iphone - CGPathRef intersection

Question

Is there a way to find out whether two CGPathRefs are intersected or not. In my case all the CGPaths are having closePath.

For example, I am having two paths. One path is the rectangle which is rotated with some angle and the other path is curved path. Two paths origin will be changing frequently. At some point they may intersect. I want to know when they are intersected. Please let me know if you have any solution.

Thanks in advance

Answer 1

Make one path the clipping path, draw the other path, then search for pixels that survived the clipping process:

// initialise and erase context
CGContextAddPath(context, path1);
CGContextClip(context);

// set fill colour to intersection colour
CGContextAddPath(context, path2);
CGContextFillPath(context);

// search for pixels that match intersection colour

This works because clipping = intersecting.

Don't forget that intersection depends on the definition of interiority, of which there are several. This code uses the winding-number fill rule, you might want the even odd rule or something else again. If interiority doesn't keep you up at night, then this code should be fine.

My previous answer involved drawing transparent curves to an RGBA context. This solution is superior to the old one because it is

1. simpler
2. uses a quarter of the memory as an 8bit greyscale context suffices
3. obviates the need for hairy, difficult-to-debug transparency code

Who could ask for more?

I guess you could ask for a complete implementation, ready to cut'n'paste, but that would spoil the fun and obfuscate an otherwise simple answer.

OLDER, HARDER TO UNDERSTAND AND LESS EFFICIENT ANSWER

Draw both CGPathRefs separately at 50% transparency into a zeroed, CGBitmapContextCreate-ed RGBA memory buffer and check for any pixel values > 128. This works on any platform that supports CoreGraphics (i.e. iOS and OSX).

In pseudocode

// zero memory

CGContextRef context;
context = CGBitmapContextCreate(memory, wide, high, 8, wide*4, CGColorSpaceCreateDeviceRGB(), kCGImageAlphaPremultipliedLast);
CGContextSetRGBFillColor(context, 1, 1, 1, 0.5);  // now everything you draw will be at 50%

// draw your path 1 to context

// draw your path 2 to context

// for each pixel in memory buffer
if(*p > 128) return true; // curves intersect  
else p+= 4; // keep looking

Let the resolution of the rasterised versions be your precision and choose the precision to suit your performance needs.