Decode MRF Images to PBM Safely with mrftopbm

Run mrftopbm image.mrf without redirecting it and you get binary image data splattered across your terminal instead of a PBM file. This covers decoding one MRF file, checking the result's dimensions, and using standard input and output in a pipeline, with Netpbm 11.05.02, packaged here as 2:11.05.02-1.1build1. Allow about ten minutes.

You need a shell, the netpbm package, and an existing MRF file. This only reads the input and writes a new output file: no elevated privileges are needed, and nothing about the source image or system configuration changes.

1. Check the installed command

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

The command takes one positional input, input.mrf in its manual page. Leave it out and mrftopbm reads MRF data from standard input instead. Either way, it writes the decoded PBM to standard output, which is the detail that catches people out: mrftopbm image.mrf on its own just dumps binary image data into the terminal rather than creating image.pbm.

Checkpoint: keep binary output out of the terminal. Pick an output path that has nothing you need in it, or check it before you overwrite it.

2. Decode one MRF file

$ mrftopbm /path/to/input.mrf > /tmp/decoded.pbm
$ printf 'exit status: %s\n' "$?"
exit status: 0

Swap in your real file for /path/to/input.mrf. A zero status means the conversion completed, but it does not by itself prove the image has the dimensions or content you intended, so check the PBM header next.

Warning: there is no overwrite prompt. The shell truncates /tmp/decoded.pbm before mrftopbm even starts. Check the destination with ls -l, or pick a new path, before repeating a conversion. If you replaced a file by accident, recovery depends on your normal backup process: mrftopbm has no undo.

3. Verify the PBM header and dimensions

Netpbm's pamfile reports on the decoded image without touching it:

$ pamfile /tmp/decoded.pbm
/tmp/decoded.pbm: PBM raw, 320 by 200

Your dimensions will differ. What matters is the format, which should be PBM, and the width and height the next tool in line expects. You can also peek at just the first few bytes:

$ od -An -c -N 20 /tmp/decoded.pbm
   P   4  \n 320 200  \n

PBM output may be plain or raw; this install produced raw PBM for the test image, header starting P4. Do not build a check that demands one particular encoding unless the next program actually requires it: check for PBM and the expected dimensions instead.

If pamfile reports a different format, an empty file, or fails to parse the result, stop and read the command's error output. Do not feed an unverified binary file into a later conversion step.

4. Use standard input and output in a pipeline

Leaving out the input pathname makes the decoder read from standard input, useful when an MRF file arrives under another name, comes from a previous command, or has passed through a decompressor:

$ cat /path/to/input.mrf | mrftopbm > /tmp/decoded-from-stdin.pbm
$ pamfile /tmp/decoded-from-stdin.pbm
/tmp/decoded-from-stdin.pbm: PBM raw, 320 by 200

For a plain file, redirection is clearer and skips an unnecessary cat process:

$ mrftopbm < /path/to/input.mrf > /tmp/decoded-from-stdin.pbm

The two redirections are independent: < feeds MRF bytes into standard input, > catches PBM bytes from standard output. Keep both. Using only < and then wondering where the binary PBM went is a common trap.

5. Keep padded edges for compressor debugging

Normal decoding returns the dimensions stored in the MRF header and ignores anything encoded outside them. -a includes those edges in the PBM output, meant for debugging an MRF compressor's edge optimisation rather than producing an ordinary image:

$ mrftopbm /path/to/input.mrf > /tmp/normal.pbm
$ mrftopbm -a /path/to/input.mrf > /tmp/with-edges.pbm
$ pamfile /tmp/normal.pbm /tmp/with-edges.pbm
/tmp/normal.pbm: PBM raw, 8 by 4
/tmp/with-edges.pbm: PBM raw, 64 by 64

That output is from an 8 by 4 test image; the exact padded dimensions depend on the source. MRF stores images in a grid of 64 by 64 squares, so edge-inclusive output can be much larger than the visible image. Treat the extra pixels as compressor-test data, not meaningful content, and only use -a when the consumer understands padded dimensions or you are investigating the compressor itself.

6. Handle failures without hiding them

Capture diagnostics separately from the PBM output. An empty input, for example, is rejected with a non-zero status:

$ mrftopbm -quiet < /dev/null > /tmp/empty.pbm
$ printf 'exit status: %s\n' "$?"
exit status: 1

The installed program's error here says the input is not an MRF image because it is shorter than the MRF header. -quiet suppresses normal Netpbm chatter, but it does not make an invalid input succeed. Keep standard error visible, or redirect it to a log when scripting.

Do not treat a zero-length output as a successful image. A safe shell check:

set -o pipefail
if mrftopbm /path/to/input.mrf > /tmp/decoded.pbm; then
    pamfile /tmp/decoded.pbm
else
    status=$?
    printf 'mrftopbm failed with status %s\n' "$status" >&2
    rm -f /tmp/decoded.pbm
    exit "$status"
fi

The rm there only removes the newly chosen temporary output after a failed conversion. Do not adapt it to a source or shared output path without checking the target first.

Done means