simple exponential). Otherwise, you use the new
syntax.
With the old syntax, you may specify a single gamma value or 3 separate
gamma values (red, green, and blue) or no gamma values. In any case,
the meanings of those parameters is the same as the more modern -gamma,
-rgamma, -ggamma, and -bgamma options described below.
OPTIONS
-bt709tolinear
Convert the image from ITU Recommendation BT.709 luminance to
radiance (sometimes called ’linear’). I.e. convert from true
PPM or PGM to a radiance-linear variation that can be used with
certain tools that need it.
This option was new in Netpbm 10.32 (February 2006).
-lineartobt709
Convert the image from radiance to ITU Recommendation BT.709
luminance. I.e. convert to true PPM or PGM from a ’linear’
variation.
This option was new in Netpbm 10.32 (February 2006).
-bt709tosrgb
Convert the image from ITU Recommendation BT.709 luminance to
SRGB luminance. I.e. convert from true PPM or PGM to an SRGB-
based variation that is required by certain tools and display
devices.
This option was new in Netpbm 10.32 (February 2006).
-srgbtobt709
Convert the image from SRGB luminance to ITU Recommendation
BT.709 luminance. I.e. convert to true PPM or PGM from an SRGB-
based variation.
This option was new in Netpbm 10.32 (February 2006).
-bt709ramp
Use the ITU-R Recommendation BT.709 gamma transfer function.
Note that it is true BT.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 none of the transfer functions the transfer func-
tion defaults to a simple power function.
This option was renamed in Netpbm 10.32 (February 2006). Before
that, its name is -cieramp.
-cieramp
This is an obsolete synonym for -bt709ramp.
The name of this option comes from a former belief that this was
a standard of CIE (International Commission On Illumination),
but it now (August 2005) looks like it never was.
-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 -bt709ramp, but with dif-
ferent 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 -bt709ramp nor -srgbramp, the transfer
function defaults to a simple power function.
-ungamma
Apply the inverse of the specified transfer function (i.e. go
from gamma-corrected luminance to radiance).
This is valid only with -bt709ramp (aka -cieramp), -srgbramp,
and the default exponential transfer function.
-gamma=float
This specifies the gamma value to use in the transfer function.
All of the transfer functions involve an exponent, and the gamma
value is that exponent.
The standards specify a particular gamma value. If you use any-
thing else, you are varying from the standard.
The default is the standard value. For the simple exponential
transfer function (which is not a standard), the default is 2.2.
If you specify one of the component-specific options (-rgamma,
etc.), that overrides the -gamma value.
With the -bt709ramp (aka -cieramp), -srgbramp, or the default
exponentional transfer function, you can’t actually use this
option, but you specify the same thing with parameters.
This option was new in Netpbm 10.32 (February 2006).
-rgamma=float
-ggamma=float
-bgamma=float
These options are just like -gamma, except they specify the
value for a particular one of the color components.
If you don’t specify this option for a particular color compo-
nent, the default is the -gamma value (or -gamma’s default if
you didn’t specify that either).
With the -bt709ramp (aka -cieramp), -srgbramp, or the default
exponentional transfer function, you can’t actually use this
option, but you specify the same thing with parameters.
This option was new in Netpbm 10.32 (February 2006).
-maxval=maxval
This is the maxval of the output image. By default, the maxval
of the output is the same as that of the input.
Because the transformation is not linear, you need a greater
maxval in the output in order not to lose any information from
the input. For example, if you convert to radiance-linear sam-
ple values with with -ungamma -bt709ramp and default gamma
value, and your maxval is 255 on both input and output, 3 dif-
ferent input sample values all generate output sample value 254.
In order to have a different output sample value for each input
sample value, you would need an output maxval at least 3 times
the input maxval.
This option was new in Netpbm 10.32 (February 2006). Before
that, you can achieve the same result by increasing the maxval
of the input or decreasing the maxval of the output using
pnmdepth.
WHAT IS GAMMA?
A good explanation of gamma is in Charles Poynton’s Gamma FAQ at
http://www.poynton.com/GammaFAQ.html (1) and Color FAQ at
http://www.poynton.com/ColorFAQ.html (1).
In brief: The simplest way to code an image is by using sample values
that are directly proportional to the radiance of the color components.
Radiance is a physical quantification based on the amount of power in
the light; it is easily measurable in a laboratory, but does not take
into account what the light looks like to a person. It wastes the sam-
ple space because the human eye can’t discern differences between low-
radiance colors as well as it can between high-radiance colors. So
instead, we pass the radiance 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 cor-
recting.
The gamma-corrected value, proportional to subjective brightness, are
known as the luminance of the pixel.
There is no precise objective way to measure luminance, since it’s psy-
chological. Also, perception of brightness varies according to a vari-
ety of factors, including the surrounding in which an image is viewed.
Therefore, there is not just one gamma transfer function.
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 radiance is done automatically by CRTs. You just apply a voltage to
the CRT’s electron gun that is proportional to the gamma-corrected sam-
ple value, and the radiance of the light that comes out of the screen
is close to the radiance 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(1)
AUTHOR
Copyright (C) 1991 by Bill Davidson and Jef Poskanzer.
netpbm documentation 18 February 2006 Pnmgamma User Manual(0)
