


pnmgamma(1)                                           pnmgamma(1)


NNAAMMEE
       pnmgamma - perform gamma correction on a PNM image


SSYYNNOOPPSSIISS
       ppnnmmggaammmmaa [--uunnggaammmmaa] [--cciieerraammpp] [_v_a_l_u_e [_p_n_m_f_i_l_e]]
       ppnnmmggaammmmaa   [--uunnggaammmmaa]   [--cciieerraammpp]   _r_e_d_g_a_m_m_a   _g_r_e_e_n_g_a_m_m_a
       _b_l_u_e_g_a_m_m_a [_p_n_m_f_i_l_e]


DDEESSCCRRIIPPTTIIOONN
       Reads a psuedo-PNM image as input.  Performs gamma correc-
       tion, and produces a PNM image as output.

       Alternatively, this program can undo gamma correction.

       The PPM specification states that the sample values in the
       image are nonlinear, i.e.  not  directly  proportional  to
       light  intensity,  i.e.  gamma corrected.  But there exist
       images that are PPM in every  respect  except  that  their
       sample  values  are  directly proportional to light inten-
       sity.  People may loosely refer to these as PPM, but  they
       are  not.   ppnnmmggaammmmaa  converts  these pseudo-PPM images to
       true PPM by performing gamma correction.  To get true PPM,
       you  must specify the --cciieerraammpp option and no gamma values.
       That causes ppnnmmggaammmmaa to  apply  the  CIE  Rec.  709  gamma
       transfer function, as specified by the PPM format specifi-
       cation.

       On the other hand, you can use the --uunnggaammmmaa option to con-
       vert  from  true PPM to linear RGB pseudo-PPM.  (Again, if
       the input is true PPM, specify the --cciieerraammpp option and  no
       gamma values).

       The  situation  for PGM images is analogous.  And ppnnmmggaammmmaa
       treats PBM images as PGM images.

       You can also apply a different  transfer  function  (which
       means you don't end up with a true PPM image) by selecting
       the gamma values as arguments  or  omitting  the  --cciieerraammpp
       option.   The  gamma value is the power to which the input
       value is raised in the transfer function.  A  value  of  1
       means  the  output is the same as the input.  A value less
       than one makes the output samples  numerically  less  than
       the input samples; A value greater than one makes the sam-
       ples numerically greater.

       Without the --cciieerraammpp option, the transfer  function  is  a
       simple power function.  With --cciieerraammpp, it is a power func-
       tion modified with a linear ramp near black, as  described
       in CIE Rec. 709.

       When you feed a linear PPM image to a display program that
       expects a true PPM, the display  appears  darker  than  it



                           11 June 2001                         1





pnmgamma(1)                                           pnmgamma(1)


       should,  so  ppnnmmggaammmmaa  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 ppnnmmggaammmmaa 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.


WWHHAATT IISS GGAAMMMMAA??
       A good explanation of gamma is in Charles  Poynton's  Gam-
       maFAQ  at <http://www.inforamp.net/~poynton/ColorFAQ.html>
       and  ColorFAQ  at   <http://www.inforamp.net/~poynton/Gam-
       maFAQ.html>

       In  brief:  The  simplest way to code an image is by using
       sample values that are directly proportional to the inten-
       sity  of the color components.  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-cor-
       rected sample value, and the intensity of light that comes
       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.



SSEEEE AALLSSOO
       ppnnmm(5)





                           11 June 2001                         2





pnmgamma(1)                                           pnmgamma(1)


AAUUTTHHOORR
       Copyright (C) 1991 by Bill Davidson and Jef Poskanzer.























































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