Captcha with mask for PHP and JS

Question

Recently I came across one interesting captcha, its difference is that you first need to "wipe" it, and then read and enter. An example .

I am interested in the method of implementation and maybe there are some examples. Interested in the "mask" itself.

In HTML5 there is such a thing as canvas, I think everything is done through it (grout).
Now she is also interested, the thing is useful, and with a new level of complexity)
It is possible to program the behavior of the server in such a way as not to post it with robots.
As for the flexibility of the second method, then yes kpcha wins, but on the attitude to the forms of filling on end users loses.
I remember that I played for a long time with a captcha consisting of a set of mini-quests (with a random choice) - to collect puzzle, turn the pictures, twist the drum) Sorry, the link was lost.

Pavel Azanov Pavel Azanov 6.569 12 thirty · Accepted Answer · 2011-07-15T18:29:25

Interesting captcha)

It looked like this scheme:

"Mask" implemented using flash
Flash requests a captcha image at http://l2top.ru/cryptImg.php?key=[ key] (the image can be downloaded only once)

It's pretty simple

There are clearly some kind of marriage games with BitmapData taking place.
I, frankly, just lazy, but generally google on the theme of "drawing as in contact" and "how to remove part of the image"

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