.TL
Pnmscalefixed User Manual
.SH 1
pnmscalefixed
.LP
Updated: 18 November 2000
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Table Of Contents
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NAME
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pnmscale - scale a PNM file quickly
.SH 2
DESCRIPTION
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.LP
This program is part of Netpbm.
\fBpnmscalefixed\fR is the same thing as \fBpnmscale\fR except that
it uses fixed point arithmetic internally instead of floating point,
which makes it run faster.  In turn, it is less accurate and may
distort the image.
.LP
Use the \fBpnmscale\fR user manual with \fBpnmscalefixed\fR.  This
document only describes the difference.
.LP
\fBpnmscalefixed\fR uses fixed point 12 bit arithmetic.  By
contrast, \fBpnmscale\fR uses floating point arithmetic which on most
machines is probably 24 bit precision.  This makes
\fBpnmscalefixed\fR run faster (30% faster in one experiment), but
the imprecision can cause distortions at the right and bottom edges.
.LP
The distortion takes the following form: One pixel from the edge of
the input is rendered larger in the output than the scaling factor
requires.  Consequently, the rest of the image is smaller than the
scaling factor requires, because the overall dimensions of the image
are always as requested.  This distortion will usually be very hard to
see.
.LP
\fBpnmscalefixed\fR with the \fB-verbose\fR option tells you how
much distortion there is.
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The amount of distortion depends on the size of the input image and how
close the scaling factor is to an integral 1/4096th.
.LP
If the scaling factor is an exact multiple of 1/4096, there is no
distortion.  So, for example doubling or halving an image causes no
distortion.  But reducing it or enlarging it by a third would cause
some distortion.  To consider an extreme case, scaling a 100,000 row
image down to 50,022 rows would create an output image with all of the
input squeezed into the top 50,000 rows, and the last row of the input
copied into the bottom 22 rows of output.
.LP
\fBpnmscalefixed\fR could probably be modified to use 16 bit or
better arithmetic without losing anything.  The modification would
consist of a single constant in the source code.  Until there is a
demonstrated need for that, though, the Netpbm maintainer wants to
keep the safety cushion afforded by the original 12 bit precision.
.LP
\fBpnmscalefixed\fR does not have \fBpnmscale\fR's \fB-nomix\fR
option.
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Table Of Contents
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