Pack PNM Images into a Checked, Reusable Montage

You will finish with one PNM image containing several smaller images, plus an optional coordinate file that tells another program where each input landed. Allow about fifteen minutes. You need Netpbm, readable PBM, PGM or PPM input files, and a writable working directory. The examples use the installed Netpbm 11.5.2 command from Debian package 2:11.05.02-1.1build1.

pnmmontage writes the composite image to standard output. That makes it easy to pipe into another Netpbm converter, but shell redirection can also truncate an existing file before the command has succeeded. Use a new output name while testing.

1. Check the installed command

Confirm that the command and package you are about to use are the expected ones. These are ordinary, read-only checks and do not need elevated privileges:

$ command -v pnmmontage
/usr/bin/pnmmontage
$ pnmmontage --version 2>&1 | sed -n '1,2p'
pnmmontage: Using libnetpbm from Netpbm Version: Netpbm 11.5.2
pnmmontage: Built from source dated 2024-03-31 09:09:47
$ dpkg-query -W -f='${Package} ${Version}\n' netpbm
netpbm 2:11.05.02-1.1build1

Checkpoint: make sure the input files are readable and really are PNM images. Do not assume that a file extension proves the format:

$ file /path/to/first.ppm /path/to/second.pgm
/path/to/first.ppm:  Netpbm image data, size 320 x 200, rawbits, pixmap
/path/to/second.pgm: Netpbm image data, size 160 x 120, rawbits, greymap
$ test -r /path/to/first.ppm && test -r /path/to/second.pgm && echo readable
readable

2. Create a basic montage

Pass two or more input paths and redirect standard output to a new file. The program accepts images with different dimensions and packs them into a minimum-area composite. Any output area not occupied by an input image is black.

$ pnmmontage /path/to/first.ppm /path/to/second.pgm > montage.ppm
$ file montage.ppm
montage.ppm: Netpbm image data, size [depends on inputs], rawbits, pixmap

Your dimensions and file wording will differ. The useful checkpoint is that montage.ppm exists, is non-empty, and has plausible dimensions. A successful exit status proves that the command completed; it does not prove that the visual arrangement is convenient.

If the inputs are in a directory and you want to include a set selected by a shell pattern, inspect the expansion first. An unmatched pattern can be passed literally on some shells, and a large set can make the packing search slow:

$ printf '%s\n' /path/to/tiles/*.ppm
/path/to/tiles/one.ppm
/path/to/tiles/two.ppm
$ pnmmontage /path/to/tiles/*.ppm > tiles-montage.ppm

3. Choose the search effort deliberately

The default quality level is -5. The numbered options run from -0, which accepts the first solution found, to -9, which performs an exhaustive search for the best packing. Higher numbers take longer. Start with -0 or the default for a large batch, then spend more time only if the arrangement matters:

$ pnmmontage -0 /path/to/first.ppm /path/to/second.pgm > quick-montage.ppm
$ pnmmontage -quality=150 /path/to/first.ppm /path/to/second.pgm > reviewed-montage.ppm

-quality=n uses a percentage-based stopping target. The default is 200; -quality=100 asks for the best possible solution and may take a very long time. The manual specifically warns that -9 is very slow except for small image sets. Do not use either setting in a time-sensitive batch without measuring it on representative inputs.

4. Record where each image landed

Use -data= when a later step needs to map source images back to positions in the composite:

$ pnmmontage -data=layout.txt \
    /path/to/first.ppm /path/to/second.pgm > montage.ppm
$ sed -n '1,3p' layout.txt
:0:0:<overall-width>:<overall-height>
/path/to/first.ppm:<x>:<y>:<width>:<height>
/path/to/second.pgm:<x>:<y>:<width>:<height>

The first line describes the composite and has an empty source name. Each later line contains the source name, the upper-left column, the upper-left row, the width and the height, separated by colons. Your order and coordinates will differ because the dimensions affect the packing. Treat this file as machine-readable data, not as a drawing of the montage.

Checkpoint: compare the source dimensions from file with the width and height recorded in layout.txt. The command packs images; it does not resize them.

5. Force a usable overall shape

Minimum area is not the same as a useful shape. A mathematically efficient result may be a tall, thin column. The documented way to impose a minimum width or height is to include a strut image: a black image one pixel high for width, or one pixel wide for height.

For example, if a black PGM named width-strut.pgm is 800 pixels wide and one pixel high, include it with the real images to make the montage at least that wide:

$ pnmmontage width-strut.pgm /path/to/first.ppm /path/to/second.pgm \
    > wide-montage.ppm
$ file wide-montage.ppm
wide-montage.ppm: Netpbm image data, size [depends on inputs], rawbits, pixmap

The strut becomes part of the output, so this method is appropriate only when a black border or spacer is acceptable. Keep the strut as a separate, deliberately named asset. Do not invent its dimensions, and check the result rather than assuming the target shape was achieved.

6. Generate C coordinates only when code needs them

-header= writes a C header containing coordinates and sizes for the component images, as well as OVERALLX and OVERALLY. Use -prefix= if those macro names need a project-specific prefix:

$ pnmmontage -header=montage-layout.h -prefix=ATLAS_ \
    /path/to/first.ppm /path/to/second.pgm > montage.ppm
$ sed -n '1,12p' montage-layout.h
$ grep -E '^#define .*OVERALL|^#define .*SZX|^#define .*SZY' montage-layout.h

The exact macro spelling is derived from the input file names. Inspect the complete header before including it, especially if names contain punctuation that is unsuitable for C identifiers. A header is configuration for a build, so review it as source code and keep it with the matching montage.

7. Avoid common failure and recovery traps

Do not run pnmmontage with no input files. It tries to read an image from standard input, so an empty or unrelated stream produces a Netpbm magic-number error. If a command fails, the safest recovery is to keep the original inputs and rerun with a new destination:

$ pnmmontage /path/to/first.ppm /path/to/second.pgm > montage.ppm.new
$ test -s montage.ppm.new && file montage.ppm.new
montage.ppm.new: Netpbm image data, size 480 x 320, rawbits, pixmap
$ mv montage.ppm.new montage.ppm

The final mv replaces the old destination only after the new file has been checked. If the command fails, leave the existing montage in place and inspect the input paths, permissions and formats. None of these operations requires sudo; elevated privileges would not repair a malformed image or a poor packing choice.