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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 3.2//EN">
<HTML><HEAD><TITLE>Pnmpsnr User Manual</TITLE></HEAD>
<BODY>
<H1>pnmpsnr</H1>
Updated: 04 March 2001
<BR>
<A HREF="#index">Table Of Contents</A>
<A NAME="lbAB"> </A>
<H2>NAME</H2>
pnmpsnr - compute the difference between two images (the PSNR)
<A NAME="lbAC"> </A>
<H2>SYNOPSIS</H2>
<B>pnmpsnr</B>
[<I>pnmfile1</I>]
[<I>pnmfile2</I>]
<A NAME="lbAD"> </A>
<H2>DESCRIPTION</H2>
<p>This program is part of <a href="index.html">Netpbm</a>.
<p><b>pnmpsnr</b> 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.
<P>If the inputs are PBM or PGM, <B>pnmpsnr</B> 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.
<P>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.
<P>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].
<P>The maximum mean square difference is identically 1.
<P>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.
<A NAME="lbAE"> </A>
<H2>SEE ALSO</H2>
<B><A HREF="pnm.html">pnm</A></B>
<HR>
<A NAME="index"> </A>
<H2>Table Of Contents</H2>
<UL>
<LI><A HREF="#lbAB">NAME</A>
<LI><A HREF="#lbAC">SYNOPSIS</A>
<LI><A HREF="#lbAD">DESCRIPTION</A>
<LI><A HREF="#lbAE">SEE ALSO</A>
</UL>
</BODY>
</HTML>
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