Convert a Netpbm Image to DjVu RLE with pamtodjvurle

You will turn a PNM or PAM image into DjVu colour RLE data and save it as a file without accidentally mixing binary output with terminal text. The command is part of Netpbm and is useful when another DjVu tool specifically requires the colour RLE format.

Prerequisites: a Linux system with the netpbm package, a readable PNM or PAM image, and a shell. No elevated privileges are needed. Allow about five minutes for a first conversion, including checking the result.

Checkpoint 1: check the local command

First confirm which executable you will run and record its Netpbm version. This matters because the installed program's argument handling should take priority over a copied example from another system.

command -v pamtodjvurle
pamtodjvurle --version

On the system used for this guide, the executable is /usr/bin/pamtodjvurle from Debian's netpbm 2:11.05.02-1.1build1, reporting Netpbm 11.5.2. The version switch prints build information and exits; it does not convert an image.

Checkpoint 2: understand the input and output

pamtodjvurle accepts a Netpbm image: a PNM image, or a PAM image carrying the equivalent PNM image data. PAM is the general Netpbm format; its header describes width, height, depth, maximum sample value and tuple type. The converter writes DjVu colour RLE to standard output.

The manual page shows an optional output filename in its synopsis, but the installed 11.5.2 binary rejects two positional filenames. Treat the output as stdout and redirect it yourself. This also makes the binary stream explicit and keeps it away from your terminal.

pamtodjvurle /path/to/input.ppm > /path/to/output.rle

Replace both paths with real values. The command above requires no root access. Redirection creates or replaces output.rle, so check the destination before running it. If the output already contains useful data, choose a new filename or make a backup first.

A successful run is normally quiet and returns exit status zero. Confirm that the file exists and is not empty:

test -s /path/to/output.rle && file /path/to/output.rle

For a small local test, a PPM can be made without changing system state:

printf 'P3\n2 1\n255\n255 255 255  255 0 0\n' > /tmp/example.ppm
pamtodjvurle /tmp/example.ppm > /tmp/example.rle
test -s /tmp/example.rle && file /tmp/example.rle

The generated file is binary data. Do not open it in a text editor or print it with cat. On this machine the first bytes begin with R6, followed by the image dimensions, which is a useful sanity check but not a substitute for a consumer-specific validation.

Checkpoint 3: choose the transparent colour

By default, white is considered transparent. That default is easy to miss: white pixels may not remain opaque when the DjVu data is consumed. If white is part of the visible artwork, choose a different transparent colour, or choose a colour that is not present in the image.

pamtodjvurle -transparent black /path/to/input.ppm > /path/to/output-black-transparent.rle

The value after -transparent is a colour name understood by Netpbm's colour parser. Use a simple name such as black or red when that is sufficient. The option changes how matching image pixels are marked in the DjVu output; it does not add an alpha channel to the source image.

Verify the conversion in the same way:

test -s /path/to/output-black-transparent.rle && file /path/to/output-black-transparent.rle

Do not assume that two output files will have the same size. Changing the transparent colour changes the encoded image, and file size is not a reliable correctness test by itself.

Common failure points

• Two filenames: do not pass an input and output filename as separate arguments on the installed Debian build. It reports that it takes at most one argument. Use shell redirection for the output.
• Binary output in the terminal: redirect stdout before running the converter. If you already sent bytes to a terminal, clear the display and rerun with redirection; the source image is not modified.
• Unexpected transparency: remember that white is the default. Repeat the conversion with -transparent set to the colour that should be transparent.
• Input errors: a missing path, unreadable file or malformed PNM/PAM input causes a non-zero exit. Check the path and format first; do not treat a zero-byte output as a valid DjVu file.
• Overwriting a result: shell redirection truncates the destination before the program runs. If conversion fails, the previous destination may already be lost. Use a new output path, or write to a temporary file and rename it only after verification.

Safer replacement workflow

For an important output, convert to a temporary file in the same directory, check it, then replace the destination. The final move is an overwrite, so keep a backup if recovery matters.

tmp=/path/to/output.rle.tmp
pamtodjvurle /path/to/input.ppm > "$tmp" && test -s "$tmp" && mv -- "$tmp" /path/to/output.rle

This is an ordinary user operation when the directory is writable. If the destination is owned by root, stop and choose a user-writable staging directory or use an approved administrative procedure. Do not add sudo merely because the conversion command failed.

If the conversion fails after creating the temporary file, inspect its error output and remove that temporary file deliberately after confirming it is the one you created. The source image remains unchanged throughout.