PDFBeads -- convert scanned images to a single PDF file
Version 1.0 (November 2010)

Copyright (C) 2010 Alexey Kryukov (amkryukov@gmail.com).
All rights reserved.

PDFBeads is a small utility written in Ruby which takes scanned
page images and converts them into a single PDF file. Unlike other
PDF creation tools, PDFBeads attempts to implement the approach
typically used for DjVu books. Its key feature is separating scanned
text (typically black, but indexed images with a small number of
colors are also accepted) from halftone pictures. Each type of
graphical data is encoded into its own layer with a specific
compression method and resolution.

The name `PDFBeads' has been selected for the package because
building PDF files from separate image is comparable to threading
beads on a string. It also seems to be a good choice for a Ruby
application.

Here's a few operations you can perform with PDFBeads:

* encode B&W images using either CCITT Group 4 Fax or JBIG2
  compression method (you'll need Adam Langley's jbig2 utility,
  available at https://github.com/agl/jbig2enc/ , for JBIG2
  compression);

* combine halftone or indexed pictures with previously binarized
  text pages, placing them into the background layer. Various
  compression methods of background images (JPEG2000, JPEG or
  PNG-styled deflate compression) are supported;

* split mixed images where binarized text is combined with color
  or grayscale pictures (such pages may be produced with ScanTailor --
  an interactive post-processing tool for scanned page, available
  at http://scantailor.sourceforge.net) and encode each layer
  separately;

* correctly process indexed images with a limited number of colors,
  encoding each color separately into the foreground layer;

* split color images into background and foreground layers (similar
  to BG44 and FG44 chunks in a DjVu file) according to a given mask;

* create PDF files with TOC and metadata;

* read text from hOCR files and create a hidden text layer in the PDF
  file.

Note that PDFBeads is intended for creating PDF files from previously
processed images, and so it can't done some operations (e. g. converting
color or grayscale scans to B&W) which should be typically performed with
a special scan processing application, such as ScanTailor.

PDFBeads requires RMagick (the Ruby bindings for the popular Magick++ image
processing library) and Nokogiri to read data from hOCR files.