This is the command pnmscale that can be run in the OnWorks free hosting provider using one of our multiple free online workstations such as Ubuntu Online, Fedora Online, Windows online emulator or MAC OS online emulator
PROGRAM:
NAME
pnmscale - scale a portable anymap
SYNOPSIS
pnmscale 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 combination 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 unspecified 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 unspecified 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 equivalent 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 pixels,
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 probably add a pnmsmooth step;
otherwise, you can see the original 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 scaling. 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 scaling down,
pixels get omitted. Note that this means the image is rather distorted. If you scale up
by 1.5 horizontally, 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 pnmenlarge 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 information, 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 blurring.
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, distort your image a little. See pnmscalefixed's man page for a complete
discussion of the difference.
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