Convert a PCX File to PPM with pcxtoppm

You will finish with a PCX image converted to a PPM file, a check that the output has the dimensions you expected, and a safe way to diagnose palette or input errors. The examples match the installed Netpbm 11.5.2 command from package version 2:11.05.02-1.1build1.

Allow about ten minutes. You need a readable PCX file, a shell, and enough free space for the uncompressed PPM output. The conversion itself normally needs no elevated privileges. Do not use sudo unless ordinary file permissions genuinely prevent access.

1. Check the installed command

Confirm which executable will run and record the local Netpbm version. These are read-only checks:

$ command -v pcxtoppm
/usr/bin/pcxtoppm
$ pcxtoppm -version 2>&1 | head -4
pcxtoppm: Using libnetpbm from Netpbm Version: Netpbm 11.5.2
pcxtoppm: Built from source dated 2024-03-31 09:09:47
pcxtoppm: Built by Debian

The command takes an optional PCX filename and writes the PPM image to standard output. That output stream is the main detail to keep in mind: a successful command does not create a file unless you redirect it.

2. Check the input before converting

Inspect the file without changing it. Replace the example path with the path to your own image:

$ INPUT='/path/to/image.pcx'
$ test -r "$INPUT" && echo 'input is readable'
input is readable
$ file "$INPUT"
/path/to/image.pcx: PCX ... image data ...

The exact file description varies with the PCX variant. pcxtoppm supports colormapped files with 2 to 16 colours, 256-colour files, 24-bit truecolour files, and 32-bit files with an intensity plane. A file can still be damaged even when its name and initial type look plausible, so keep the original until the converted image has been checked.

If the test produces no output, the file is not readable by your current user. Fix the path or permissions deliberately. Do not jump to root access as a diagnostic shortcut.

3. Convert to a new PPM path

Choose a destination that does not contain useful work. The shell redirection operator truncates an existing destination before pcxtoppm starts, so this example uses a new filename:

$ OUTPUT='/path/to/image.ppm'
$ pcxtoppm "$INPUT" > "$OUTPUT"
$ printf 'exit status: %s\n' "$?"
exit status: 0

Exit status 0 means the converter completed successfully. It does not guarantee that the image is visually correct. If the command fails, the redirected file may be empty or incomplete. Remove or rename that failed output only after checking that it is the new file, not your original PCX or a previous good PPM.

There is no persistent configuration to undo. To discard a conversion you no longer need, use an explicit path check before deleting the generated PPM. Keep the PCX source untouched.

4. Verify the PPM header and dimensions

Use file or Netpbm's pamfile to inspect the result:

$ file "$OUTPUT"
/path/to/image.ppm: Netpbm image data, size 640 x 480, rawbits, pixmap
$ pamfile "$OUTPUT"
/path/to/image.ppm: PPM raw, 640 by 480  maxval 255

Your wording may differ. Look for a PPM result, non-zero dimensions, and dimensions that match the PCX. A raw PPM normally begins with the magic number P6 followed by its width, height and maximum channel value:

$ head -3 "$OUTPUT"
P6
640 480
255

Only the header is text. The remaining pixel data is binary, so do not open the complete file in a text editor or rely on a full cat to inspect it.

5. Use diagnostics when the result is unexpected

Pass -verbose when you need to see the PCX header details that pcxtoppm decoded. Keep the image output redirected separately because diagnostic text is reported on the terminal:

$ pcxtoppm -verbose "$INPUT" > "$OUTPUT"
pcxtoppm: Version: 3
pcxtoppm: BitsPerPixel: 8
pcxtoppm: Planes: 1    BytesPerLine: 640    PaletteInfo: 1

The values depend on the file. This mode is useful for distinguishing a 16-colour packed or bitplane image from a 256-colour or truecolour image. If the header describes a different format from the one you expected, inspect the original file and the program that produced it rather than forcing a guessed option.

pcxtoppm has no resize option. Its output dimensions come from the PCX image bounds. A wrong size usually points to malformed input or an input that is not a PCX format this version recognises.

6. Handle a misleading 16-colour palette

The -stdpalette option makes pcxtoppm use its predefined standard palette even when a 16-colour PCX supplies its own palette. This is specifically useful when an old or limited producer wrote a random palette into the file rather than a meaningful one:

$ pcxtoppm -stdpalette "$INPUT" > /path/to/image-standard-palette.ppm
$ pamfile /path/to/image-standard-palette.ppm
/path/to/image-standard-palette.ppm: PPM raw, 640 by 480  maxval 255

Do not add this option by habit. It is meaningful only for the 16-colour paletted format. For a valid file with a deliberate custom palette, replacing that palette changes the colours and may make the result less accurate. Compare the normal and standard-palette outputs with an image viewer when the source's palette is in doubt.

7. Convert a batch without destroying earlier output

For multiple files, write each result to a temporary name in the same directory, verify it, then rename it into place. This keeps a previous PPM available if a conversion fails:

$ pcxtoppm /path/to/image.pcx > /path/to/image.ppm.new
$ pamfile /path/to/image.ppm.new
/path/to/image.ppm.new: PPM raw, 640 by 480  maxval 255
$ mv /path/to/image.ppm.new /path/to/image.ppm

The final mv replaces an existing destination on the same filesystem. Treat that replacement as deliberate and check the two paths before running it. If validation fails, leave the old output in place and remove the specifically named .new file during your normal cleanup. Do not use a broad wildcard in a cleanup command.