PBM to DJVU Converter

PBM (Portable Bitmap) is the monochrome (black and white, 1-bit) member of the Netpbm family of image formats, created by Jef Poskanzer in 1988 as part of the Pbmplus toolkit for Unix systems. The format exists in two variants: ASCII (magic number P1), where each pixel is represented as a text character '0' (white) or '1' (black) separated by whitespace, and binary (magic number P4), where pixels are packed eight per byte for compact storage. Both variants begin with a plain-text header specifying the magic number, image width and height, and optional comments. PBM was designed as the simplest possible image format — a bridge format for converting between the many incompatible raster formats that proliferated across different Unix systems and applications during the 1980s. The Netpbm philosophy was to convert any source format to PBM/PGM/PPM as an intermediate step, then convert to the target format, using the portable formats as a universal exchange layer. One advantage is extreme simplicity — the ASCII variant can be literally typed by hand in a text editor, and both variants are trivial to parse and generate in any programming language without external libraries. The format's role as a universal image processing intermediate is another strength: hundreds of Netpbm command-line tools accept PBM input, enabling complex image manipulation pipelines through Unix pipes. PBM remains used in computer science education, OCR preprocessing, and any context where a dead-simple monochrome image representation is needed.