NAME

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

SYNOPSIS

       pnmpsnr

       [pnmfile1]

       [pnmfile2]

       [-rgb] [-machine] [-max=n] [-target=n] [-target1=n] [-target2=n] [-target3=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
       image compression papers to rate the distortion between original and decoded 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 luminance 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

       In addition to the options common to all programs based on libnetpbm (most notably  -quiet,  see   Common
       Options ), pnmpsnr recognizes the following command line options:

       -rgb   This option causes pnmpsnr to compare the red, green, and blue components of the color rather than
              the luminance and chrominance 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 information.

              The output is a single line.  It contains  one  floating  point  decimal  number  for  each  color
              component,  with  a single space between 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 order.  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 effect 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 computed PSNRs (luminance for a PBM or PGM;
              luminance and chrominance or red, green, and blue for PPM) exceed n, the program prints 'match' to
              Standard Output.  Otherwise, it prints 'nomatch'.

              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 deciding whether the images match.

              If  the  image  is  a  PBM  or  PGM,  these  options have no effect, except 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 pnmpsnr 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(1)

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(1)