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Pnmscalefixed User Manual(0)                      Pnmscalefixed User Manual(0)



NAME
       pnmscale - scale a PNM file quickly


DESCRIPTION
       This program is part of Netpbm(1).

       pnmscalefixed is like pamscale 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.  It also lacks many of  the
       features of pamscale.

       Use  the pamscale user manual with pnmscalefixed.  This document only describes the difference.  Avoid any fea-
       ture mentioned in the pamscale manual as not existing before Netpbm 9.9.

       pnmscalefixed uses fixed point 12 bit arithmetic.  By contrast, pamscale uses floating point  arithmetic  which
       on  most machines is probably 24 bit precision.  This makes pnmscalefixed run faster (30% faster in one experi-
       ment), but the imprecision can cause distortions at the right and bottom edges.

       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.

       pnmscalefixed with the -verbose option tells you how much distortion there is.

       The amount of distortion depends on the size of the input image and how close the scaling factor is to an inte-
       gral 1/4096th.

       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.

       pnmscalefixed could probably be modified to use 16 bit or better arithmetic without losing anything.  The modi-
       fication  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.

       pnmscalefixed does not have pamscale's -nomix option.



netpbm documentation           18 November 2000   Pnmscalefixed User Manual(0)