Convert Netpbm Image Streams to Garmin SRF with pamtosrf

Need a Garmin vehicle bitmap and all you have is a stack of Netpbm images? pamtosrf reads a Netpbm image stream and writes SRF data to standard output, leaving your source images alone. Redirection is the entire interface, so if your input stream already exists, budget about ten minutes.

You need Linux, the netpbm package, and a readable PAM, PBM, PGM, or PPM input stream. The examples below use Netpbm 11.05.02, installed as package version 2:11.05.02-1.1build1 on this machine. Its manual page is dated 27 May 2011, so check the installed command whenever exact behaviour matters on a different release.

1. Check the installed command

Confirm which binary will run and ask it for local help, none of this needs elevated privileges:

$ command -v pamtosrf
/usr/bin/pamtosrf
$ pamtosrf --help
pamtosrf: Use 'man pamtosrf' for help.

The documented syntax is pamtosrf [-verbose] [netpbmfile], one input-file argument at most. Leave it out and the program reads standard input instead. Output is always standard output, there is no output-file argument to add, no matter how tempting it looks.

Checkpoint: make sure the binary command -v found is the one you actually mean to run, and keep the input path clearly separate from the output path.

2. Convert one Netpbm file

Swap in a real image file and redirect the SRF stream to a new destination:

$ pamtosrf /path/to/input.pam > vehicle.srf
$ printf 'exit status: %s\n' "$?"
exit status: 0

Status 0 just means the converter finished. It says nothing about whether the image has the dimensions or layout the consuming program actually needs. Keep the original input around until the result has been checked.

SRF is commonly associated with Garmin vehicle information files, but the converter itself accepts any image stream without enforcing a Garmin vehicle layout. That means a file can be syntactically valid SRF while still being useless to a particular receiver or application.

3. Inspect the result before using it

Use a file-identification tool and, if it is installed, the reverse converter. Both are ordinary, unprivileged reads:

$ file vehicle.srf
vehicle.srf: Garmin Bitmap file, version 1.00, 2x1
$ srftopam vehicle.srf > roundtrip.pam
$ printf 'round-trip status: %s\n' "$?"
round-trip status: 0
$ pamfile roundtrip.pam
roundtrip.pam: PAM, 2 by 1 by 4 maxval 255
    Tuple type: RGB_ALPHA

Your own dimensions and PAM tuple type will differ. What actually matters: a zero exit status, a non-empty destination, and an SRF identification consistent with the image you fed in. A successful reverse conversion is a format check, nothing more, it says nothing about visual correctness or device compatibility.

Not keeping the output? Remove only the file you created, and only after checking its path, that deletion is irreversible. Never remove the source image as part of a cleanup pass.

4. Convert a stream from standard input

Standard input earns its keep when another Netpbm command is already producing the image stream. For a file that already exists, this is equivalent to naming the path directly:

$ cat /path/to/input.pam | pamtosrf > vehicle.srf
$ printf 'exit status: %s\n' "$?"
exit status: 0

Prefer a direct pipeline from the producing command wherever you can:

$ pamcat -lr /path/to/view-a.pam /path/to/view-b.pam | pamtosrf > vehicle.srf

The pipe hands pamtosrf exactly one Netpbm stream. Do not assume that concatenating arbitrary binary files makes a valid multi-image stream, each image has to be genuinely valid Netpbm, and the producer defines the order they appear in.

5. Preserve multiple images deliberately

An SRF file can hold several images, and each image in a multi-image Netpbm input stream becomes one image in the SRF output. For a Garmin vehicle information file specifically, the manual describes two: one for 3D oblique views and one for overhead views, each a horizontal concatenation of 36 square views rotated 10 degrees apart.

That layout is a requirement of the Garmin format, not something pamtosrf does for you automatically. Build the views with other Netpbm tools, then hand the resulting stream to pamtosrf. pamcat -lr makes the horizontal concatenation, and pnmrotate creates the rotated views. Check dimensions and ordering before you convert.

Checkpoint: expecting two images in your input? Run with diagnostics on and count the conversion messages:

$ pamtosrf -verbose /path/to/two-image-stream.pam > vehicle.srf
pamtosrf: Converting Image 0
pamtosrf: reading 2x1 image
pamtosrf: Converting Image 1
pamtosrf: reading 1x1 image

Dimensions in the messages will be yours, and image numbering starts at zero. These go to standard error, so they never corrupt the SRF bytes written to standard output. Reach for -verbose when checking image counts or diagnosing a stream, then drop it in a quiet script once you no longer need the noise.

6. Avoid truncating a useful SRF file

Shell > truncates an existing destination before pamtosrf ever runs, so if the conversion then fails, the old file is already gone. Use a temporary destination and only replace the old file once you have validated the new one:

$ umask 077
$ pamtosrf -verbose /path/to/input.pam > vehicle.srf.new
$ srftopam vehicle.srf.new > /tmp/vehicle-check.pam
$ pamfile /tmp/vehicle-check.pam
$ mv -i vehicle.srf.new vehicle.srf

The -i prompt protects an existing vehicle.srf, answer n if the names or validation result are not what you expect. On failure, inspect the diagnostic and remove only the incomplete vehicle.srf.new, the original destination stays exactly where it was. The temporary PAM holds real image data, so remove it too if it is sensitive:

$ rm -- /tmp/vehicle-check.pam

Do not reach for sudo here for conversion or validation, unless the input directory genuinely denies your normal account access. Running as root fixes neither a malformed Netpbm stream nor an SRF layout that is simply wrong for a device.

7. Diagnose the common failures

An error about the first byte of a Netpbm magic number usually means the input is empty or is not Netpbm data at all. Check path, readability and file type without modifying anything:

$ test -r /path/to/input.pam && echo readable
readable
$ file /path/to/input.pam
$ wc -c /path/to/input.pam

Empty file? Regenerate whatever produces it. file reports another format entirely? Convert it to Netpbm first. Do not feed a PNG, JPEG or an existing SRF file to pamtosrf just because it has an image extension.

If srftopam rejects the output, keep the pamtosrf diagnostic and check that redirection did not accidentally capture an error message from a shell wrapper. With -verbose, normal diagnostics go to standard error and the output file should contain only the SRF stream. If dimensions or view order are wrong, fix the step that generated the input, pamtosrf will not resize or rearrange images for you.