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 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)