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)