ppmdim

Dim the specified PPM image by dimfactor

$ ppmdim [0.6] [path/to/input.ppm] > [path/to/output.ppm]

ppmdim [-x=float] [-y=float] [ppmfile]
ppmdim [scale_factor] [ppmfile]

scale_factor
    A floating-point number representing a uniform scaling factor applied to both the horizontal and vertical dimensions. If this is the only numeric argument, it scales uniformly.

-x=float
    Specifies the horizontal scaling factor as a floating-point number.

-y=float
    Specifies the vertical scaling factor as a floating-point number.

ppmfile
    The path to the PPM image file to be processed. If omitted, ppmdim reads from standard input.

ppmdim is a specialized utility within the Netpbm image processing suite, designed to dim or scale Portable Pixmap (PPM) images. It reads a PPM image from standard input or a specified file and outputs a scaled version to standard output. While the name "dim" might suggest only reducing brightness, ppmdim primarily functions as a scaling tool. It achieves the "dimming" effect by averaging pixels, effectively reducing the resolution or detail, thereby making the image appear "dimmer". This can be useful for quickly generating thumbnails, previews, or for reducing image data size for specific applications where precise high-quality scaling isn't the primary concern. It supports independent horizontal and vertical scaling factors, allowing for non-uniform resizing.

ppmdim is specific to the PPM (Portable Pixmap) format. It will not work directly with other image formats like JPEG, PNG, or GIF unless they are first converted to PPM using other Netpbm tools (e.g., jpegtoppm, pngtoppm).
The "dimming" effect is a result of resolution reduction and pixel averaging, not a direct adjustment of brightness or color values.
Scaling up significantly with ppmdim may result in a pixelated or blocky appearance due to its simple averaging algorithm, unlike more sophisticated image scaling algorithms that interpolate pixels for smoother results. For higher quality scaling, consider pamscale.

ppmdim is frequently used in shell pipelines due to Netpbm's design philosophy. For example, to convert a JPEG image to a scaled-down PPM, you might use:
jpegtoppm image.jpg | ppmdim 0.5 > scaled_image.ppm
This allows for complex image processing workflows by chaining multiple Netpbm utilities.

ppmdim is a legacy tool from the Netpbm project, which originated from Jef Poskanzer's PBMPlus package in the late 1980s. It was one of the early specialized utilities for manipulating Portable Pixmap (PPM) images. While ppmdim specifically "dims" (scales down and averages), more general-purpose scaling tools like pnmscale and later pamscale (which handles all Netpbm formats: PBM, PGM, PPM, PAM) have largely superseded it for common scaling tasks. Its continued presence in the Netpbm suite reflects its historical significance and specific, albeit simple, scaling methodology.

pamscale(1), pnmscale(1), pnmcut(1), ppmtojpeg(1), netpbm(1)