CGContextAddArc counterclockwise instead of clockwise - ios

Question

I'm having an issue when drawing an arc inside of my drawing function inside of a CALayer subclass. The implementation for that drawing function is as follows:
-(void)drawInContext:(CGContextRef)ctx
{
CGPoint center = CGPointMake(self.bounds.size.width/2, self.bounds.size.height/2);
CGFloat radius = MIN(center.x, center.y);
CGContextBeginPath(ctx);
CGContextAddArc(ctx, center.x, center.y, radius, DEG2RAD(0), DEG2RAD(90), YES);
CGContextSetStrokeColorWithColor(ctx, [UIColor whiteColor].CGColor);
CGContextSetLineWidth(ctx, 5);
CGContextDrawPath(ctx, kCGPathStroke);
}
Nothing too groundbreaking here, but the weird thing is that it's drawing the arc counterclockwise even though I specified that it shoud be clockwise. Conversely, when I specify NO for the clockwise parameter, it draws the arc clockwise. I did some research into flipped coordinate systems and figured that might be what the issue here is, but I didn't want to just hardcode the opposite bool parameter of what I meant. Any suggestions on how to fix this?
Thanks!

Answer 1

This is indeed due to the flipped coordinate system of UIKit vs. Quartz. From the docs for CGContext:
The clockwise parameter determines the direction in which the arc is created; the actual direction of the final path is dependent on the current transformation matrix of the graphics context. For example, on iOS, a UIView flips the Y-coordinate by scaling the Y values by -1. In a flipped coordinate system, specifying a clockwise arc results in a counterclockwise arc after the transformation is applied.
You can alleviate this in your code by using the transformation matrix to flip your context:
CGContextTranslateCTM(ctx, 0.0, self.bounds.size.height);
CGContextScaleCTM(ctx, 1.0, -1.0);
You probably want to flip it back when you are finished with your drawing i.e.
CGContextSaveGState(ctx);
CGContextTranslateCTM(ctx, 0.0, self.bounds.size.height);
CGContextScaleCTM(ctx, 1.0, -1.0);
// Draw...
CGContextRestoreGState(ctx);