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Asked: 2026-05-13T17:20:37+00:00

I am creating a CGPath to define an area in my game like this:

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I am creating a CGPath to define an area in my game like this:

CGPathMoveToPoint   ( myPath, NULL, center.x, center.y );
CGPathAddLineToPoint( myPath, NULL,center.x + 100, center.y);
CGPathAddLineToPoint( myPath, NULL, center.x + 100, center.y + 100);
CGPathAddLineToPoint( myPath, NULL, center.x,  center.y + 100);
CGPathCloseSubpath  ( myPath );

I know this is just a square and that I could just use another CGRect but the path I wish to actually create is not actually a rectangle (I am just testing at the moment). And then detecting the touch area simply with a:

if (CGPathContainsPoint(myPath, nil, location, YES))

This all works fine, the problem is that the CGPath may be moving up to 40 times per second. How can I move it without having to create a new one? I know I can do something like this to “move” it:

center.y += x;
CGPathRelease(myPath);
myPath = CGPathCreateMutable();
CGPathMoveToPoint   ( myPath, NULL, center.x, center.y );
CGPathAddLineToPoint( myPath, NULL,center.x + 100, center.y);
CGPathAddLineToPoint( myPath, NULL, center.x + 100, center.y + 100);
CGPathAddLineToPoint( myPath, NULL, center.x,  center.y + 100);
CGPathCloseSubpath  ( myPath );

But I have to release and create a new path up to 40 times per second, which I think might have a performance penalty; is this true?

I would like to be able to move it just like I am currently moving some CGRects by simply setting the origin to a different value, is this possible with CGPath?

Thank you.

EDIT: I forgot to mention I don’t have a GraphicsContext as I am not drawing on a UIView.

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1 Answer

  1. Editorial Team

    Applying a transformation to the CGPath and test against a point, is equivalent to applying the inverse transformation to the point.

    Therefore, you can use

    CGPoint adjusted_point = CGPointMake(location.x - center.x, location.y - center.y);
if (CGPathContainsPoint(myPath, NULL, adjusted_point, YES))

    But CGPathContainsPoint already takes a CGAffineTransform parameter (which you have NULL-ed it), so you can also use

    CGAffineTransform transf = CGAffineTransformMakeTranslation(-center.x, -center.y);
if (CGPathContainsPoint(myPath, &transf, location, YES))

    If you are drawing, instead of changing the path, you can change the CTM in your drawing code directly. 

    CGContextSaveGState(c);
CGContextTranslateCTM(c, center.x, center.y);
// draw your path
CGContextRestoreGState(c);

    Use a CAShapeLayer if you need performance.

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