Pnmgamma User Manual(0) Pnmgamma User Manual(0)NAMEpnmgamma - perform gamma adjustment on a PNM image
SYNOPSISpnmgamma {
-bt709tolinear |
-lineartobt709 |
-bt709tosrgb |
-srgbtobt709 } [-gamma=float] [-rgamma=float] [-ggamma=float]
[-bgamma=float]
[pnmfile]
pnmgamma [
-bt709ramp |
-srgbramp ] [-ungamma] [{gamma | redgamma greengamma bluegamma} [pnm‐
file]]
DESCRIPTION
This program is part of Netpbm(1)
Pnmgamma performs gamma adjustment 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. BT.709 luminance as a
function of radiance is a power function modified with a linear ramp
near black.
However, people sometimes work with approximations of PPM and PGM where
the sample values represent intensity in different ways:
In one common variation, the sample value is directly proportional to
radiance (often called 'linear intensity').
Another popular variation is to make the samples proportional to lumi‐
nance as defined by the International Electrotechnical Commission (IEC)
SRGB standard. The SRGB gamma transfer function is like the BT.709 one
except with different constants in it.
Note that SRGB is often spelled 'sRGB'. In this document, we use stan‐
dard English typography, though, which doesn't allow for that kind of
capitalization.
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 adjustment 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 mix‐
ture 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 BT.709 luminance to radiance. I.e. con‐
vert 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 BT.709 luminance. I.e. con‐
vert to true PPM or PGM from a radiance-linear variation.
You get true BT.709 (ergo true PPM or PGM) only if you use the
default gamma value (i.e. don't specify -gamma, etc.).
This option was new in Netpbm 10.32 (February 2006).
-bt709tosrgb
Convert the image from 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.
You get true SRGB only if you use the default gamma value (i.e.
don't specify -gamma, etc.).
This option was new in Netpbm 10.32 (February 2006).
-srgbtobt709
Convert the image from SRGB luminance to 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
Same as -lineartobt709, but using the old syntax.
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
Convert the image from radiance to SRGB luminance. Note that it
is true SRGB only if you use the default gamma value (i.e. don't
specify any gamma parameters).
This is an old syntax option. There is no equivalent in the new
syntax because it really shouldn't be a function of pnmgamma at
all. It exists solely for backward compatibility. The reason
it shouldn't exist is that the way to do this conversion consis‐
tent with the Netpbm philosophy is do a -lineartobt709 followed
by a -bt709tosrgb. It's exactly analogous to the way you have
to convert from PNG to TIFF by doing a pngtopam followed by a
pnmtotiff. The -srgbramp option actually dates to before there
was a standard definition of what the sample values of a Netpbm
image measure, and pnmgamma considered radiance-linear to be the
proper intermediate format.
-ungamma
Apply the inverse of the specified transfer function (i.e. go
from gamma-adjusted 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.
In the -bt709tosrgb and -srgbtobt709 conversions there are two
exponents. -gamma affects the 'to' function; the 'from' func‐
tion always uses the standard gamma value.
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
exponential transfer function, you can't actually use this
option, but you specify the same thing with parameters.
⟨#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
exponential transfer function, you can't actually use this
option, but you specify the same thing with parameters.
⟨#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
pamdepth.
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
adjusting.
The gamma-adjusted 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-
adjusted 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-adjusted 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-adjusted 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-adjusted sample values.
SEE ALSOpnm(1)AUTHOR
Copyright (C) 1991 by Bill Davidson and Jef Poskanzer.
netpbm documentation 30 June 2007 Pnmgamma User Manual(0)
