How to implement a gradient color picker in code?

I'开发者_如何转开发m wanting to program something like Photoshop's gradient color picker. I've been googling but can't find anything. Do you guys know how to do it?

It's a bit out of date (and .NET if that's your choice of framework), but in the past I've implemented an app with this ColorPicker.

I remember hacking up something like it with a static image and getPixel.