Reduce Every Image Column with pamsummcol

You will finish with a one-row Netpbm image whose pixels contain a per-column sum, mean, minimum or maximum. The examples use pamsummcol from Netpbm 11.5.2, provided by Ubuntu package netpbm version 2:11.05.02-1.1build1.

Allow about fifteen minutes. You need a shell and a PNM or PAM image. The workflow only reads the input and writes a new result to standard output, so it does not need elevated privileges and does not alter the source image.

1. Check the installed command

Confirm that the command on your PATH is the one you intend to run:

$ command -v pamsummcol
/usr/bin/pamsummcol
$ pamsummcol -version
pamsummcol: Using libnetpbm from Netpbm Version: Netpbm 11.5.2
... additional build details ...

The exact build-detail lines can vary. The useful checkpoint is the Netpbm version. This guide describes the installed 11.5.2 behaviour, while the command's basic interface is also documented by its local pamsummcol(1) manual page.

2. Inspect the input before reducing it

pamsummcol accepts PNM or PAM input. Inspect the file with pamfile before choosing an operation:

$ pamfile /path/to/input.pgm
/path/to/input.pgm: PGM raw, 640 by 480  maxval 255

Replace /path/to/input.pgm with an existing file. The width is the number of values in the output row. The height is reduced to one row. For a PGM or PPM image, the operation uses stored sample values, not a conversion to perceived light intensity. A colour image is processed independently in each plane, so each output tuple keeps the input depth.

Checkpoint: stop if pamfile reports a format other than PNM or PAM, or if the dimensions and maxval are not what you expected. A visually similar image can still have different sample values.

3. Produce one result row

Choose exactly one of the four operations. Redirect the result to a new file so the input remains available for comparison:

$ pamsummcol -mean /path/to/input.pgm > /tmp/input-column-means.pgm
$ pamfile /tmp/input-column-means.pgm
/tmp/input-column-means.pgm: PGM raw, 640 by 1  maxval 255

The output has the same width and depth as the input, but one row. The command writes the image to standard output, which is why the shell redirection is part of the example. It does not display the image or create a file by itself.

Use the other operations in the same form:

$ pamsummcol -sum  /path/to/input.pgm > /tmp/input-column-sums.pgm
$ pamsummcol -min  /path/to/input.pgm > /tmp/input-column-minima.pgm
$ pamsummcol -max  /path/to/input.pgm > /tmp/input-column-maxima.pgm

Do not provide more than one operation. The program requires exactly one of -sum, -mean, -min or -max. The option names can be abbreviated to a unique prefix, but spelling them in full makes scripts easier to review.

4. Verify the numbers with a tiny fixture

When a production result looks surprising, test the command with a small known image. This creates only a temporary file under /tmp:

$ printf '%s\n' 'P2' '3 2' '10' '1 2 3' '4 5 6' > /tmp/pamsummcol-check.pgm
$ pamsummcol -sum /tmp/pamsummcol-check.pgm > /tmp/pamsummcol-sum.pgm
$ pamfile /tmp/pamsummcol-sum.pgm
/tmp/pamsummcol-sum.pgm: PGM raw, 3 by 1  maxval 10

The three column sums are 5, 7 and 9. The corresponding outputs for this fixture are 3, 4 and 5 with -mean, 1, 2 and 3 with -min, and 4, 5 and 6 with -max. To inspect a result with another Netpbm tool, keep the pipeline binary and make sure the next tool reads the image format you produced.

There is no persistent undo step here. The command does not modify its input. If you no longer need the test output, remove only the named temporary files after checking them:

$ rm -- /tmp/pamsummcol-check.pgm /tmp/pamsummcol-sum.pgm

This removal is irreversible. Check each pathname before pressing Enter, and never replace /tmp/pamsummcol-check.pgm with a broad wildcard in a cleanup command.

5. Account for sums that exceed maxval

A sum can be larger than the image's maximum sample value. pamsummcol clips such a result to the input maxval. For an 8-bit-style PGM with maxval 255, a column sum of 400 is emitted as 255, not 400. This is a data-loss boundary, not an error message.

If the sum itself matters, do not treat the output as an exact accumulator without checking this limit. Consider -mean for an average-valued image, or choose a workflow that explicitly handles a larger numeric range before reducing the image. Test the chosen workflow with values near the input maxval and inspect the resulting maxval with pamfile.

6. Change the direction or scope deliberately

pamsummcol works vertically, combining all rows in each column. To reduce by row instead, run the image through pamflip first. If you want one column as the final shape, flip the one-row result again. Check the dimensions after each stage rather than relying on the visual orientation.

For a summary over the whole image, the manual's workflow is to use pamsumm, flip its summary row into a column, then use pamsumm again. That is a different job from pamsummcol. Keep the tools separate in scripts so a later reader can see whether the intended result is per column, per row or one value per tuple.

7. Diagnose the common failures

• A missing filename or unreadable path usually means the shell opened no input. Check the path and permissions first. Do not use sudo as a substitute for correcting a typo.
• An error about the operation means exactly one summary option was not supplied. Add one full option and remove any second operation.
• A result with one row but unexpected values may reflect clipping, sample-versus-light interpretation, or independent colour planes. Recheck maxval, format and depth with pamfile.
• If the output is empty or corrupt, check the redirection target and the exit status immediately: pamsummcol -mean /path/to/input.pgm > /tmp/result.pgm followed by printf 'status: %s\n' "$?". A non-zero status means the result should not be used.