pgmdeshadow

Remove grey shadows from a PGM image

$ pgmdeshadow [path/to/input_file.pgm] > [path/to/output_file.pgm]

pgmdeshadow [pgmfile]

pgmdeshadow is a specialized command within the Netpbm toolkit designed to eliminate shadows from Portable Graymap (PGM) image files.

The program reads a PGM image as input and produces a shadow-free PGM image as output. Its core algorithm works by identifying the darkest regions within the image, which are assumed to be shadows, and then intelligently lightening them. Crucially, while it lightens these areas, it simultaneously strives to preserve the overall contrast and visual integrity of the image. This prevents the output from appearing flat or washed out after shadow removal.

The command is particularly effective when dealing with images where the shadows are fairly uniform in their darkness and the image content outside the shadows is relatively consistent. It's often used in pipelines with other Netpbm utilities for more complex image manipulation tasks.

The effectiveness of pgmdeshadow relies on specific assumptions about the input image. It works best when the image is relatively uniform except for shadows, and when the shadows themselves are consistent in their darkness. If these conditions are not met, the algorithm may not produce optimal results, potentially leading to undesired artifacts or an incomplete shadow removal. It's not a universal shadow removal tool for all types of images and complex lighting conditions.

pgmdeshadow is an integral part of the Netpbm project, a widely used toolkit for image manipulation. Its original algorithm was derived from a program called despeckle, authored by Miguel de Icaza. The specific adaptation and integration into Netpbm, along with further modifications and improvements, were carried out by Bryan Henderson. Its inclusion enhances Netpbm's capabilities for automated image correction, particularly for graymap data.

pgm(5), pgmtopgm(1), pnmgamma(1), pnmnorm(1), pnm(5)