getCrossings() returns touching paths which do not cross each other

[richnicholls404]

In the documentation, Path.getCrossings() says

Crossings are intersections where the paths actually are crossing each other, as opposed to simply touching.

However, with the following code, these non-intersecting but touching paths are reported as crossing:

var path = new Path({
    segments: [[20, 20], [20, 80], [80, 80], [80, 20]],
    fillColor: 'blue',
    closed: true
});

var secondPath = new Path({
    segments: [[80, 20], [80, 80], [140, 80], [140, 20]],
    fillColor: 'green',
    closed: true
});

var crossings = path.getCrossings(secondPath);
console.log('crossings', crossings);
for (var i = 0; i < crossings.length; i++) {
	new Path.Circle({
		center: crossings[i].point,
		radius: 4,
		fillColor: 'red'
	});
}

Loving paper.js! It's absolutely amazing and I am very thankful for such a great library! Thank you!!

[lehni]

Yes there are a ton of edge cases where this doesn't work as it should. Unfortunately I have very little time available to further improve anything on paper.js : /

[richnicholls404]

I can completely understand. It's a very complex area of the library too. In case it helps you or anyone else who has this issue, I'll detail my fix.

In order to find out if a path overlapped another path (but without returning as an overlap if it happened to be touching), I ended up using something like this:

const intersectingPath = PathItem.intersect(secondPath);
If (intersectingPath.segments.length === 0) console.log('does not overlap');

Thanks for your speedy feedback!

[renschler]

I'm not sure if this should be logged as a separate issue.

But if you take two identical simple paths, and run getCrossings, should this return empty? Today it does not.

For example, see this sketch

[lehni]

@renschler yeah this is a a bit of a workaround that was included for the boolean operations to work correctly, but it shouldn't affect the public API like this. See the TODO here:

paper.js/src/path/PathItem.js

Lines 354 to 360 in b51a4be

	// TODO: Only return overlaps that are actually crossings! For this
	// we need proper overlap range detection / merging first...
	// But as we call #resolveCrossings() first in boolean operations,
	// removing all self-touching areas in paths, this currently works
	// as it should in the known use cases.
	// The ideal implementation would deal with it in a way outlined in:
	// https://github.com/paperjs/paper.js/issues/874#issuecomment-168332391

You can work around this for now by doing this instead:

const crossings = path1.getIntersections(path2, inter => inter.isCrossing());

Updated Sketch