pnmscalefixed

pnmscalefixed(1)            General Commands Manual           pnmscalefixed(1)

NAME
       pnmscalefixed - scale a portable anymap quickly, but less accurate

DESCRIPTION
       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 de-
       scribes the difference.

       pnmscalefixed uses fixed point 12 bit arithmetic.   By  contrast,  pnm-
       scale 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 re-
       quires.  Consequently, the rest of the image is smaller than the  scal-
       ing  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  dis-
       tortion.   So,  for example doubling or halving an image causes no dis-
       tortion.  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 arith-
       metic  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 cush-
       ion afforded by the original 12 bit precision.

       pnmscalefixed does not have pnmscale 's -nomix option.

                               18 November 2000               pnmscalefixed(1)