pjtoppm

Convert a HP PaintJet file to PPM

$ pjtoppm [path/to/input.pj] > [path/to/output.ppm]

pjtoppm [pjpegfile]

pjtoppm is a specialized utility from the Netpbm project, a comprehensive toolkit for image format conversion and manipulation. Its core function is to convert a PJPEG (Progressive JPEG) image file into a PPM (Portable Pixmap) format image file. Unlike traditional baseline JPEGs, Progressive JPEGs store image data in multiple scans, displaying a rough image quickly and then progressively refining its quality.

The PPM format is a simple, uncompressed representation of pixel data, making it an ideal intermediate format for subsequent processing by other Netpbm tools or custom scripts. pjtoppm typically reads the PJPEG data from standard input (or an optionally specified file) and writes the resulting PPM data to standard output. This design makes it highly suitable for use in Unix-style pipelines.

While more general converters like jpegtopnm exist, pjtoppm offers a focused conversion for progressive JPEGs, fitting seamlessly into the Netpbm philosophy of small, single-purpose tools that can be combined to achieve complex operations.

pjtoppm is designed specifically for Progressive JPEG files. Attempting to convert a baseline JPEG or a malformed file may result in errors or unexpected output. As a Netpbm tool, it assumes a pipeline-oriented workflow, meaning error messages are usually written to standard error and output to standard output.

pjtoppm heavily relies on standard input and output, making it highly suitable for chaining with other commands in a Unix shell. For example, to convert a progressive JPEG and then resize it, you might use:
cat image.pjp | pjtoppm | pnmscale 0.5 | pnmtopng > output.png

As part of the Netpbm suite, pjtoppm produces a PPM (Portable Pixmap) output, which is a common intermediate format for many Netpbm utilities. This allows for seamless transitions between various image processing steps within the Netpbm framework.

pjtoppm is a component of the venerable Netpbm toolkit, which traces its origins back to the pbmplus package from the late 1980s. Netpbm's development has always emphasized modularity and the Unix philosophy of small, single-purpose tools that do one thing well. pjtoppm likely emerged to specifically address the conversion of Progressive JPEG formats, which became more common over time, into Netpbm's native intermediate formats. Its creation reflects the toolkit's ongoing evolution to support various image standards and facilitate interoperation within the Netpbm ecosystem.

jpegtopnm(1), ppmtojpeg(1), pnm(5), netpbm(1)