Convert a PBM Image to MGR Bitmap with pbmtomgr

pbmtomgr is a small, single-purpose converter: feed it a PBM image and it writes an MGR bitmap to standard output, nothing more.

The examples use Netpbm 11.5.2 from the installed netpbm package. Allow about ten minutes if you already have a PBM file; the only prerequisite is a shell and a readable input image. It does not display the image, choose a destination file, or touch system configuration, which is handy in pipelines but also means shell redirection is part of the job and deserves attention.

1. Check the installed command

Start with ordinary, read-only checks. No elevated privileges are needed:

$ command -v pbmtomgr
/usr/bin/pbmtomgr
$ pbmtomgr -version
pbmtomgr: Using libnetpbm from Netpbm Version: Netpbm 11.5.2
pbmtomgr: Built from source dated 2024-03-31 09:09:47
pbmtomgr: Built by Debian

The exact build details can differ between distributions. The installed command accepts Netpbm's common -version option; the pbmtomgr-specific manual defines no conversion options of its own.

2. Confirm that the input is PBM

PBM is a monochrome Netpbm format. A raw PBM normally starts with P4; a plain-text PBM starts with P1. The file name is only a convention, so inspect the content before converting it:

$ file /path/to/input.pbm
/path/to/input.pbm: Netpbm image data, size 8 x 4, rawbits, bitmap
$ test -r /path/to/input.pbm && echo 'input is readable'
input is readable

Replace /path/to/input.pbm with your real path. Do not use a JPEG, PNG or greyscale PGM file just because someone gave it a .pbm suffix: convert that source to PBM first with an appropriate Netpbm tool, then check the result.

3. Convert to a new MGR file

Redirect standard output to a new destination. This keeps the source untouched:

$ pbmtomgr /path/to/input.pbm > /path/to/output.mgr
$ printf 'exit status: %s\n' "$?"
exit status: 0

A status of zero means the converter finished successfully. MGR is a binary bitmap format, so do not open output.mgr in a text editor and do not expect readable text in a terminal. It is not a portable PBM image and cannot be inspected with PBM tools unless you convert it back.

$ pbmtomgr /path/to/input.pbm > /path/to/output.mgr.new
$ test -s /path/to/output.mgr.new
$ mv /path/to/output.mgr.new /path/to/output.mgr

The final mv only changes the destination once a non-empty output file exists. If conversion fails, leave the original file in place and inspect the error; remove the incomplete .new file only after confirming it is the temporary one you meant to discard.

4. Verify the binary result

For a quick smoke test, check that the file exists and contains bytes:

$ stat -c '%n: %s bytes' /path/to/output.mgr
/path/to/output.mgr: 12 bytes
$ xxd -g1 -l32 /path/to/output.mgr
00000000: 79 7a 20 28 20 24 21 20 66 99 3c c3              yz ( $! f.<.

Size and byte values depend on the input dimensions and pixels, so do not compare them against this small example unless your PBM is identical. What matters is a successful exit status and a non-empty output file. Neither proves the source had the picture you intended, so inspect the image with an MGR-aware reader or the reverse converter if it is installed.

5. Use standard input in a pipeline

The input file argument is optional. With none, pbmtomgr reads PBM data from standard input while the MGR bitmap still goes to standard output:

$ pnminvert /path/to/input.pbm | pbmtomgr > /path/to/inverted.mgr
$ test -s /path/to/inverted.mgr
inverted.mgr

This assumes pnminvert is installed and deliberately changes the pixels before conversion. For a direct pipeline, cat /path/to/input.pbm | pbmtomgr > /path/to/output.mgr works too, though passing the filename is simpler.

• Keep diagnostics separate from the bitmap. Do not redirect standard error into standard output with 2>&1, or text diagnostics could corrupt the binary stream.
• Test each pipeline stage separately when it fails. Your shell may report only the last command's status unless it is configured with a pipeline-failure option:

$ pnminvert /path/to/input.pbm > /tmp/inverted.pbm
$ pbmtomgr /tmp/inverted.pbm > /path/to/inverted.mgr
$ printf 'converter status: %s\n' "$?"
converter status: 0

The temporary PBM in this diagnostic workflow can be discarded after verification. Do not replace your original image during an experiment.

Common traps

If the input cannot be opened, check its path and permissions as the same user who will run the conversion. If the converter reports an invalid PBM, go back to the format check in step 2: a damaged header, a truncated raster or a file in another format needs repairing or converting before pbmtomgr can use it.

Do not add flags such as width, height, colour or output format: this installed command has no pbmtomgr-specific options. Netpbm's shared options include -quiet, which suppresses informational messages, and -version. Neither changes the MGR format or fixes a bad input; use pbmtomgr -quiet input.pbm > output.mgr only when you specifically want a quiet standard error stream.

Nothing in this workflow needs sudo unless the input or output directory is deliberately restricted. Running the converter as root will not improve the image and can leave root-owned output you cannot conveniently replace later.