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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]
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 prints 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.
If the inputs are PBM or PGM, pnmpsnr prints the PSNR of the luminance
only. Otherwise, it prints the separate PSNRs of the luminance, and
chrominance (Cb and Cr) components of the colors.
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.
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 grayscale 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 grayscale 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 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).
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 23 January 2016 Pnmpsnr User Manual(0)
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