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 arith-
       metic 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 difference.  Avoid any feature mentioned in the pamscale
       manual as not existing before Netpbm 9.9.

       pnmscalefixed uses fixed point 12 bit arithmetic.   By  contrast,  pam-
       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
       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 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 pamscale's -nomix option.



netpbm documentation	       18 November 2000	  Pnmscalefixed User Manual(0)