yourfriendzak - 6 months ago 26

iOS Question

I'm creating a

`CAShapeLayer`

`UIView`

`UIBezierPath`

`#define degreesToRadians(x) ((x) * M_PI / 180.0)`

...

// "layer" refer's to the UIView's root layer

CAShapeLayer *maskLayer = [CAShapeLayer layer];

maskLayer.frame =

layer.presentationLayer ?

((CAGradientLayer *)layer.presentationLayer).bounds : layer.bounds;

maskLayer.fillRule = kCAFillRuleEvenOdd;

maskLayer.needsDisplayOnBoundsChange = YES;

CGFloat maskLayerWidth = maskLayer.bounds.size.width;

CGFloat maskLayerHeight = maskLayer.bounds.size.height;

CGPoint maskLayerCenter =

CGPointMake(maskLayerWidth/2,maskLayerHeight/2);

UIBezierPath *path = [UIBezierPath bezierPath];

[path moveToPoint:maskLayerCenter];

[path addArcWithCenter:maskLayerCenter radius:(maskLayerWidth/2)

startAngle:degreesToRadians(0) endAngle:degreesToRadians(90) clockwise:YES];

[path closePath];

maskLayer.path = path.CGPath;

layer.mask = maskLayer;

The end result is that a pie slice is drawn in the lower right corner. The first point of the arc is drawn at the 90 degrees, then down to 180. Why does it do this even though I'm using 0 and 90 degree angles?

Answer

This image comes straight from the documentation.

And there is a discussion of how it works (given the default coordinate system)

Discussion

This method adds the specified arc beginning at the current point. The created arc lies on the perimeter of the specified circle. When drawn in the default coordinate system, the start and end angles are based on the unit circle shown inFigure 1. For example, specifying a start angle of 0 radians, an end angle of π radians, and setting the clockwise parameter to`YES`

draws the bottom half of the circle. However, specifying the same start and end angles but setting the`clockwise`

parameter set to`NO`

draws the top half of the circle.

This is how the angles work for `addArcWithCenter:radius:startAngle:endAngle:clockwise:`

. If that isn't what you are seeing then you are calculating your angles incorrectly.