Convert a PPM Image to Macintosh PICT with ppmtopict

You will convert a PPM image into a Macintosh PICT data fork, check that the output is non-empty, and keep the original files recoverable if the conversion fails. The examples use Netpbm 11.5.2, the version installed on this machine. Allow about ten minutes for a single image.

You need the netpbm package, a readable PPM input, and a directory where you can create the output. This is an ordinary user workflow. It should not need sudo; elevated privileges are only relevant if your input or destination permissions already require them.

1. Check the installed command

Confirm that the shell will run the Netpbm program you expect and record its package version:

$ command -v ppmtopict
/usr/bin/ppmtopict
$ dpkg-query -W -f='${Package} ${Version}\n' netpbm
netpbm 2:11.05.02-1.1build1
$ ppmtopict --version
ppmtopict: Using libnetpbm from Netpbm Version: Netpbm 11.5.2
ppmtopict: Built from source dated 2024-03-31 09:09:47

The version output is diagnostic text rather than a conversion option. The manual page for this installed package documents one optional operand, ppmfile, and no ppmtopict-specific options. With no file operand, the program reads the PPM image from standard input.

2. Inspect the PPM before converting it

Check the input path without changing it:

$ test -r /path/to/input.ppm && echo 'input is readable'
input is readable
$ file /path/to/input.ppm
/path/to/input.ppm: Netpbm image data, size = 800 x 600, pixmap

The exact file wording depends on the file utility and whether the PPM is plain text or binary. You are checking that the file exists and is identified as a Netpbm image, not trying to prove that every pixel is correct.

The installed converter rejects an image narrower than eight pixels. That boundary is easy to hit with a tiny test image, so use a real image or check its dimensions before starting. A successful command still depends on the PPM being structurally valid.

3. Write a PICT file without destroying an existing one

Redirection sends ppmtopict's standard output to a file. Because > truncates an existing destination before the program starts, convert to a new temporary name first:

$ ppmtopict /path/to/input.ppm > /path/to/output.pict.new
ppmtopict: computing colormap...
ppmtopict: 247 colors found
$ test -s /path/to/output.pict.new
$ mv /path/to/output.pict.new /path/to/output.pict

The colour count is informational and varies with the image. The useful success signal is the command's exit status, followed by the non-empty-file check. The final mv changes the destination directory, so only run it when the new file is the replacement you intend. If conversion fails, the .new file may be incomplete: leave the old output alone and remove the failed temporary file using your normal file-management tool.

If output.pict already exists, pause before the final move. Rename the old file to a dated backup or choose another destination. Do not make an irreversible deletion merely to tidy up a test.

4. Confirm the output and its format boundary

PICT is not a text format, so inspect it with file-size and byte-oriented tools rather than a text editor:

$ wc -c /path/to/output.pict
652 /path/to/output.pict
$ test -s /path/to/output.pict && echo 'PICT data fork is non-empty'
PICT data fork is non-empty

Your byte count will differ. This check confirms that output was written, not that the image looks correct. Open it with a Macintosh-compatible PICT reader or pass it to a tool that explicitly supports PICT. Do not rename the file and assume that a different extension converts its contents.

For a repeatable smoke test, the converter can read standard input. These two commands should produce byte-for-byte identical output when they receive the same PPM:

$ ppmtopict /path/to/input.ppm > /tmp/from-file.pict
$ cat /path/to/input.ppm | ppmtopict > /tmp/from-stdin.pict
$ cmp /tmp/from-file.pict /tmp/from-stdin.pict
$ echo $?
0

The temporary files are ordinary data and can be removed after the comparison. Keep the source PPM until the PICT has been checked by the software that will consume it.

5. Handle the Macintosh file-format limitation

ppmtopict writes only the data fork of a picture. The manual warns that this is not, by itself, a complete MacBinary or BinHex-wrapped file with the metadata needed by classic Mac OS to identify a PICT file. If you need that packaging, use a suitable tool such as mcvert after checking that it is installed and that its input and output conventions match your target system.

Do not treat a successful ppmtopict exit status as proof that an old Macintosh workflow will accept the result. First verify the PICT data, then test the packaging step separately. Keeping those stages separate makes it clear whether a failure is in image conversion or file wrapping.

6. Diagnose a failed conversion

If ppmtopict reports that it cannot open the input, check the exact path and read permission:

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

If it reports that the input is too narrow, use an image at least eight pixels wide. If it reports malformed PPM data, regenerate or repair the PPM with the tool that created it rather than guessing at binary bytes. If no output is produced, check the command status immediately and do not move the temporary file into place.

There is no persistent configuration to undo in this workflow. A failed conversion affects only the newly redirected file. If you used the temporary-name pattern, the existing PICT remains available until you deliberately replace it.