How to draw stars using Quartz Core?

Question

I\'m trying to adapt an example provided by Apple in order to programmatically draw stars in line, the code is the following:

CGContextRef context = UIGraphi         


        

Answer 1

I prefer using a CAShaperLayer to implementing drawRect, as it can then be animated. Here's a function that will create a path in the shape of a 5 point star:

func createStarPath(size: CGSize) -> CGPath {
    let numberOfPoints: CGFloat = 5

    let starRatio: CGFloat = 0.5

    let steps: CGFloat = numberOfPoints * 2

    let outerRadius: CGFloat = min(size.height, size.width) / 2
    let innerRadius: CGFloat = outerRadius * starRatio

    let stepAngle = CGFloat(2) * CGFloat(M_PI) / CGFloat(steps)
    let center = CGPoint(x: size.width / 2, y: size.height / 2)

    let path = CGPathCreateMutable()

    for i in 0..<Int(steps) {
        let radius = i % 2 == 0 ? outerRadius : innerRadius

        let angle = CGFloat(i) * stepAngle - CGFloat(M_PI_2)

        let x = radius * cos(angle) + center.x
        let y = radius * sin(angle) + center.y

        if i == 0 {
            CGPathMoveToPoint(path, nil, x, y)
        }
        else {
            CGPathAddLineToPoint(path, nil, x, y)
        }
    }

    CGPathCloseSubpath(path)
    return path
}

It can then be used with a CAShapeLayer like so:

let layer = CAShapeLayer()
layer.path = createStarPath(CGSize(width: 100, height: 100))
layer.lineWidth = 1
layer.strokeColor = UIColor.blackColor().CGColor
layer.fillColor = UIColor.yellowColor().CGColor
layer.fillRule = kCAFillRuleNonZero

Answer 2

Here is code that will draw 3 stars in a horizontal line, it's is not pretty but it may help:

-(void)drawRect:(CGRect)rect
{
    int aSize = 100.0;
    const CGFloat color[4] = { 0.0, 0.0, 1.0, 1.0 }; // Blue
    CGColorRef aColor = CGColorCreate(CGColorSpaceCreateDeviceRGB(), color);
    CGContextRef context = UIGraphicsGetCurrentContext();
    CGContextSetLineWidth(context, aSize);
    CGFloat xCenter = 100.0;
    CGFloat yCenter = 100.0;

    float  w = 100.0;
    double r = w / 2.0;
    float flip = -1.0;

    for (NSUInteger i=0; i<3; i++) 
    {
        CGContextSetFillColorWithColor(context, aColor);
        CGContextSetStrokeColorWithColor(context, aColor);

        double theta = 2.0 * M_PI * (2.0 / 5.0); // 144 degrees

        CGContextMoveToPoint(context, xCenter, r*flip+yCenter);

        for (NSUInteger k=1; k<5; k++) 
        {
            float x = r * sin(k * theta);
            float y = r * cos(k * theta);
            CGContextAddLineToPoint(context, x+xCenter, y*flip+yCenter);
        }
        xCenter += 150.0;
    }
    CGContextClosePath(context);
    CGContextFillPath(context);
}

Answer 3

Here's an algorithm to implement what buddhabrot implied:

- (void)drawStarInContext:(CGContextRef)context withNumberOfPoints:(NSInteger)points center:(CGPoint)center innerRadius:(CGFloat)innerRadius outerRadius:(CGFloat)outerRadius fillColor:(UIColor *)fill strokeColor:(UIColor *)stroke strokeWidth:(CGFloat)strokeWidth {
    CGFloat arcPerPoint = 2.0f * M_PI / points;
    CGFloat theta = M_PI / 2.0f;

    // Move to starting point (tip at 90 degrees on outside of star)
    CGPoint pt = CGPointMake(center.x - (outerRadius * cosf(theta)), center.y - (outerRadius * sinf(theta)));
    CGContextMoveToPoint(context, pt.x, pt.y);

    for (int i = 0; i < points; i = i + 1) {
        // Calculate next inner point (moving clockwise), accounting for crossing of 0 degrees
        theta = theta - (arcPerPoint / 2.0f);
        if (theta < 0.0f) {
            theta = theta + (2 * M_PI);
        }
        pt = CGPointMake(center.x - (innerRadius * cosf(theta)), center.y - (innerRadius * sinf(theta)));
        CGContextAddLineToPoint(context, pt.x, pt.y);

        // Calculate next outer point (moving clockwise), accounting for crossing of 0 degrees
        theta = theta - (arcPerPoint / 2.0f);
        if (theta < 0.0f) {
            theta = theta + (2 * M_PI);
        }
        pt = CGPointMake(center.x - (outerRadius * cosf(theta)), center.y - (outerRadius * sinf(theta)));
        CGContextAddLineToPoint(context, pt.x, pt.y);
    }
    CGContextClosePath(context);
    CGContextSetLineWidth(context, strokeWidth);
    [fill setFill];
    [stroke setStroke];
    CGContextDrawPath(context, kCGPathFillStroke);
}

Works for me for most basic stars. Tested from 2 points (makes a good diamond!) up to 9 stars.

If you want a star with point down, change the subtraction to addition.

To draw multiples, create a loop and call this method multiple times, passing a new center each time. That should line them up nicely!