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    <p>The following draws an HSL color wheel in a UIView subclass. It does this by generating a bitmap by computing, for each pixel, the correct color value. This is not exactly what you're trying to do (looks like it's just hue varies in the circle with a constant luminance/saturation), but you should be able to adapt it for your needs.</p> <p>Note that this may not have optimal performance, but it should get you started. Also, you can use <code>getColorWheelValue()</code> to handle user input (clicks/touches at a given coordinate).</p> <pre><code>- (void)drawRect:(CGRect)rect { int dim = self.bounds.size.width; // should always be square. bitmapData = CFDataCreateMutable(NULL, 0); CFDataSetLength(bitmapData, dim * dim * 4); generateColorWheelBitmap(CFDataGetMutableBytePtr(bitmapData), dim, luminance); UIImage *image = createUIImageWithRGBAData(bitmapData, self.bounds.size.width, self.bounds.size.height); CFRelease(bitmapData); [image drawAtPoint:CGPointZero]; [image release]; } void generateColorWheelBitmap(UInt8 *bitmap, int widthHeight, float l) { // I think maybe you can do 1/3 of the pie, then do something smart to generate the other two parts, but for now we'll brute force it. for (int y = 0; y &lt; widthHeight; y++) { for (int x = 0; x &lt; widthHeight; x++) { float h, s, r, g, b, a; getColorWheelValue(widthHeight, x, y, &amp;h, &amp;s); if (s &lt; 1.0) { // Antialias the edge of the circle. if (s &gt; 0.99) a = (1.0 - s) * 100; else a = 1.0; HSL2RGB(h, s, l, &amp;r, &amp;g, &amp;b); } else { r = g = b = a = 0.0f; } int i = 4 * (x + y * widthHeight); bitmap[i] = r * 0xff; bitmap[i+1] = g * 0xff; bitmap[i+2] = b * 0xff; bitmap[i+3] = a * 0xff; } } } void getColorWheelValue(int widthHeight, int x, int y, float *outH, float *outS) { int c = widthHeight / 2; float dx = (float)(x - c) / c; float dy = (float)(y - c) / c; float d = sqrtf((float)(dx*dx + dy*dy)); *outS = d; *outH = acosf((float)dx / d) / M_PI / 2.0f; if (dy &lt; 0) *outH = 1.0 - *outH; } UIImage *createUIImageWithRGBAData(CFDataRef data, int width, int height) { CGDataProviderRef dataProvider = CGDataProviderCreateWithCFData(data); CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB(); CGImageRef imageRef = CGImageCreate(width, height, 8, 32, width * 4, colorSpace, kCGImageAlphaLast, dataProvider, NULL, 0, kCGRenderingIntentDefault); UIImage *image = [[UIImage alloc] initWithCGImage:imageRef]; CGDataProviderRelease(dataProvider); CGColorSpaceRelease(colorSpace); CGImageRelease(imageRef); return image; } // Adapted from Apple sample code. See http://en.wikipedia.org/wiki/HSV_color_space#Comparison_of_HSL_and_HSV void HSL2RGB(float h, float s, float l, float* outR, float* outG, float* outB) { float temp1, temp2; float temp[3]; int i; // Check for saturation. If there isn't any just return the luminance value for each, which results in gray. if(s == 0.0) { *outR = l; *outG = l; *outB = l; return; } // Test for luminance and compute temporary values based on luminance and saturation if(l &lt; 0.5) temp2 = l * (1.0 + s); else temp2 = l + s - l * s; temp1 = 2.0 * l - temp2; // Compute intermediate values based on hue temp[0] = h + 1.0 / 3.0; temp[1] = h; temp[2] = h - 1.0 / 3.0; for(i = 0; i &lt; 3; ++i) { // Adjust the range if(temp[i] &lt; 0.0) temp[i] += 1.0; if(temp[i] &gt; 1.0) temp[i] -= 1.0; if(6.0 * temp[i] &lt; 1.0) temp[i] = temp1 + (temp2 - temp1) * 6.0 * temp[i]; else { if(2.0 * temp[i] &lt; 1.0) temp[i] = temp2; else { if(3.0 * temp[i] &lt; 2.0) temp[i] = temp1 + (temp2 - temp1) * ((2.0 / 3.0) - temp[i]) * 6.0; else temp[i] = temp1; } } } // Assign temporary values to R, G, B *outR = temp[0]; *outG = temp[1]; *outB = temp[2]; } </code></pre>
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    1. COWaste of battery power to calculate this repeatedly. Nice coding, but only not great fir user battery.
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