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Pnmgamma User Manual(0)				       Pnmgamma User Manual(0)

       pnmgamma - perform gamma adjustment on a PNM image

       pnmgamma {
	-bt709tolinear |
	-lineartobt709 |
	-bt709tosrgb |
	-srgbtobt709	}   [-gamma=float]   [-rgamma=float]   [-ggamma=float]


       pnmgamma [
	-bt709ramp |
	-srgbramp ] [-ungamma] [{gamma | redgamma greengamma bluegamma}	 [pnm‐

       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 Internation 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

       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.

       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

       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

       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.

	      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).

	      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).

	      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).

	      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).

	      Same as -lineartobt709, but using the old syntax.

	      This option was renamed in Netpbm 10.32 (February 2006).	Before
	      that, its name is -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.

	      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.

	      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.

	      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.

	      This option was new in Netpbm 10.32 (February 2006).



	      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.

	      This option was new in Netpbm 10.32 (February 2006).

	      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

       A  good	explanation  of	 gamma	is  in Charles Poynton's Gamma FAQ at   (1)   and	  Color	  FAQ	at (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

       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.


       Copyright (C) 1991 by Bill Davidson and Jef Poskanzer.

netpbm documentation		 30 June 2007	       Pnmgamma User Manual(0)

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