Going from simple to super CAPTCHA

PHP General List,

With a little help from the web, and help from this list, I have a simple CAPTCHA image that works within the content system I'm building.

But it's *really* simple. Basically white text on a black background, with a couple of white lines to obscure the text a little.

I'm pretty sure that in its current state, my CAPTCHA image could be cracked by OCR software from the 1950s.

So I'm hoping to take it up to the next level.

I might be able to figure out how to add more lines, more colours, and those kind of basic changes.

But what I definitely can't comprehend is how some CAPTCHAs have that really warped and distorted text.

Is that possible with PHP? Do I have to make a library of pre-warped text images or something?

Is it possible to make backgrounds that are really crazy with textures and gradients. Would that also rely on a pre-built library of images?

Can someone maybe point me to an online instructional on how to get a CAPTCHA image that really works?

Tips on what differentiates a good CAPTCHA from a bad one would also be really sweet.

Thank you for any advice.

--
Dave M G
Ubuntu Feisty 7.04
Kernel 2.6.20-15-386
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