Blend Two PPM Images Predictably with ppmmix

You will blend two matching PPM images with Netpbm's ppmmix, save the result without destroying an existing file, and check that the inputs meet the command's restrictions. The examples use Netpbm 11.5.2, installed here as Debian package netpbm version 2:11.05.02-1.1build1.

Allow about fifteen minutes. You need a shell, two readable PPM files, and enough disk space for a new output. This is an ordinary image conversion workflow: it does not need sudo, does not alter the input images, and does not change system configuration.

1. Check the installed command

Confirm which executable will run and record its Netpbm version:

$ command -v ppmmix
/usr/bin/ppmmix
$ ppmmix --version
ppmmix: Using libnetpbm from Netpbm Version: Netpbm 11.5.2
...

The version output contains build details as well as the useful library version, so the exact trailing lines can differ. The installed manual describes the command as ppmmix fadefactor ppmfile1 ppmfile2. There are no ppmmix-specific long options. The command does recognise options common to Netpbm programs, but none is needed for this workflow.

2. Check the two input images

ppmmix requires both images to have the same width, height and maximum colour value, usually called maxval in PPM documentation. In the installed Netpbm release, images of different PPM types can be mixed; older Netpbm releases before 10.54 also required the types to match.

Use a read-only inspection first:

$ file /path/to/first.ppm /path/to/second.ppm
/path/to/first.ppm: PPM image data, size 1920 x 1080, rawbits, pixmap
/path/to/second.ppm: PPM image data, size 1920 x 1080, rawbits, pixmap

Your file description may use different wording. The useful part is that both dimensions agree. If either file is missing, unreadable or not identified as a PPM, fix that input problem before testing the blend. Do not assume that two files with similar names have compatible headers.

For a more exact header check, inspect the beginning of each file. Do not use a text editor on a raw PPM because the pixel data can be binary:

$ head -c 64 /path/to/first.ppm
P6
1920 1080
255
...

The magic number is commonly P6 for a binary PPM or P3 for an ASCII PPM. The dimensions and maxval are in the header, although comments can appear there, so avoid parsing a production file with a one-line text trick unless you understand the PPM grammar.

3. Choose a fade factor

The fade factor is a decimal from 0.0 to 1.0. At 0.0, the result contains only the first image. At 1.0, it contains only the second. A value such as 0.5 gives an even blend between them.

The calculation is applied to perceived brightness, not physical light intensity. That is useful for a simple visual transition, but it is not a colour-managed or physically linear compositing operation. If you need different image sizes, a transparency mask, or intensity-based compositing, inspect pamcomp instead of trying to force those requirements into ppmmix.

4. Write a new blended image

Redirect standard output to a new filename. This example gives the second image 35 percent of the blend:

$ ppmmix 0.35 /path/to/first.ppm /path/to/second.ppm > blended.ppm
$ printf 'exit status: %s\n' "$?"
exit status: 0

There is normally no progress message: the image is written to standard output. A zero exit status means the command completed and produced output. It does not prove that the images were the right pair or that the visual result is suitable, so inspect the output before using it downstream.

On this Netpbm 11.5.2 installation, the output is a raw P6 PPM even when the small test inputs are ASCII P3 files:

$ head -c 16 blended.ppm
P6
1920 1080
255
...

Do not put an existing valuable output path after > casually. Shell redirection truncates that file before ppmmix starts. Use a new name, or create a backup and replace the destination only after checking the new file:

$ cp --preserve=all blended.ppm blended.ppm.bak
$ ppmmix 0.35 /path/to/first.ppm /path/to/second.ppm > blended.ppm.new
$ file blended.ppm.new
$ mv blended.ppm.new blended.ppm

The mv is the state-changing step. If conversion fails, leave the original in place and remove the incomplete blended.ppm.new after checking that no process is still writing it. The backup is your recovery copy; keep it until the replacement has passed a visual check. Deleting the backup is irreversible.

5. Diagnose rejected inputs

A dimensions mismatch is a normal input error. Test it safely by writing diagnostics to the terminal or a temporary file, not over a useful image:

$ ppmmix 0.5 /path/to/first.ppm /path/to/different-size.ppm > /tmp/ppmmix-test.ppm
ppmmix: image sizes are different
$ printf 'exit status: %s\n' "$?"
exit status: 1

Other header differences, including different maxval values, can also make the inputs incompatible. Convert or normalise both images with a suitable Netpbm tool, then check the resulting headers again. Keep those conversions in new files until the results are verified.

An invalid factor fails before a useful image is produced:

$ ppmmix 1.5 /path/to/first.ppm /path/to/second.ppm > /tmp/ppmmix-test.ppm
ppmmix: fade factor must be in the range from 0.0 to 1.0
$ printf 'exit status: %s\n' "$?"
exit status: 1

Also check the shell's exit status immediately. A later file, mv or inspection command can hide the status that matters. If output redirection created an incomplete temporary file, do not promote it into place.