Mohamed Magdy wrote:
Is it possible to generate video on the fly containing random characters and numbers and use it instead of the still images? or may be generate a .gif animations (with characters dancing up and down in a colorful way :))?
I assume that can slow ocr (forcing it to convert the video to still images first then do the ocr) but i don't know about video, may be it is possible to make the video gives garbage when converted to still images.. if you google video character recognition you get a lot of hits but i don't think spammers capable of developing vcr for their needs ;)..
That would most likely make a compatibility nightmare (maybe except for animated gifs), while not solving any of the before mentioned acccessibility problems.
I suspect that video captchas in fact can be worse, at least if you don't consider decoding time, since you give the adversary more information. If you take all the still images and try to decrypt the captcha from each of them, you may be able to decode some of the still pictures, and even interframe information can be used. In a still image you only have one chance.
On could of cause think of animation that made a word "emerge from the noise", utilising the human ability to merge a set of still pictures each too noisy to interpret into an image, but I would suggest to stay with still images.