Provided by: netpbm_10.0-12.2_i386 bug


       pnmscalefixed - scale a portable anymap quickly, but less accurate


       pnmscalefixed  is  the same thing as pnmscale 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.

       Use  the  pnmscale  man  page  with  pnmscalefixed.  This man page only
       describes the difference.

       pnmscalefixed  uses  fixed  point  12  bit  arithmetic.   By  contrast,
       pnmscale  uses  floating  point  arithmetic  which  on most machines is
       probably 24 bit precision.  This makes pnmscalefixed  run  faster  (30%
       faster in one experiment), 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

       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 pnmscale 's -nomix option.

                               18 November 2000               pnmscalefixed(1)