Randomly Shift PPM Rows with ppmshift

You will finish with a PPM image whose rows have been shifted left or right by random amounts, plus a checked output file that you can pass to another image tool. The examples use ppmshift from Netpbm package version 2:11.05.02-1.1build1, as installed on this machine.

Allow about ten minutes. You need a readable PPM image, a shell, and enough free space for a second image. This is an ordinary image transformation: it does not need sudo, does not alter the input, and does not change system configuration.

1. Check the installed command

Confirm that the binary is available and record the package version before building a script around it:

$ command -v ppmshift
/usr/bin/ppmshift
$ dpkg-query -W -f='${Package} ${Version}\n' netpbm
netpbm 2:11.05.02-1.1build1

The command has no useful option summary of its own. Its interface is a required shift value followed by an optional PPM file:

$ man ppmshift

The first argument is a range in pixels, not a fixed displacement. Each row is moved left or right by a random amount within that range, with a uniform distribution centred on zero. A value of 0 therefore requests no movement. The output is written to standard output.

Checkpoint: you should have a readable source file and a numeric shift range chosen for the effect you want. For a subtle distortion on a large image, start with a small value such as 10 rather than guessing with a large displacement.

2. Inspect the source without changing it

Use file or your normal image inspection tool to check that the input is actually PPM and to see its dimensions:

$ file /path/to/input.ppm
/path/to/input.ppm: Netpbm image data, size = 800 x 600, rawbits, pixmap

The wording can vary between file versions. Look for a PPM or Netpbm pixmap and the expected width and height. Do not assume that a file named .ppm is valid. If the file cannot be read, fix its path or permissions first; running the filter with elevated privileges is not a substitute for checking the input.

Keep the original. ppmshift reads it and emits a new image, so there is no reason to overwrite the source during a first test.

3. Write a shifted image to a new file

Choose a new destination and redirect standard output there:

$ ppmshift 10 /path/to/input.ppm > /path/to/input-shifted-10.ppm
$ printf 'exit status: %s\n' "$?"
exit status: 0

A zero exit status means the command completed. It does not prove that the visual result is suitable, so verify the destination before using it elsewhere:

$ file /path/to/input-shifted-10.ppm
/path/to/input-shifted-10.ppm: Netpbm image data, size = 800 x 600, rawbits, pixmap
$ test -s /path/to/input-shifted-10.ppm && echo 'output is non-empty'
output is non-empty

ppmshift may emit a rawbits PPM even when the source is ASCII PPM. That is still a valid PPM representation. Check the image in a viewer or convert it with a separate Netpbm tool before deleting the source.

4. Use standard input in a pipeline

Omit the optional input filename when another command supplies the PPM on standard input. This is useful when you want to crop or otherwise prepare the image first:

$ pamcut -width 800 -height 300 /path/to/input.ppm | \
    ppmshift 10 > /path/to/cropped-shifted.ppm
$ file /path/to/cropped-shifted.ppm
/path/to/cropped-shifted.ppm: Netpbm image data, size = 800 x 300, rawbits, pixmap

Every stage must write an image format that the next stage accepts. If a pipeline fails, test each command separately so that a diagnostic from one tool is not mistaken for a ppmshift error. The pipeline above remains unprivileged unless you deliberately read or write a protected path.

5. Avoid accidental overwrites

Shell redirection with > truncates an existing destination before ppmshift starts. That is destructive if the destination contains the only copy of a useful result. Use a temporary name, inspect it, then replace the old file only when you are satisfied:

$ ppmshift 10 /path/to/input.ppm > /path/to/input-shifted-10.ppm.new
$ test -s /path/to/input-shifted-10.ppm.new
$ file /path/to/input-shifted-10.ppm.new
/path/to/input-shifted-10.ppm.new: Netpbm image data, size = 800 x 600, rawbits, pixmap
$ mv /path/to/input-shifted-10.ppm.new /path/to/input-shifted-10.ppm

mv replaces the old destination if it already exists. If you need an undo path, make a backup first:

$ cp --preserve=all /path/to/input-shifted-10.ppm /path/to/input-shifted-10.ppm.bak

After checking the replacement, restore that backup with mv if necessary. Do not remove the backup until you have another verified copy. Deleting it is irreversible within this workflow.

6. Diagnose the common failures

A negative range is rejected by the installed program:

$ ppmshift -1 /path/to/input.ppm > /tmp/unused.ppm
ppmshift: shift factor must be 0 or more

Use zero or a positive integer. If the input path is missing or unreadable, ppmshift exits non-zero and reports that it cannot open the file. Check it without changing anything:

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

Keep the shift argument numeric and put the input filename in the optional second position. A typo in the first argument is not a safe way to express a filename, and can make a command behave differently from the one you intended. In scripts, quote paths that may contain spaces:

$ ppmshift 10 "/path/to/source image.ppm" > "/path/to/source image-shifted.ppm"

Repeated runs are expected to differ because the displacement is random. The manual notes that versions before Netpbm 10.37 could produce identical output when run twice in the same second. The installed package is newer than that boundary, but do not treat randomness as a reproducibility mechanism: retain the generated file when a particular result matters.