I always liked playing with graphics. I made some demo's myself in the 486 era (nothing special) and I still like to play with particles, alpha-shading, etcetera. So creating a captcha was something I could do quite easily, and I decided to make cracking mine hard by using a picture as a background and overlaying the text kinda vaguely. I decided to release the sourcecode under the LGPL, so you may use it too. The source is retrievable here.
The source needs a directory, 'pics', where large pictures can be dumped into. It'll use these as a backdrop. I'm not quite sure how this approach scales if one were to get hit by thousands of visitors, though. Edit: I do now. My site has been slashdotted, and the server (shared hosting) survived about 100K hits in one day without failing. I'd say it's pretty stable.
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