ppmtojpeg

View documentation for the current command

$ tldr pnmtojpeg

ppmtojpeg [options] [netpbmfile]

-quality N
    Sets the JPEG compression quality, where N is an integer from 0 (lowest quality, smallest file) to 100 (highest quality, largest file). The default is usually 75.

-grayscale
    Converts the input image to grayscale before JPEG encoding, even if the input is a color PPM file. The output JPEG will be grayscale.

-optimize
    Performs entropy coding optimization (optimizes Huffman tables). This typically results in a slightly smaller JPEG file size but requires more processing time during compression.

-progressive
    Creates a progressive JPEG file. Progressive JPEGs display a low-resolution version of the image first and gradually improve quality as more data is loaded, which is useful for web display.

-comment "text"
    Adds a comment string to the JPEG header. The comment must be enclosed in quotes.

-density N
    Sets the horizontal and vertical density (in DPI) to N in the JPEG header. This information is used by some display programs for scaling.

-icc FILE
    Embeds the International Color Consortium (ICC) color profile from FILE into the JPEG header. This helps maintain color accuracy across different devices.

-strict
    Treats all warnings as fatal errors. The command will exit immediately if any warning occurs during processing.

-smooth N
    Applies a smoothing filter to the input image before compression. N is a value from 1 to 100, where higher values indicate more smoothing.

-sample HxV
    Controls chrominance subsampling. H and V specify the horizontal and vertical sampling factors, e.g., 2x1, 1x1. Common options are 2x2 (standard, 4:2:0) or 1x1 (no subsampling, larger file).

-restart N
    Inserts restart markers into the JPEG stream every N Minimum Coded Units (MCUs). This can help with error recovery in corrupted files.

-arithmetic
    Uses arithmetic coding instead of Huffman coding. While it can produce slightly smaller files, arithmetic coding is less widely supported by JPEG decoders and patents may apply.

-baseline
    Ensures the output JPEG is a baseline JPEG, which is the most widely compatible JPEG format.

-verbose
    Prints detailed information about the compression process to standard error.

-qtable FILE
    Uses custom quantization tables defined in FILE. This allows for very specific control over compression.

-quant-table N
    Uses one of the predefined quantization tables (e.g., 0 for default, 1 for less aggressive, 2 for more aggressive, etc.).

The ppmtojpeg command, a utility from the Netpbm toolkit, is designed to convert images in the Netpbm formats – Portable Pixmap (PPM), Portable Graymap (PGM), or Portable Bitmap (PBM) – into the widely used JPEG image format.

It functions by reading image data from a specified input file or, by default, from standard input. The converted JPEG data is then written to standard output or a specified output file. This versatile tool offers extensive control over the JPEG compression process, allowing users to adjust parameters such as compression quality, progressive encoding, color handling, and more. It is particularly valuable for scripting image conversion tasks within Linux and Unix-like environments.

Using the -arithmetic option may result in smaller files but can lead to compatibility issues as not all JPEG decoders support arithmetic coding. For maximum compatibility, ensure the -baseline option is implicitly or explicitly used. Choosing the optimal -quality setting often requires experimentation to balance file size and visual fidelity.

By default, ppmtojpeg reads image data from standard input (stdin). If an input netpbmfile is provided as an argument, it reads from that file instead. The resulting JPEG data is written to standard output (stdout) by default, which can be redirected to a file using shell redirection (e.g., ppmtojpeg input.ppm > output.jpg).

The command offers fine-grained control over JPEG compression parameters. The -quality option is the most frequently used, allowing a straightforward trade-off between file size and visual fidelity; higher quality values retain more detail but result in larger files. Other options like -optimize, -progressive, and -sample allow for further optimization and the creation of specific JPEG features tailored to different use cases, such as web delivery or archival.

The ppmtojpeg utility is an integral part of the Netpbm image manipulation toolkit, which traces its origins back to the Pbmplus package developed in the late 1980s. Netpbm serves as a foundational suite of tools for various image conversions and manipulations in Unix-like systems. Specifically, ppmtojpeg leverages the widely used libjpeg library for its JPEG encoding capabilities, providing robust and industry-standard compression. Its continuous development is intertwined with the evolution of the Netpbm project, ensuring its functionality and compatibility with modern image processing requirements and standards.

jpegtopnm(1), pnm(5), netpbm(1), cjpeg(1), djpeg(1)