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Welcome. Clara OCR is a free OCR, written for systems supporting the C library and the X Windows System. Clara OCR is intended for the cooperative OCR of books. There are some screenshots available at http://www.claraocr.org/.
This documentation is extracted automatically from the comments of the Clara OCR source code. It is known as "The Clara OCR Advanced User's Manual". It's currently unfinished. First-time users are invited to read "The Clara OCR Tutorial". Developers must read "The Clara OCR Developer's Guide".
| CONTENTS |
| 1. Welcome to Clara OCR |
Clara is an optical character recognition (OCR) software, that is, a software that tries to identify the graphic images of the characters on a scanned document, converting them from their images to ASC, ISO or other codes.
The name Clara stands for "Cooperative Lightweight chAracter Recognizer".
Clara offers a revision interface that may be used from a standalone GUI or through the WWW by various different reviewers simultaneously. Because of this feature it's a "cooperative" OCR (it's also "cooperative" in the sense of its free/open status and development model).
| 1.1 Early historical notes |
For some years now we have tested and used OCR softwares, mainly for old books. Popular OCR softwares (those bundled with scanners) are useful tools. However, OCR is not a simple task. The results obtained using those programs vary largely depending on the the printed document, and, for most texts we're interested on, the results are really poor or even unusable. In fact, it's not a surprise that many digitalization projects prefer not to use OCR, but typists only.
For a programmer, it is somewhat intuitive that OCR could achieve good results even from low quality texts, when an add-hoc approach is used, focusing one specific book (for instance). Within this approach, OCR becomes a matter of finding one software adequate for the texts you're trying to OCR, or perhaps develop a new one. So a free and easy to customize OCR (on the source code level) would be a valuable resource for text digitalization projects.
Dealing with graphics is not among our main occupations, but after analysing many scanned materials, we began to write some simple and specialized recognition tools. More recently (in the third quarter of 1999) a simple X interface linked to a naive bitmap comparison heuristic was written. From that prototype, Clara OCR evolved. Since then, many new ideas from various persons helped to make it better.
| 1.2 Design notes |
It's not a bad idea to enumerate some principles that have driven Clara OCR development. They'll make easier to understand the features and limitations of the software (these principles may change along time).
1. Clara is an OCR for printed texts, not for handwritten texts.
2. Clara was not designed to be used to OCR one or two single pages, but to OCR a large number of documents with the same graphic characteristics (font, size, etc). So it can take advantage of a fine (and perhaps expensive) training. This will be tipically the case when OCRing an entire book.
3. We chose not support directly multiple graphic formats, but only Jeff Poskanzer's raw PBM. Non-PBM files will be read through filters.
4. Clara OCR wants to be a tool that makes viable the sum and reuse of human revision effort. Because of this, on the OCR model implemented by Clara, training and revision are one same thing. The revision is a sum of punctual and independent acts and alternates with reprocessing steps along a refinement process.
5. The Clara GUI was implemented and behaves like a minimalistic HTML viewer. This is just an easy and standard way to implement a forms interface.
6. We have tried to make the source code portable across platforms that support the C library and the Xlib. Clara has no special provision to be ported to environments that do not support the Xlib. We avoided to use a higher level graphic environment like Motif, GTK or Qt, but we do not discourage initiatives to add code to Clara OCR adapt or adapt better to these or other graphic environments.
7. We generally try to make the code efficient in terms of RAM usage. CPU and disk usage (for session files) are less prioritary.
| 1.3 Supported Alphabets |
Clara OCR focuses the Latin Alphabet ("a", "b", "c", ...), used by most European languages, and the indo-arabic digits ("0", "1", "2", ...), but we're trying to support as many alphabets as possible.
To say that Clara OCR supports a given alphabet means that Clara OCR
(a) is able to be trained from the keyboard for the symbols of that alphabet, eventually applying some transliteration from that alphabet to latin. For instance, when OCRing a greek text, if the user presses the latin "a" key (assuming that the keyboard has latin labels), Clara is expected to train the current symbol as "alpha".
(b) knows the vertical alignment of each letter of that alphabet, for instance, knows that the bottom of an "e" is aligned at the baseline;
(c) knows which letters accept or require which signs (accents and others, like the dot found on "i" and "j");
(d) contains code to help avoiding common mistakes, like recognizing "e" as "c", "l" as "1", etc.
To say that Clara OCR supports a given alphabet does not necessarily mean that Clara OCR
(a) knows some particular encoding (ISO-8859-X, Unicode, etc) for that alphabet;
(b) contains or is able to use fonts for that alphabet to display the OCR output on the PAGE (OUTPUT) window.
Even ignoring the standard encondings for one given alphabet (e.g. ISO-LATIN-7 for Greek), Clara eventually will be able to produce output using TeX macros, like {\Alpha}.
| 1.4 Clara vs the others |
Clara differs from other OCR softwares in various aspects:
1. Most known OCRs are non-free and Clara is free. Clara focus the X Windows System. Clara offers batch processing, a web interface and supports cooperative revision effort.
2. Most OCR softwares focus omnifont technology