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  ⟨index.html#commonoptions⟩  ),  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