Trim/subtract CGPath from CGPath?

Question

I have some CGPath (a poly with two circle) carried out on a CAShapeLayer.

Can I trim/subtract paths like this?

Melvamelvena

Short answer: No. Long answer: You can but it's pretty hard. – Balling

Thanks :) I can do it with by adding layer mask as a fallback, but I wanted to supress rasterization passes, would prefer math. – Hanley

Thanks there. It was made during design process, but was a perfect fit for this question. :) – Hanley

Answer 1

Instead of do the math, the result can be achieved at drawing time using kCGBlendModeClear.

// Create the poly.
CGContextBeginPath(context);
CGContextMoveToPoint       (context, a_.x, a_.y);
CGContextAddLineToPoint    (context, b_.x, b_.y);
CGContextAddLineToPoint    (context, c_.x, c_.y);
CGContextAddLineToPoint    (context, d_.x, d_.y);
CGContextClosePath(context);

// Draw with the desired color.
CGContextSetFillColorWithColor(context, self.color.CGColor);
CGContextFillPath(context);

// Create a circle.
CGContextBeginPath(context);
CGContextAddEllipseInRect(context, (CGRect){
    b_.x - self.radius,
    b_.y - self.radius,
    self.radius * 2.0,
    self.radius * 2.0 });
CGContextClosePath(context);

// Draw with Clear (!) blend mode.
CGContextSetBlendMode(context, kCGBlendModeClear);
CGContextSetFillColorWithColor(context, self.color.CGColor);
CGContextFillPath(context);

Melvamelvena

Answer 2

Based on Geri's answer, using Swift 5

context.addPath(path1) // your original CGPath
context.setFillColor(color1) // your color
context.fillPath()
        
context.addPath(path2) // your clearing CGPath
context.setBlendMode(.clear) // draw with clear mode
context.fillPath()

Conjoint