djpeg

djpeg [options] [jpegfile]

-bmp
    Output a Windows BMP file.

-colors N
    Reduce image to at most N colors.

-dither
    Apply Floyd-Steinberg dithering to color quantization.

-grayscale
    Force grayscale output.

-os2
    Use OS/2 BMP file format.

-scale fraction
    Scale the image by the specified fraction (e.g., 0.5 or 1/4).

-ppm
    Output a PPM (Portable Pixmap) file.

-pgm
    Output a PGM (Portable Graymap) file.

-rgb
    Force RGB output even for grayscale JPEG input.

-targa
    Output a Targa file.

-verbose
    Enable verbose output.

-version
    Display version information.

-dct int
    Use integer DCT method (default).

-dct fast
    Use fast integer DCT method.

-dct float
    Use floating-point DCT method.

-fast
    Equivalent to -dct fast -dither -nosmooth.

-nosmooth
    Disable smoothing filter.

The `djpeg` command is part of the Independent JPEG Group's (IJG) software suite, enabling users to decompress JPEG images. It converts JPEG files into various other image formats, such as PPM (Portable Pixmap), PGM (Portable Graymap), BMP (Bitmap), and Targa.

This tool is useful for converting JPEG images for use with image processing software or display tools that do not directly support JPEG. `djpeg` offers various options to control the decompression process, including scaling, color quantization, and error handling. It is a command-line utility primarily intended for batch processing or integration into scripts.

`djpeg` operates by decoding the compressed JPEG data and reconstructing the original image based on the JPEG compression algorithm. It provides control over the output format, allowing users to choose the format most suitable for their needs. It's often used as a component in larger image manipulation workflows.

JPEG images can suffer from loss of quality during compression and decompression cycles. Repeated compression and decompression can lead to noticeable artifacts.

The available options and output formats might vary slightly depending on the specific version of the IJG JPEG library installed.

djpeg attempts to handle errors gracefully. However, severely corrupt JPEG files may cause the program to terminate unexpectedly. Using the -verbose option can provide more information about errors encountered during decompression.

The choice of output format depends on the intended use of the decompressed image. PPM and PGM are simple, uncompressed formats suitable for further processing. BMP and Targa are more widely supported by image editing software but may be larger in size.

djpeg was developed as part of the Independent JPEG Group's (IJG) software. The IJG library has been a cornerstone of JPEG handling on Unix-like systems since the early 1990s.

It emerged to provide a simple and reliable way to decompress JPEG images. Over time, it has been updated to support newer JPEG features and to improve performance and compatibility. Its widespread use has established it as a standard tool for image manipulation.

cjpeg(1), jpegtran(1)