jpegtopnm

Convert JPEG/JFIF image to a PPM or PGM image

$ jpegtopnm [path/to/file.jpg] > [path/to/file.pnm]

$ jpegtopnm [[-v|-version]]

jpegtopnm [-verbose] [-plain] [-grayscale] [-rgb] [-scale N] [-maxmemory M] [-dct method] [-no-block-smoothing] [-icc] [jpegfile]

-verbose
    Displays informational messages about the decoding process, including image dimensions, color space, and memory usage.

-plain
    Outputs the PNM image in plain (ASCII) text format rather than the default raw (binary) format. Plain format files are larger but human-readable.

-grayscale
    Converts the output image to grayscale, producing a PGM (Portable Graymap) file even if the input JPEG was color.

-rgb
    Forces the output to be a color PPM (Portable Pixmap) in RGB format, even if the input JPEG was a grayscale image.

-scale N
    Scales the output image down by a factor of 1/N. Valid values for N are 1, 2, 4, or 8, resulting in 100%, 50%, 25%, or 12.5% size respectively. Useful for quick previews or processing large images.

-maxmemory M
    Limits the maximum amount of memory (in kilobytes) that the command will use for decompression buffers. Useful for preventing excessive memory consumption on systems with limited RAM.

-dct method
    Specifies the Discrete Cosine Transform (DCT) method to use for decompression. method can be 'fast' (fast integer), 'accurate' (accurate integer), or 'float' (floating-point, most accurate but slowest).

-no-block-smoothing
    Disables the block-smoothing post-processing step during decompression. This can sometimes result in sharper images but may show more block artifacts.

-icc
    Reads and processes any embedded ICC color profile information present in the JPEG file. This can help ensure accurate color reproduction.

jpegtopnm is a utility from the Netpbm suite that converts JPEG image files into one of the Netpbm portable pixmap formats: PBM (Portable Bitmap), PGM (Portable Graymap), or PPM (Portable Pixmap). This conversion is crucial for interoperability, allowing JPEG images to be processed by the extensive collection of Netpbm tools, which operate exclusively on the PNM family of formats.

The command reads a JPEG image from a specified file or standard input and writes the corresponding PNM data to standard output. This design makes jpegtopnm highly suitable for piping operations in shell scripts, enabling complex image manipulation workflows. It handles various JPEG subformats and supports options for scaling, grayscale conversion, and controlling the decompression algorithm for optimal balance between speed and accuracy.

jpegtopnm is designed specifically for converting JPEG to PNM. It does not perform encoding or advanced image editing (like cropping, resizing beyond simple scaling, or applying filters) directly; these operations are typically handled by other Netpbm utilities after conversion to PNM. Since JPEG is a lossy compression format, repeated cycles of JPEG encoding and decoding (e.g., JPEG -> PNM -> JPEG) can lead to cumulative quality degradation.

By default, jpegtopnm reads a JPEG image from standard input (stdin) if no jpegfile argument is provided. It writes the resulting PNM data (PPM, PGM, or PBM) to standard output (stdout). This stream-based I/O design facilitates piping the output directly to other Netpbm commands or redirecting it to a file.

As a member of the Netpbm family, jpegtopnm serves as a vital bridge between the common JPEG format and the internal PNM formats used by Netpbm tools. This allows users to leverage the vast array of Netpbm utilities (e.g., pamscale, pnmgamma, pnmflip) for processing JPEG images, simply by piping the output of jpegtopnm to another Netpbm command.

PNM is a family of pixelmap formats: PBM (Portable Bitmap) for black & white images, PGM (Portable Graymap) for grayscale images, and PPM (Portable Pixmap) for color images. jpegtopnm automatically determines the appropriate PNM format based on the input JPEG's content and specified options (like -grayscale).

jpegtopnm is an integral part of the Netpbm project, a comprehensive suite of graphics programs that originated in the early Unix environment. Its core functionality relies on the widely used libjpeg library for JPEG decompression. The command's consistent presence and utility reflect the Netpbm project's enduring commitment to providing foundational tools for image manipulation and format conversion in a modular, Unix-philosophy manner.

pnmtojpeg(1), anytopnm(1), pnm(5), netpbm(1), cjpeg(1), djpeg(1)