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Pnmpsnr	User Manual(0)					Pnmpsnr	User Manual(0)

NAME
       pnmpsnr - compute the difference	between	two images (the	PSNR)

SYNOPSIS
       pnmpsnr

       [pnmfile1]

       [pnmfile2]

       [-rgb] [-machine] [-max=n] [-target=n] [-target1=n] [-target2=n]	[-tar-
       get3=n]

       Minimum unique abbreviations of options are acceptable.	 You  may  use
       double hyphens instead of single	hyphen to denote options.  You may use
       white space in place of the equals sign to separate an option name from
       its value.

DESCRIPTION
       This program is part of Netpbm(1).

       pnmpsnr	reads two PBM, PGM, or PPM files, or PAM equivalents, as input
       and computes the	magnitude of difference	between	the two	 images	 as  a
       peak  signal-to-noise ratio (PSNR) This metric is typically used	in im-
       age compression papers to rate the distortion between original and  de-
       coded image.

       pnmpsnr	either	prints these values or compares	them to	thresholds you
       specify.

       The PSNR	of a given component is	the ratio of the maximum  mean	square
       difference  of component	values that could exist	between	the two	images
       (a measure of the information content in	an image) to the  actual  mean
       square  difference  for	the  two subject images.  It is	expressed as a
       decibel value.

       The mean	square difference of a component for two images	 is  the  mean
       square difference of the	component value, comparing each	pixel with the
       pixel in	the same position of the other image.	For  the  purposes  of
       this computation, components are	normalized to the scale	[0..1].

       The maximum mean	square difference is identically 1.

       So the higher the PSNR, the closer the images are.  A luminance PSNR of
       20 means	the mean square	difference of the luminances of	the pixels  is
       100 times less than the maximum possible	difference, i.e. 0.01.

       Note  that the word "peak" is a misnomer; there is no maximum involved;
       the metric is a mean.  But "peak	signal to noise	 ratio"	 is  for  some
       reason the common term for this measurement.

       If  the	inputs	are PBM	or PGM,	pnmpsnr	computes the PSNR of the lumi-
       nance only.  Otherwise, it computes three separate  PSNRs:  either  the
       luminance,  and chrominance (Cb and Cr) components of the colors	or the
       red, green, and blue components.

       By default, the program prints the PSNRs	to Standard Output  in	human-
       friendly	form.

       With the	-machine option, the program prints the	PSNRs, but in machine-
       friendly	form.

       With a -target[x] option, the program just prints 'match' or 'nomatch',
       depending on whether the	PSNRs exceed targets you specify.

       pnmpsnr	reports	 the PSNR either in human-friendly form	or in machine-
       friendly	form (see -machine).

OPTIONS
       -rgb   This option causes pnmpsnr to compare the	red, green,  and  blue
	      components  of  the  color rather	than the luminance and chromi-
	      nance components.	 It has	no effect on a monotone	image.

	      This option was new in Netpbm 10.71 (June	2015).

       -machine
	      This option causes pnmpsnr  to  report  the  PSNRs  in  machine-
	      friendly	form,  so  another program can easily use the informa-
	      tion.

	      The output is a single line.  It	contains  one  floating	 point
	      decimal number for each color component, with a single space be-
	      tween every two.	(This means there are either 1 or 3  numbers).
	      For  the	YCbCr  color space (no -rgb), they are in the order Y,
	      Cb, Cr.  For the RGB color space (-rgb), they are	in R, G, B or-
	      der.  For	a monotone image, there	is one number.

	      Where  the  component does not differ between the	images,	so the
	      PSNR is infinite,	the number is inf

	      But note that the	number displayed is also modified by  the  ef-
	      fect of -max.  In	particular, with -max, you will	never see inf.

	      This option has no effect	when you also specify -target[n].

	      This option was new in Netpbm 10.74 (March 2016).

       -max=n This is meaningful only with -machine.

	      It  specifies  the  maximum number pnmpsnr will print as a PSNR.
	      If the PSNR is greater than n, pnmpsnr just prints n.   n	 is  a
	      decimal  floating	 point number.	An infinite PSNR is considered
	      greater than any number.

	      This is mainly useful to deal with infinite PSNRs.  It is	 often
	      much more	convenient to have a program process only numbers than
	      to make it deal with infinity, and often a very large number has
	      the same effect on a program as infinity.

	      Note  that  the  output is logarithmic, which means you will not
	      see really large but finite numbers.  If you specify  -max=1000,
	      the  only	 way you will see 1000 in the output is	if the PSNR is
	      really infinite.	Two images with	as many	pixels	as  there  are
	      electrons	in the universe, differing in only one pixel, and only
	      in the smallest amount representable in the Netpbm format,  have
	      a	PSNR less than 1000.

	      This option was new in Netpbm 10.74 (March 2016).

       -target=n

	      This  option  causes  pnmpsnr to run in comparison mode -	rather
	      than print the PSNRs, it just tells you whether the PSNRs	exceed
	      n	 (a  floating  point number), i.e. whether the compared	images
	      are the same within a given margin of error.  If	all  the  com-
	      puted  PSNRs  (luminance for a PBM or PGM; luminance and chromi-
	      nance or red, green, and blue for	PPM)  exceed  n,  the  program
	      prints  'match'  to  Standard Output.  Otherwise,	it prints 'no-
	      match'.

	      If you also specify any of -target1, -target2, or	-target3,  and
	      the images are color, pnmpsnr ignores -target.

	      This  is	mainly useful for use in a program.  If	you're running
	      pnmpsnr manually,	you could just run pnmpsnr without -target and
	      compare the PSNRs	to your	targets	yourself.

	      This option was new in Netpbm 10.82 (March 2018).

       -target{1,2,3}=n
	      Like  -target,  these options cause pnmpsnr to run in comparison
	      mode.  But they provide  separate	 targets  for  the  individual
	      color component PSNRs.  -target1,	target-2, and -target3 are for
	      either the Y, Cb,	and Cr components, respectively, or  the  red,
	      green, and blue components, respectively,	depending upon whether
	      you specified -rgb.

	      If you don't specify the corresponding  -targetn	option	for  a
	      component,  pnmpsnr ignores the PSNR of that component in	decid-
	      ing whether the images match.

	      If the image is a	PBM or PGM, these options have no effect,  ex-
	      cept  that  it  stilll  selects comparison mode, so if you don't
	      also specify -target, and	the image is PBM or PGM,  the  program
	      fails.

	      Note that	the options are	defined	so that	you could code a pnmp-
	      snr command in a program that works on both color	 and  monotone
	      images,  specifying individual PSNR targets for use on the color
	      images and the single target for use on the monotone images.

	      These options were new in	Netpbm 10.82 (March 2018).

SEE ALSO
       pnm(5)

DOCUMENT SOURCE
       This manual page	was generated by the Netpbm tool 'makeman'  from  HTML
       source.	The master documentation is at

	      http://netpbm.sourceforge.net/doc/pnmpsnr.html

netpbm documentation		06 January 2018		Pnmpsnr	User Manual(0)

NAME | SYNOPSIS | DESCRIPTION | OPTIONS | SEE ALSO | DOCUMENT SOURCE

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