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Pnmnorm User Manual(0)							   Pnmnorm User Manual(0)

       pnmnorm - normalize the contrast in a Netbpm image


       [-bpercent=N | -bvalue=N]

       [-wpercent=N | -wvalue=N]





       [-luminosity | -colorvalue | -saturation]


       All  options  can be abbreviated to their shortest unique prefix.  You may use two hyphens
       instead of one to designate an option.  You may use either white space or an  equals  sign
       between an option name and its value.

       This program is part of Netpbm(1)

       pnmnorm	reads  a PNM image (PBM, PGM, or PPM).	It normalizes the contrast by forcing the
       brightest pixels to white, the darkest pixels to black, and  spreading  out  the  ones  in
       between.   It  produces the same kind of file as output.  This is pretty useless for a PBM

       The program offers two ways of spreading out the pixels in between the darkest and bright-
       est:  linear and quadratic.  In the quadratic case, you specify some in between brightness
       and specify what brightness that should become in  the  output.	 With  those  three  con-
       straints:  the  brightness  that becomes black, the brightness that becomes white, and the
       brightness that becomes that middle value, pnmnorm computes a quadratic equation that maps
       all the other brightnesses from input values to output values.

       The program first determines a mapping of old brightness to new brightness.  For each pos-
       sible brightness of a pixel, the program determines a  corresponding  brightness  for  the
       output image.

       Then  for each pixel in the image, the program computes a color which has the desired out-
       put brightness and puts that in the output.  With a color image, it is not always possible
       to compute such a color and retain any semblance of the original hue, so the brightest and
       dimmest pixels may only approximate the desired brightness.

       For a PPM image, you have a choice of three ways to define brightness:

       o      luminosity

       o      color value

       o      saturation

	      In the case of saturation, 'brightness' is pretty much a misnomer, but you can  use
	      the brightness analogy to see what it does.  In the analogy, bright means saturated
	      and dark means unsaturated.

       Note that all of these are different from separately normalizing the individual color com-

       An  alternative	way  to  spread out the brightnesses in an image is pnmhisteq.	pnmhisteq
       stretches the brightest pixels to white and the darkest pixels to black, but  rather  than
       linearly  adjusting the ones in between, it adjusts them so that there are an equal number
       of pixels of each brightness throughout the range.  This gives you more contrast than pnm-
       norm does, but can considerably change the picture in exchange.

       By  default,  the darkest 2 percent of all pixels are mapped to black, and the brightest 1
       percent are mapped to white.  You can override these percentages by  using  the	-bpercent
       and -wpercent options, or you can specify the exact pixel values to be mapped by using the
       -bvalue and -wvalue options.  You  can  get  appropriate  numbers  for  the  options  from
       ppmhist.   If  you  just want to enhance the contrast, then choose values at elbows in the
       histogram; e.g. if value 29 represents 3% of the image but value 30 represents 20%, choose
       30  for	bvalue.   If you want to brighten the image, then set bvalue to 0 and just fiddle
       with wvalue; similarly, to darken the image, set wvalue to maxval and play with bvalue.

       If you specify both -bvalue and -bpercent, pnmnorm uses the one that  produces  the  least
       change.	 The  same  goes for -wvalue and -wpercent.  (In Netpbm 10.26 (January 2005), the
       -bvalue/-wvalue takes precedence, and before that, it's a syntax error to specify both).

       If you want to maximize the change instead of minimizing it, just cascade two runs of pnm-
       norm, specifying values for the first and percentages for the second.

       -bpercent  and -wpercent values are floating point decimal.  Zero is valid and is the same
       as -bvalue=0 or -wvalue=maxval, respectively.

       Because there are whole numbers of pixels at  each  brightness,	pnmnorm  obviously  can't
       guarantee  the exact percentage, so it arranges that at least the percentage of pixels you
       specify get remapped as promised.

       It is possible for your -bpercent or -wpercent to overlap your -wvalue or -bvalue, respec-
       tively.	 For  example, you say -bpercent=20 and -wvalue=100 for an image in which only 10
       percent of the pixels are darker than 100.  In that case, pnmnorm adjusts  the  percentile
       value as required.  In the example, it uses 99 as the black value (like -bvalue=99).

       It is also possible for your -bpercent and -wpercent options to select the same brightness
       value for the stretch-to-white and stretch-to-black value because of the fact that pnmnorm
       can't  subdivide a histogram cell.  E.g. if an image is all brightness 100, then no matter
       what -bpercent and -wpercent values you	choose,  it's  the  same  as  saying  -bvalue=100
       -wvalue=100.   In  that case, pnmnorm changes one of the values by 1 to make it legal.  In
       the example, pnmnorm would either make the black value 99 or the white value 101.

       Before Netpbm 10.43 (June 2008), pnmnorm fails if the -wpercent	and/or	-bpercent  values
       specify an overlap.

       The  stretch points are further constrained by the -maxexpand option.  Sometimes, too much
       contrast is a bad thing.  If your intensities are all concentrated in the  middle,  -bper-
       cent=2  and  -wpercent=1  might	mean that an intensity of 60 gets stretched up to 100 and
       intensity of 20 gets stretched down to zero, for a range expansion of 150% (from  a  range
       of  40  to  a  range of 100).  That much stretching means two adjacent pixels that used to
       differ in intensity by 4 units now differ by 10, and that might be unsightly.

       So that you can put a limit on the amount of expansion without having to examine the image
       first, there is the -maxexpand option.  It specifies the maximum expansion you will toler-
       ate, as an additional percentage.  In the example above, you could  say	-maxexpand=50  to
       say  you  want the range to expand by at most 50%, regardless of your other options.  pnm-
       norm figures out what intensity to stretch to full intensity and what intensity to stretch
       to  zero intensity as described above, and then raises the former and lowers the latter as
       needed to limit the expansion to the amount you specified.

       When pnmnorm limits the expansion due to -maxexpand, it tells you about it with a  message
       like this:
	   limiting expansion of 150% to 50%

       In any case, pnmnorm tells you exactly what expansion it's doing, like this:

	   remapping 25..75 to 0..100

       Before  Netpbm  10.26 (December 2004), it was not valid to specify both -bvalue and -bper-
       cent or -wvalue and -wpercent.

       -maxexpand was new in Netpbm 10.32 (February 2006).

       The -keephues option says to keep each pixel the same hue as it	is  in	the  input;  just
       adjust  its  brightness.   You  normally  want this; the only reason it is not the default
       behavior is backward compatibility with a design mistake.

       By default, pnmnorm normalizes contrast in each component independently (except	that  the
       meaning	of  the  -wpercent and -bpercent options are based on the overall brightnesses of
       the colors, not each component taken separately).   So  if  you	have  a  color	which  is
       intensely  red but dimly green, pnmnorm would make the red more intense and the green less
       intense, so you end up with a different hue than you started with.

       When you specify -midvalue=N, pnmnorm uses a quadratic function to map old brightnesses to
       new  ones,  making sure that an old brightness of N becomes 50% bright in the output.  You
       can override that 50% default with -middle.  The value of -middle is a floating point num-
       ber  in	the  range 0 through 1 with 0 being full darkness and 1 full brightness.  If your
       -midvalue and -middle indicate an ambiguous or impossible quadratic function  (e.g.  -mid-
       value  is  the same as -bvalue, so an infinite number of quadratic functions fit), pnmnorm
       just ignores your -midvalue and maps linearly.

       -midvalue and -middle were new in Netpbm 10.57 (December 2011).

       If you specify -keephues, pnmnorm would likely leave this pixel alone, since  its  overall
       brightness is medium.

       -keephues  can  cause  clipping,  because  a certain color may be below a target intensity
       while one of its components is saturated.  Where that's the case, pnmnorm uses the maximum
       representable  intensity for the saturated component and the pixel ends up with less over-
       all intensity, and a different hue, than it is supposed to have.

       This option is meaningless on grayscale images.

       When you don't specify -keephues, the -luminosity, -colorvalue,	and  -saturation  options
       affect  the  transfer  function	(which is the same for all three RGB components), but are
       meaningless when it comes to applying the transfer function (since it is applied  to  each
       individual RGB component).

       Before Netpbm 9.25 (March 2002), there was no -keephues option.

       -luminosity,  -colorvalue,  and	-saturation determine what property of the pixels pnmnorm
       normalizes.  I.e., what kind of brightness.  You cannot specify more than one of these.

       The -luminosity option says to use the luminosity (i.e. the 'Y' in the YUV or YCbCr  color
       space)  as  the pixel's brightness.  The luminosity is a measure of how bright a human eye
       would find the color, taking into account the fact that the human eye is more sensitive to
       some RGB components than others.

       This option is default.

       This option is meaningless on grayscale images.

       Before  Netpbm  10.28  (August 2005), there was no -luminosity option, but its meaning was
       still the default.

       Before Netpbm 10.28 (August 2005), there was no -colorvalue option.

       The -colorvalue option says to use the color value (i.e. the 'V' in the HSV  color  space)
       as  the	pixel's  brightness.  The color value is the gamma-adjusted intensity of the most
       intense RGB component.

       This option is meaningless on grayscale images.

       Before Netpbm 10.28 (August 2005), there was no -colorvalue option.

       The -saturation option says to use the saturation (i.e. the 'S' in the HSV color space) as
       the  pixel's brightness.  The saturation is the ratio of the intensity of the most intense
       RGB component to the difference between the intensities of the most and least intense  RGB
       component (all intensities gamma-adjusted).

       In  this case, 'brightness' is more of a metaphor than anything.  'bright' means saturated
       and 'dark' means unsaturated.

       This option is meaningless on grayscale images.

       Before Netpbm 10.28 (August 2005), there was no -colorvalue option.

       pnmhisteq(1) , ppmhist(1) , pgmhist(1) , pnmgamma(1) , ppmbrighten(1) , ppmdim(1) , pnm(1)

netpbm documentation			  6 January 2006		   Pnmnorm User Manual(0)
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