Resize Netpbm Images Predictably with pamscale

pamscale resizes a Netpbm image on the command line, and getting the aspect ratio and exact pixel count right first time saves a second pass later. The examples use pamscale from Netpbm 11.5.2, provided here by package netpbm 2:11.05.02-1.1build1.

Allow about fifteen minutes. You need a shell, the Netpbm tools and a PNM, PGM, PPM or PAM input image. The commands write new files in the current directory and need no elevated privileges unless the directory or source is protected. Do not use sudo just to scale an image.

1. Check the installed command and source image

Confirm which executable will run, then inspect the input's current dimensions:

$ command -v pamscale
/usr/bin/pamscale
$ pamscale --version
pamscale: Using libnetpbm from Netpbm Version: Netpbm 11.5.2
$ pamfile /path/to/source.ppm
/path/to/source.ppm: PPM raw, 400 by 300

The exact pamfile wording depends on the input format. If the file is not a Netpbm image, convert it first with an appropriate Netpbm converter. pamscale reads standard input or a named file and writes the scaled image to standard output, so redirecting to a new name is the safe pattern that keeps your original intact.

2. Preview an aspect-preserving fit

Use -xyfit when an image has to fit inside a bounding box without distortion. This plans the largest result that fits within 800 by 600 pixels, but creates nothing yet:

$ pamscale -xyfit 800 600 -reportonly /path/to/source.ppm
400 300 1.000000 1.000000 400 300

The six fields are input width, input height, horizontal factor, vertical factor, output width and output height. The sample line above is for a 400 by 300 source; your numbers will differ. -reportonly emits one line per input image and sends no scaled image to standard output.

• Source larger than the box. Both factors will normally come out below 1.
• Source already smaller than the box. The fit can leave it unchanged, because pamscale never enlarges just to fill a bounding box.

3. Write the fitted image to a new file

Repeat the same options minus -reportonly and redirect standard output:

$ pamscale -xyfit 800 600 /path/to/source.ppm > fitted.ppm
$ pamfile fitted.ppm
fitted.ppm: PPM raw, 400 by 300

On a source that doesn't already fit, pamfile will show the reduced dimensions. The aspect ratio survives because only the fit box, not independent width and height values, drives the scale.

4. Choose exact dimensions or one dimension

Use -width and -height for absolute output dimensions. Supply both and pamscale scales them independently, which can stretch the image:

$ pamscale -width 800 -height 600 /path/to/source.ppm > exact.ppm
$ pamfile exact.ppm
exact.ppm: PPM raw, 800 by 600

• Supply only one absolute dimension and pamscale calculates the other to preserve the aspect ratio.
• -xsize and -ysize are equivalent names for width and height. Prefer the clearer -width and -height in new scripts.
• A scale factor like -xscale 0.5 halves the width and leaves the height alone, since factors work independently unless you say otherwise.
• Want both dimensions scaled the same: use a single positional factor such as 0.5, or set both -xscale and -yscale explicitly. The positional form still works for compatibility, but named options are easier to audit later.

5. Reduce by a pixel budget

Use -pixels when a downstream limit cares about total pixel count rather than a particular width. It scales down to no more than the requested number of pixels and keeps the aspect ratio:

$ pamscale -pixels 1000000 /path/to/source.ppm > preview.ppm
$ pamfile preview.ppm
preview.ppm: PPM raw, 1154 by 866

The example output is illustrative for a suitably large source; check your own result with pamfile. An input already below the limit gets copied rather than enlarged, which makes -pixels a good maximum-size rule but a poor way to guarantee a specific thumbnail size.

6. Pick the sampling behaviour deliberately

By default pamscale mixes pixels, which usually gives smoother reductions and can create new intermediate colours. Add -nomix for discrete sampling instead: it keeps samples from the source palette but can make thin lines uneven or vanish during reduction.

$ pamscale -nomix -width 200 /path/to/source.ppm > palette-safe.ppm
$ pamfile palette-safe.ppm
palette-safe.ppm: PPM raw, 200 by 150

This matters most for palette-limited workflows: mixing may need a later colour-quantisation step before conversion to a restricted format. Discrete sampling isn't automatically higher quality, it's just what you want when preserving exact source values beats smooth averaging.

For finer control, -filter=function selects a resampling filter, with names such as box, triangle, sinc and lanczos. A filter still needs a scale selection alongside it:

$ pamscale -width 800 -filter=box /path/to/source.ppm > filtered.ppm

Common traps

• PBM input, unexpected PGM output. pamscale normally keeps the input's image type, but a PBM input becomes PGM with maxval 255 so pixel mixing has meaningful intermediate values. Convert back explicitly with pamditherbw, or consider pbmreduce, if you need PBM again.
• Multi-image streams. pamscale scales each image in the stream independently on this Netpbm version. That behaviour changed after Netpbm 10.49, so check the package version on any older installation before relying on it.
• Unusual PAM tuple types. Anything outside RGB, grayscale, black-and-white and their alpha variants has no defined general result.
• Command fails outright. Check the input is readable, every option has a value, and the output directory is writable. Add -verbose for scaling messages on standard error, and keep diagnostics separate from standard output, which is the image stream, not text.