Convert Images to PFM with pamtopfm

pamtopfm turns a Netpbm image into a PFM file, the floating-point format built for light intensity data rather than plain display colour. You will pick a byte order and scale factor, inspect the header you get, then convert back to PAM with pfmtopam as a round-trip check.

Allow about fifteen minutes. You need pamtopfm, pfmtopam, a shell, and a readable PNM or PAM image. The examples use Netpbm 11.5.2 from the installed netpbm package. The workflow is unprivileged: do not use sudo unless your input or output directory has deliberately restricted permissions.

1. Check the installed converters

Confirm which binaries will run before you build a script around them:

$ command -v pamtopfm
/usr/bin/pamtopfm
$ command -v pfmtopam
/usr/bin/pfmtopam
$ pamtopfm --version
pamtopfm: Using libnetpbm from Netpbm Version: Netpbm 11.5.2

The local manpages describe the format and the converter options. PFM looks similar to PPM, but its samples are 32-bit IEEE floating-point values rather than integers with a declared maximum. That makes it suited to data such as generic light intensity, not just display-ready colour.

2. Create a small test image

Use a tiny grayscale PGM so the conversion has a known input and does not overwrite anything useful:

printf 'P2\n2 1\n255\n0 255\n' > /tmp/pfm-input.pgm

This is a two-pixel image, one black sample and one white sample. The temporary path keeps the example separate from your working files. For a real conversion, replace the input path with an existing PNM or PAM file and keep the original unchanged.

Verify the input before converting it:

$ file /tmp/pfm-input.pgm
/tmp/pfm-input.pgm: Netpbm image data, size 2 x 1, ASCII, grayscale

3. Convert the image to PFM

Write the output to a new name. This example requests little-endian samples and a scale factor of 2:

$ pamtopfm -endian=little -scale=2 /tmp/pfm-input.pgm > /tmp/pfm-output.pfm
$ file /tmp/pfm-output.pfm
/tmp/pfm-output.pfm: data

• Redirection creates the file. pamtopfm writes PFM to standard output; the shell handles the rest.
• Colour follows the source. RGB input produces colour PFM; anything else, like the grayscale file above, produces non-colour PFM.
• Defaults are implicit. Scale defaults to 1.0 and byte order defaults to the machine's native order, convenient locally but less explicit for an interchange file. Set both options when another program or machine must consume the result.

The scale is metadata for interpreting sample units. It is not a request to brighten, resize, or otherwise display-adjust the image.

cp --preserve=all /path/to/output.pfm /path/to/output.pfm.bak
pamtopfm -endian=little -scale=2 /path/to/input.pgm > /path/to/output.pfm.new
mv /path/to/output.pfm.new /path/to/output.pfm

4. Read the PFM header

A PFM file begins with three ASCII fields, followed immediately by binary raster data. Inspect only the header, not the whole file:

$ od -An -tx1 -c -N 32 /tmp/pfm-output.pfm
  50  66  0a  32  20  31  0a  2d  32  2e  30  30  30  30  30  30
   P   f  \n   2       1  \n   -   2   .   0   0   0   0   0   0

• Pf means grayscale, PF means colour. That is the first field.
• The next line is width and height. Here, 2 1.
• The sign of the scale line is the byte order. Negative means little-endian raster data; positive means big-endian. Its absolute value is the scale factor, here 2.0.

Do not treat the raster as text. Each grayscale pixel has one four-byte sample; each colour pixel has three. Rows are stored bottom to top, and pixels within a row run left to right. That bottom-to-top order is an easy source of an apparently upside-down image when a consumer assumes ordinary top-to-bottom storage.

5. Round-trip through PAM

Use pfmtopam to check that the header can be read and that the converter recognises the format:

$ pfmtopam -verbose /tmp/pfm-output.pfm > /tmp/pfm-roundtrip.pam
pfmtopam: width: 2, height: 1
pfmtopam: color: NO
pfmtopam: endian: LITTLE
pfmtopam: scale factor: 2.000000
$ sed -n '1,7p' /tmp/pfm-roundtrip.pam
P7
WIDTH 2
HEIGHT 1
DEPTH 1
MAXVAL 255
TUPLTYPE GRAYSCALE
ENDHDR

The PAM header confirms the dimensions, grayscale depth, byte order and scale. The pixel bytes after ENDHDR are binary, so unreadable characters after the header are expected. The default PAM maximum value is 255; choose another with -maxval only when the receiving workflow requires it.

For a production conversion, compare dimensions and colour mode with the source as well as checking that pfmtopam exits successfully. A successful parse does not prove that an unrelated application interprets the scale units or row order as you intend.

Common traps

• PFM is not an official Netpbm format. The local manual describes the form understood by these two converters, based on behaviour deduced from an earlier program. Test any external reader with a small known image before exchanging valuable data.
• No units are built in. PFM has no extra delimiter or trailer around the raster, and no built-in unit such as watts per square metre. The scale only tells a reader how sample values are scaled; separate domain knowledge still gives those values meaning.
• Keep your notes with the file. Record the scale factor, endianness, dimensions and colour mode alongside your processing notes.