Question

I have the following code:

- (void)drawRect:(CGRect)rect {
    CGContextRef c = UIGraphicsGetCurrentContext();

    CGContextSetFillColorWithColor(c, [UIColor blackColor].CGColor);
    CGContextFillRect(c, rect);

    CGContextSetLineJoin(c, kCGLineJoinRound);
    CGContextSetLineCap(c, kCGLineCapRound);
    CGContextSetLineWidth(c, 50.0);

    CGContextSetStrokeColorWithColor(c, [UIColor redColor].CGColor);
    CGContextBeginPath(c);
    CGContextMoveToPoint(c, 60, 60);
    CGContextAddLineToPoint(c, 60, 250);
    CGContextAddLineToPoint(c, 60, 249);
    CGContextStrokePath(c);

    CGContextSetStrokeColorWithColor(c, [UIColor blueColor].CGColor);
    CGContextBeginPath(c);
    CGContextMoveToPoint(c, 160, 60);
    CGContextAddLineToPoint(c, 160, 250);
    CGContextAddLineToPoint(c, 160.01, 249);
    CGContextStrokePath(c);
}

This generates the following output:

Is there a good reason that the red shape’s bottom edge is not rounded? Or is it a bug in Core Graphics when the line exactly doubles back on itself?

Answer 1

It’s definitely a bug. If you try adding another line to the path, you can see how Core Graphics is unable to handle it.

CGContextMoveToPoint(c, 60.0, 60.0);
CGContextAddLineToPoint(c, 60.0, 250.0);
CGContextAddLineToPoint(c, 60.0, 249.0);
CGContextAddLineToPoint(c, 60.0, 250.0);

It’s as if the masking that creates the rounded caps and joins gets inverted when it’s doubled.