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 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 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  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 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)