Here are a few examples of my work.
Don't let the terrible layout of this page turn you off; I'd rather spent my time working on things that matter than wasting it on things like designing an example page.
Remember, any feature can be implemented on any CAPTCHA

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A gradient background, text gradienting in the opposite direction.
Distorted by a linear and irregular sinusoidal lines.
Uses distortion on each character
Numbers only due to readability issues.

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This drops the background, but adds another sinusoidal line.
Uses variety letters for increased security.

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Sinusoidal waves and numbers only for a simple CAPTCHA.

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Avoids using certain functions that aren't installed on some servers.

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This gives users a few options (about 6, eg 'Apple', 'Phone', 'Pencil', 'Flower', etc). Very difficult to crack; To prevent random guessing by bots, incorrect clicks can be tracked.