Limiting a pan gesture recognizer to a specific circle - ios

Question

I am using a circular gradient, drawn in an image view, as a color wheel for my color picker. I want to limit the pan gesture recognizer's shouldReceiveTouch: to the gradient's position so that the rest of the view is not affected by the pan gesture recognizer.
How do I do specify circular bounds matching the gradient's radius?
EDIT: I have tried the following code without effect :
if (sender.numberOfTouches)
{
CGSize size = CGSizeMake(self.view.bounds.size.width, self.view.bounds.size.height/3);
float radius = MIN(size.width, size.height)/2;
[alphaSlider addTarget:self action:#selector(changeOpacity:) forControlEvents:UIControlEventValueChanged];
[hellSlider addTarget:self action:#selector(changeBrightness:) forControlEvents:UIControlEventValueChanged];
[saturationSlider addTarget:self action:#selector(saturate:) forControlEvents:UIControlEventValueChanged];
[rSlider addTarget:self action:#selector(redSlider:) forControlEvents:UIControlEventValueChanged];
[gSlider addTarget:self action:#selector(greenSlider:) forControlEvents:UIControlEventValueChanged];
[bSlider addTarget:self action:#selector(blueSlider:) forControlEvents:UIControlEventValueChanged];
CGPoint lastPoint = [sender locationOfTouch: sender.numberOfTouches - 1 inView: gradientView];
CGPoint center = CGPointMake((size.width/2), (size.height /2));
CGPoint delta = CGPointMake(lastPoint.x - center.x, lastPoint.y - center.y);
CGFloat angle = (delta.y == 0 ? delta.x >= 0 ? 0 : M_PI : atan2(delta.y, delta.x));
angle = fmod(angle, M_PI * 2.0);
angle += angle >= 0 ? 0 : M_PI * 2.0;
if((lastPoint.x - center.x) + (lastPoint.y - center.y)/2 < radius)
{
UIColor *color = [UIColor colorWithHue: angle / (M_PI * 2.0) saturation:saturationSlider.value brightness:hellSlider.value alpha:alphaSlider.value];
if ([color getRed: &r green: &g blue:&b alpha: &a])
{
NSLog(#"Color value - R : %g G : %g : B %g", r*255, g*255, b*255);
}
float red = r;
float green = g;
float blue = b;
rText.text = [NSString stringWithFormat:#"%.0f",rSlider.value];
gText.text = [NSString stringWithFormat:#"%.0f",green*255];
bText.text = [NSString stringWithFormat:#"%.0f",blue*255];
colorView.backgroundColor = [UIColor colorWithRed:(rText.text.floatValue/255) green:(gText.text.floatValue/255) blue:(bText.text.floatValue/255) alpha:alphaSlider.value];
rSlider.value = red*255;
gSlider.value = green*255;
bSlider.value = blue*255;
alphaText.text = [NSString stringWithFormat:#"%.2f",alphaSlider.value];
brightnessText.text = [NSString stringWithFormat:#"%.2f",hellSlider.value];
saturationText.text = [NSString stringWithFormat:#"%.2f",saturationSlider.value];
}
}
}
It seems to work on the right side of the gradient and quits receiving color updates, left and lower side are still updating. I need to limit the panGesture appropriately in order for it to stop interfering with my UISliders.
Any ideas?

Answer 1

You just need to hit-test the touch event to see if it's within the circle's area. To do this you'll need to know the center point of your circle, and its radius.
A general equation for doing this is covered on this existing question:
Equation for testing if a point is inside a circle