Drawing a Linear Gradient in Quartz and turn it into a circle

I want to draw a Linear Gradient and turn it into a circle, so that it looks like this at the end.

Since this isn't a开发者_运维技巧 usual linear gradient, nor a real radial gradient, I do not have a real idea what to do and where to start.

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This is the exact example you are looking for. Download it source code and try it.