Operation
Scanned text is subjected to analysis by two different optical character recognition programs. Their respective outputs are then aligned with each other by standard string-matching algorithms and compared both to each other and to an English dictionary. Any word that is deciphered differently by both OCR programs or that is not in the English dictionary is marked as "suspicious" and converted into a CAPTCHA. The suspicious word is displayed, out of context, along with a control word already known. The system assumes that if the human types the control word correctly, then the response to the questionable word is accepted as probably valid. If enough users were to correctly type the control word, but incorrectly type the 2nd word which OCR had failed to recognize, then the digital version of documents could end up containing the incorrect word. Thus, due to human error in distinguishing between the word "Internet" and the French name "Infernet", references to Captain Infernet have occasionally become Captain Internet. The identification performed by each OCR program is given a value of 0.5 points, and each interpretation by a human is given a full point. Once a given identification hits 2.5 points, the word is considered valid. Those words that are consistently given a single identity by human judges are later recycled as control words.
The original reCAPTCHA method was designed to show the questionable words separately, as out-of-context correction, rather than in use, such as within a phrase of 5 words from the original document. Also, the control word might mislead context for the 2nd word, such as a request of "/metal/ /fife/" being entered as "metal file" due to the logical connection of filing with a metal tool being considered more common than the musical instrument "fife".
In 2012, reCAPTCHA began using photographs of house numbers taken from Google's Street View project, in addition to scanned words.
Read more about this topic: Re CAPTCHA
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