iphone - 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 gradient, which draws a gradient in a straight line like this:

and a radial gradient, which draws the gradient starting at point and radiating out in all directions like this:

I want to draw a linear gradient that rotates around a point like this:

Answer 1

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.

Note that this may not have optimal performance, but it should get you started. Also, you can use getColorWheelValue() to handle user input (clicks/touches at a given coordinate).

- (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 < widthHeight; y++)
    {
        for (int x = 0; x < widthHeight; x++)
        {
            float h, s, r, g, b, a;
            getColorWheelValue(widthHeight, x, y, &h, &s);
            if (s < 1.0)
            {
                // Antialias the edge of the circle.
                if (s > 0.99) a = (1.0 - s) * 100;
                else a = 1.0;

                HSL2RGB(h, s, l, &r, &g, &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 < 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 < 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 < 3; ++i)
    {
        // Adjust the range
        if(temp[i] < 0.0)
            temp[i] += 1.0;
        if(temp[i] > 1.0)
            temp[i] -= 1.0;


        if(6.0 * temp[i] < 1.0)
            temp[i] = temp1 + (temp2 - temp1) * 6.0 * temp[i];
        else {
            if(2.0 * temp[i] < 1.0)
                temp[i] = temp2;
            else {
                if(3.0 * temp[i] < 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];
}