Question

I have a problem regarding positioning an image according to the touches location, however limited to a circle.

It works for the most part, but if the angle (from the touches location to the desired location) is less than 0, it positions the image on the wrong side of the circle.

Perhaps it’s some maths that I’ve done wrong.

Anyway, here’s the code:

float newHeight, newWidth, centerPointX, centerPointY;
newHeight = -(invertedY.y - (view.frame.origin.y+view.frame.size.height/2));
newWidth = -(invertedY.x - (view.frame.origin.x+view.frame.size.width/2));

float tangent = newHeight/newWidth; 
float calculatedAngle = atanf(tangent);
float s, c, d, fX, fY;

d = view.frame.size.width/2+30;

    if (calculatedAngle < 0) {
        s = sinf(calculatedAngle) * d;
        c = cosf(calculatedAngle) * d;
    } else {
        s = -sinf(calculatedAngle) * d;
        c = -cosf(calculatedAngle) * d;
    }

    fX = view.center.x + c;
    fY = view.center.y + s;

    [delegate setPoint:CGPointMake(fX, fY)];

    NSLog(@"angle = %.2f", calculatedAngle);

Any help appreciated.

Answer 1

I think the best way to limit location to a circle is calculate vector from center to touch location. Calculate vector length then divide it by that length so it would be normalized. Then multiply normalized vector by radius of circle and finally add this vector to the center to compute new location.

CGPoint touch, center;
CGPoint vector = CGPointMake(touch.x-center.x, touch.y-center.y);

float length = sqrtf(vector.x*vector.x + vector.y*vector.y); 
// Normalize and multiply by radius (r)
vector.x = r * vector.x / length;
vector.y = r * vector.y / length;

[delegate setPoint:CGPointMake(center.x + vector.x, center.y + vector.y)];