Convert Netpbm Image Streams to Garmin SRF with pamtosrf

You will finish with an SRF file made from one or more Netpbm images, without changing the source images. The command reads a Netpbm image stream and writes the SRF data to standard output, so shell redirection is part of the normal workflow. Allow about ten minutes if your input stream already exists.

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. The manual page is dated 27 May 2011, so check the installed command when behaviour matters on a different release.

1. Check the installed command

Confirm which executable will run and ask it for its local help. This does not need elevated privileges:

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

The documented syntax is pamtosrf [-verbose] [netpbmfile]. There is at most one input-file argument. If you omit it, the program reads standard input. The output is always standard output; there is no output-file argument to add.

Checkpoint: make sure the file named by command -v is the one you intend to use, and keep the input path separate from the output path.

2. Convert one Netpbm file

Replace the placeholder with an existing 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 means the converter completed. It does not prove that the image has the dimensions or layout required by the program that will consume the SRF file. Keep the original input until the result has been checked.

SRF is commonly associated with Garmin vehicle information files. The converter itself accepts image streams without enforcing a Garmin vehicle layout, so an output file can be syntactically valid while still being unsuitable for a particular receiver or application.

3. Inspect the result before using it

Use a file-identification tool and, when available, the reverse converter. These checks 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 dimensions and PAM tuple type will depend on the input. The useful checks are a zero exit status, a non-empty destination, and an SRF identification consistent with the image you supplied. A successful reverse conversion is a format check, not a visual or device-compatibility check.

If the output is not needed, remove only the file you created after checking its path. That deletion is irreversible. Do not remove the source image as part of cleanup.

4. Convert a stream from standard input

Standard input is useful when another Netpbm command produces the image stream. For a file that already exists, this is equivalent to giving the file path:

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

Prefer a direct pipeline from the producing command when possible, for example:

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

The pipe passes one Netpbm stream to pamtosrf. Do not assume that concatenating arbitrary binary files creates a valid multi-image stream; each image must be a valid Netpbm image, and the producer should define the intended order.

5. Preserve multiple images deliberately

An SRF file can contain multiple images. Each image in a multi-image Netpbm input stream becomes one image in the SRF output. For a Garmin vehicle information file, the manual describes two images: one for 3D oblique views and one for overhead views. It also describes each image as a horizontal concatenation of 36 square views, each rotated by 10 degrees.

That layout is a requirement of the consuming Garmin format, not an automatic transformation performed by pamtosrf. Prepare the views with other Netpbm tools, then pass the resulting stream to pamtosrf. For example, pamcat -lr can make a horizontal concatenation, while pnmrotate can create rotated views. Verify the dimensions and ordering before conversion.

Checkpoint: if your input should contain two images, run with diagnostics enabled 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

The exact dimensions in the messages will be yours. The image numbers start at zero. Informational messages go to standard error, so they do not corrupt the SRF bytes written to standard output. Use -verbose when checking image count or diagnosing an input stream, then omit it in a quiet script if you do not need the messages.

6. Avoid truncating a useful SRF file

Shell > truncates an existing destination before pamtosrf runs. If conversion then fails, the old file is already gone. Use a temporary destination and replace the old file only after validation:

$ 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. If conversion fails, inspect the diagnostic and remove only the incomplete vehicle.srf.new; the original destination remains in place. The temporary PAM contains image data, so remove it after checking if it is sensitive:

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

Do not use sudo for conversion or validation unless the input directory genuinely denies your normal account access. Running as root does not repair a malformed Netpbm stream or make an SRF layout compatible with 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 a Netpbm image. Check the path, readability and file type without modifying it:

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

If the file is empty, regenerate the producer's output. If file identifies another format, convert it to a Netpbm format first. Do not feed a PNG, JPEG or an existing SRF file to pamtosrf merely because it has an image extension.

If srftopam rejects the output, retain the pamtosrf diagnostic and check that the redirection did not capture an error message from a shell wrapper. With -verbose, normal diagnostics belong on standard error; the output file should contain only the SRF stream. If the dimensions or view order are wrong, fix the input-generation step rather than expecting pamtosrf to resize or rearrange images.