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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 in-
ternally instead of floating point, which makes it run faster. In turn, it is less accu-
rate and may distort the image.
Use the pnmscale man page with pnmscalefixed. This man page only describes the differ-
ence.
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 pnm-
scalefixed run faster (30% faster in one experiment), but the imprecision can cause dis-
tortions 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 im-
age is smaller than the scaling factor requires, because the overall dimensions of the im-
age 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 ex-
ample 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)
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On Apache
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