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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 differ-
       ence.  Avoid any feature 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  pnm-
       scalefixed  run	faster (30% faster in one experiment), but the imprecision can cause dis-
       tortions 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 integral 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 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.

       pnmscalefixed does not have pamscale's -nomix option.

netpbm documentation			 18 November 2000	     Pnmscalefixed User Manual(0)
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