How to draw a color wheel in objective-c

Question

I am trying to draw a color wheel for an iPhone, but I can\'t get the gradient to rotate around a point. I am trying to use gradients but objective-c provides a linear grad

Answer 1

You'll have to make a bitmap as suggested by hotpaw2. The most efficient way to calculate this is to use the HSL color space. That will allow you to create a function that takes as input a pixel location and spits out an RGB value, using only a few basic arithmetic operations and a little trig to calculate it.

If you want arbitrary colors along the "spokes" of the wheel, well, that takes more work. You need to calculate blending values for each pixel, and it involves a fair sight more trigonometry, and isn't as fast. When I did this, I had to vectorize and parallelize it to remove the slight (though perceptible) refresh delay from recalculating the bitmap--on a MacBook Pro. On the iPhone you don't have those options, so you'll have to live with the delay.

Let me know if you need more detailed explanations of these techniques; I'll be happy to oblige.