pbmtojpeg

pbmtojpeg [-quality=n] [-grayscale] [-optimize] [-progressive] [-comment=text] [-iccfile=filename] [-verbose]

-quality=n
    Specifies the JPEG compression quality, a number from 0 to 100. Higher numbers mean better quality but larger files. Default is typically 75.

-grayscale
    Converts the input image to grayscale before JPEG compression, even if the input was color. This can reduce file size significantly.

-optimize
    Performs Huffman coding optimization during compression. This can result in a smaller file at the cost of slightly slower compression time.

-progressive
    Creates a progressive JPEG file, which allows an image to be displayed gradually as it downloads over a slow connection.

-comment=text
    Embeds the specified text as a comment in the JPEG file's metadata.

-iccfile=filename
    Embeds the ICC color profile from the specified filename into the JPEG output.

-smooth=n
    Performs a smoothing operation on the input data with a factor of n before compression. Useful for noisy input.

-restart=n
    Inserts restart markers every n MCUs (Minimum Coded Units). Useful for error resilience in transmission.

-maxmemory=n
    Limits the memory usage for compression to n kilobytes.

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

The `pbmtojpeg` command is a utility within the Netpbm toolkit, designed to convert images from any of the Netpbm formats – PBM (Portable Bitmap), PGM (Portable Graymap), or PPM (Portable Pixmap) – into the JPEG (Joint Photographic Experts Group) format.

It typically reads the Netpbm image data from standard input and writes the resulting JPEG image data to standard output, making it highly suitable for use in shell pipelines. `pbmtojpeg` provides various options to control the compression quality, optimize the output file size, enable progressive encoding for web display, and embed metadata like comments or ICC profiles.

As JPEG is a lossy compression format, `pbmtojpeg` is best used when file size reduction is a priority and some loss of image fidelity is acceptable. It is a fundamental tool for image format conversion and processing in Unix-like environments.

JPEG is a lossy compression format. Each time an image is saved in JPEG format, some data is permanently discarded. Repeatedly editing and re-saving an image as JPEG will degrade its quality over time. For images requiring exact pixel representation or lossless compression, consider using PNG or other formats.

`pbmtojpeg` relies on an underlying JPEG library (commonly libjpeg or libjpeg-turbo); its capabilities and performance are tied to the features and efficiency of that library.

By default, `pbmtojpeg` reads PBM, PGM, or PPM image data from standard input (stdin) and writes the compressed JPEG data to standard output (stdout). This design facilitates its use in pipelines, for example: `cat image.ppm | pbmtojpeg -quality=85 > image.jpg`.

The functionality of `pbmtojpeg` heavily relies on the presence of a JPEG compression library, most notably libjpeg or its faster variant, libjpeg-turbo. These libraries handle the complex Discrete Cosine Transform (DCT) and Huffman encoding/decoding processes inherent in JPEG compression.

The `pbmtojpeg` command is a component of the Netpbm project, a comprehensive toolkit for image manipulation. Netpbm itself evolved from the pbmplus package, originally created by Jef Poskanzer in the late 1980s. As JPEG emerged as a dominant image format, `pbmtojpeg` was developed to provide seamless conversion capabilities between the Netpbm family of formats and JPEG, integrating the widely used libjpeg library. Its design as a filter, reading from standard input and writing to standard output, reflects the Unix philosophy of chaining small, specialized tools, making it a versatile part of many command-line image processing workflows since the early 1990s.

jpegtopnm(1), pnm(5), anytopnm(1), pnmscale(1), cjpeg(1), convert(1) (ImageMagick)