Scale PNM Images Fast with pnmscalefixed

pnmscalefixed resizes a PNM image to a width and height you choose, trading a little precision for speed. Allow about ten minutes to resize a test image, check the header, and see where the fixed-point maths can nudge an edge out of place. The input is never touched; only the output path changes.

1. Check the installed command

This guide uses the Netpbm command installed on this machine: Netpbm 11.5.2, Debian package 2:11.05.02-1.1build1. The manual page is dated 1 July 2020, so check the local manual before moving a script to another release.

$ command -v pnmscalefixed
/usr/bin/pnmscalefixed
$ pnmscalefixed --version
pnmscalefixed: Using libnetpbm from Netpbm Version: Netpbm 11.5.2
pnmscalefixed: Built from source dated 2024-03-31 09:09:47

• It is the fast alternative to pamscale. 12-bit fixed-point arithmetic beats floating point on speed but not on accuracy.
• Reach for pamscale instead when precision or extra features matter more than raw speed; it also has a -nomix option that pnmscalefixed does not.

2. Scale to exact output dimensions

Use -xsize and -ysize (also spelled -width and -height) when you know the target dimensions. This example halves a four-column test image and doubles its height:

$ pnmscalefixed -xsize 2 -ysize 6 /path/to/input.pgm > /path/to/output.pgm

Supply both values and they are treated independently, so the aspect ratio can change. If the source shape matters, work out the matching dimension yourself before typing both numbers.

$ file /path/to/output.pgm
$ head -n 3 /path/to/output.pgm
P5
2 6
255

Netpbm may pick a binary encoding, so file is safer than reading the whole image in a terminal. Look for a non-empty file, the dimensions you asked for, and a valid PNM header; do not treat the pixel bytes after the header as text.

3. Scale by a factor instead of a size

Use -xscale and -yscale when the requirement is a ratio, not an absolute size. A factor of 2 doubles a dimension; 0.5 halves it:

$ pnmscalefixed -xscale 0.5 -yscale 0.5 /path/to/input.pgm > /path/to/half-size.pgm
$ file /path/to/half-size.pgm
/path/to/half-size.pgm: Netpbm image data, size 640 x 480, rawbits, greymap

Your own dimensions will differ; treat the example above as a shape, not a promise. Some rules worth remembering:

• One scale factor only? The other dimension stays as it is.
• One absolute dimension only? The other is adjusted to keep the aspect ratio.
• Both absolute dimensions supplied? That deliberately overrides ratio preservation.

The older -xysize and -pixels options still exist. Use the full option names in scripts so a later reader can see what was intended, and check the installed manual before relying on the less common forms.

4. See the fixed-point calculation

Add -verbose to see what the command actually worked out. Diagnostics go to standard error while the scaled image still goes to standard output:

$ pnmscalefixed -xsize 2 -ysize 6 -verbose /path/to/input.pgm > /path/to/output.pgm
pnmscalefixed: Scaling by 2048/4096 = 0.500000 horizontally to 2 columns.
pnmscalefixed: Scaling by 8192/4096 = 2.000000 vertically to 6 rows.

That denominator, 4096, is the command's 12-bit fixed-point arithmetic showing through. When your requested factor is an exact multiple of 1/4096, the manual says there is no fixed-point distortion at all; doubling and halving both qualify. Other ratios can push one input edge pixel into slightly more output space than it deserves, usually at the right or bottom edge.

That is a precision boundary, not a broken conversion. Invisible for a thumbnail, worth a second look for measurement work, repeated resampling, or anything where edge placement matters. Compare against pamscale and pick whichever meets your accuracy bar.

5. Protect an existing output file

$ pnmscalefixed -xsize 640 -ysize 480 /path/to/input.pgm > /path/to/output.pgm.new
$ test -s /path/to/output.pgm.new
$ file /path/to/output.pgm.new
$ mv /path/to/output.pgm.new /path/to/output.pgm

That final mv is the step that actually replaces the old file, so double-check the path before running it. If the conversion fails or the dimensions look wrong, leave the original alone and just remove the .new file. Once a good replacement has happened, the only way back is a backup or a snapshot; pnmscalefixed has no undo.

None of this needs elevated privileges. Reach for sudo only if the input or output directory is genuinely off-limits to your account, and check the path first.

6. Diagnose the common failures

An error opening the input almost always means a wrong path or unreadable file. Check without changing anything:

$ ls -l /path/to/input.pgm
$ test -r /path/to/input.pgm && echo readable

• Header dimensions look wrong? Check you did not mix up absolute sizes and scale factors: -xsize 640 means 640 columns, -xscale 640 means a factor of 640, which is a very different request.
• Image looks stretched? Both absolute dimensions were probably supplied without preserving the source ratio.
• Only one edge looks displaced? Rerun with -verbose, compare the factors, and for a precision-sensitive job run the same request through pamscale too.

Keep the original input around until the chosen output has been checked properly.