Is it a graphic, thus requiring bulky OCR which most bots lack (apparently)? Even so, it is a fixed graphic and would not really require OCR'ing just simple pattern matching against a library of one item. I just don't get how it poses an insurmountable hurdle to frustrate bots.

  • Which type are you specifically asking about? Last one I saw had 9 photos and asked to click on the ones associated with "football", which includes the US type and soccer. I think even google images wouldn't be very helpful to a bot – Xen2050 Aug 30 '17 at 1:08
  • Google's no captcha recaptcha has a lot of parameters it uses to identify you; check it out here : stackoverflow.com/questions/27286232/… – Ibrahim Hassan Aug 30 '17 at 1:51

The captcha monitors mouse behaviour. While it is easy for a bot to click a button on a form, it is hard to simulate the erratic movement of a mouse moved by a human.

However, it is not impossible: https://www.youtube.com/watch?v=fsF7enQY8uI

  • 1
    Yet only for the one-click one. The youtube video is a gold good addition to your answer.... – El8dN8 Aug 30 '17 at 1:50
  • I should have been more specific about which kind of captcha I was discussing. Thanks, this addressed the one I intended. – gbambo Sep 3 '17 at 6:59

Text in captcha's will have been failed to be recognised by OCR. OCR used standard rules of what text is to recognise text.

Suitable Text usually be distorted, not parallel of run in straight lines parallel; to the horizon and contain random junk which OCR can't handle.

ie fail the usual rules of what text should look like.

With Deep Learning becoming more common its only a matter of time before Captchas will not work..

There are many different captcha's, some require choosing a number of graphics which have a theme (eg. which are parts of a sign which can later be further refined and then added to the library method below once the whole sign is assembled and the text graphic extracted by the same method) which a computer won't be able to discern. With this type you are always asked to identify a known scenario and usually an unknown for addition to the library of knowns once enough identical answers are received.

The most common use images in 2 ways:

A single randomly distorted image generated from a word then extra junk added to confuse OCR. Like "Salting" a password List by adding a "random" junk word to stop a rainbow attack.

Another form is using photos (usually of words) of something which people had to decide what it is because image is too complex to recognise automatically. Generally it's outside computer programming parameters of what defines text (or a sign or whatever) and is often surrounded by from a random environment.
This requires A large library of Photos with known "text" or other parameters like which are part of a sign etc.

Side note:
The library for the second method is increased by providing 2 images which users try to identify correctly.
1 image is a known and another an unknown.
Correctly solving the known proves you aren't a robot.
Enough people matching/answering the unknown with the same answer then means that one is now known and can be added to the known library.
This is how Google Maps identifies what actually are street/place name signs (and then later the text they contain) and project Gutenberg texts which failed OCR were corrected.

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