pnmscale(1)pnmscale(1)NAMEpnmscale - scale a PNM image
SYNOPSISpnmscale scale_factor [pnmfile]
pnmscale-reduce reduction_factor [pnmfile]
pnmscale [{-xsize=cols | -width=cols | -xscale=factor}]
[{-ysize=rows | -height=rows | -yscale=factor}] [pnmfile]
pnmscale-xysize cols rows [pnmfile]
pnmscale-pixels n [pnmfile]
Miscellaneous options:
-verbose -nomix
Minimum unique abbreviation of option is acceptable. You
may use double hypens instead of single hyphen to denote
options. You may use white space in place of the equals
sign to separate an option name from its value.
DESCRIPTION
Reads a PBM, PGM, or PPM image as input, scales it by the
specified factor or factors and produces a PGM or PPM
image as output. If the input file is in color (PPM), the
output will be too, otherwise it will be grayscale (PGM).
This is true even if the input is a black and white bitmap
(PBM), because the process of scaling can turn a combina
tion of black and white pixels into a gray pixel.
If you want PBM output, use pgmtopbm to convert pnmscale's
output to PBM. Also consider pbmreduce.
You can both enlarge (scale factor > 1) and reduce (scale
factor < 1).
When you specify an absolute size or scale factor for both
dimensions, pnmscale scales each dimension independently
without consideration of the aspect ratio.
If you specify one dimension as a pixel size and don't
specify the other dimension, pnmscale scales the unspeci
fied dimension to preserve the aspect ratio.
If you specify one dimension as a scale factor and don't
specify the other dimension, pnmscale leaves the unspeci
fied dimension unchanged from the input.
If you specify the scale_factor parameter instead of
dimension options, that is the scale factor for both
dimensions. It is equivalent to -xscale=scale_factor
-yscale=scale_factor .
Specifying the -reduce reduction_factor option is equiva
lent to specifying the scale_factor parameter, where
scale_factor is the reciprocal of reduction_factor.
-xysize specifies a bounding box. pnmscale scales the
input image to the largest size that fits within the box,
while preserving its aspect ratio.
-pixels specifies a maximum total number of output pixels.
pnmscale scales the image down to that number of pixels.
If the input image is already no more than that many pix
els, pnmscale just copies it as output; pnmscale does not
scale up with -pixels.
If you enlarge by a factor of 3 or more, you should proba
bly add a pnmsmooth step; otherwise, you can see the orig
inal pixels in the resulting image.
When the scale factor is not an integer (including all
cases of scaling down), there are two ways to do the scal
ing. Which one pnmscale does is controlled by its -nomix
option.
By default, pnmscale mixes the colors of adjacent pixels
to produce output pixels that contain information from
multiple input pixels. This makes the image look more
like it would if it had infinite resolution. Note that it
means the output may contain colors that aren't in the
input at all.
But if you specify -nomix, pnmscale never mixes pixels.
Each output pixel is derived from one input pixel. If
you're scaling up, pixels get duplicated. If you're scal
ing down, pixels get omitted. Note that this means the
image is rather distorted. If you scale up by 1.5 hori
zontally, for example, the even numbered input pixels are
doubled in the output and the odd numbered ones are copied
singly.
When the scale factor is an integer (which means you're
scaling up), the -nomix option has no effect -- output
pixels are always just N copies of the input pixels. In
this case, though, consider using pamstretch instead of
pnmscale to get the added pixels interpolated instead of
just copied and thereby get a smoother enlargement.
pnmscale with -nomix is faster than without, but pnmen
large is faster still. pnmenlarge works only on integer
enlargements.
A useful application of pnmscale is to blur an image.
Scale it down (without -nomix ) to discard some informa
tion, then scale it back up using pamstretch.
Or scale it back up with pnmscale and create a "pixelized"
image, which is sort of a computer-age version of blur
ring.
PRECISION
pnmscale uses floating point arithmetic internally. There
is a speed cost associated with this. For some images,
you can get the acceptable results (in fact, sometimes
identical results) faster with pnmscalefixed, which uses
fixed point arithmetic. pnmscalefixed may, however, dis
tort your image a little. See pnmscalefixed's man page
for a complete discussion of the difference.
SEE ALSOpnmscalefixed(1), pamstretch(1), pbmreduce(1), pnmen
large(1), pnmsmooth(1), pnmcut(1), pnm(5)AUTHOR
Copyright (C) 1989, 1991 by Jef Poskanzer.
04 November 2000 pnmscale(1)
