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Completely Automated Public Turing test to tell Computers and Humans Apart (commonly abbreviated as CAPTCHA) and its next-generation evolution Re-Captcha is any Turing test designed to prove that an individual is a human not a machine.

In particular, the most common Captcha implementation is a word, text, or other random assortment of alphanumeric characters scrambled and/or written over and obscured so that it would be very difficult for an image scanning program (i.e. robot, or bot) to scan and understand the text. For those with vision impairments, an "Audio captcha" is typically suggested to be made available as well in order to comply with A11Y guidelines and ensure the accessibility of your products & services.

Recently, the "Re-Captcha" project introduced more robust options such as picking out objects in a set of images, commonly of the format "choose the images that contain _________":

  • street signs
  • stop lights
  • buses
  • cars
  • cyclists
  • people

As Google acquired the original Re-Captcha project, it was speculated and quite likely that the results of each Google users' Captcha solving was being used to help Google build their Image Recognition algorithms for their products/services such as Maps, StreetView, and possibly even as Machine Learning inputs to their "Self-Driving Cars" program.

Despite the Google acquisition, the Re-Captcha movement in general inspired further more interactive/intuitive options for users though, such as:

  • drag the shape into the slot in which it fits
  • match the colors and shapes
  • drag the puzzle pieces to make an image of something
  • drag the labelled item onto the appropriate image


Captchas are commonly used to protect user login and registration forms, but may also be used for providing an added level of security on content publishing and/or editing services. They can be somewhat effective in reducing spam and fraud online in a web/desktop application or website, by mathematically increasing the complexity of automating such activities.


[1] [2]

External Links


  1. Top 20 Captcha and related anti-spam solutions:
  2. 21 free Captcha sources: (ways to generate, online tools, OSS libs, etc)

See Also

Google | Microsoft | Image Recognition | Security