'polygonIntersectsPolygon' fails detecting intersecting polygons in special cases

[robertkovacs]

In version 2.2.1, using the following two polygons:

[[180, 140], [180, 205]] and

[[160, 180], [170, 181.72], [180, 187.64], [185, 193.41]]

'polygonIntersectsPolygon' gives false, though these two polygons clearly do intersect. The problem is in the 'lineIntersectsLine' function, where replacing the original

return 0 < lambda && lambda < 1 && 0 < gamma && gamma < 1;

line by

return 0 <= lambda && lambda <= 1 && 0 <= gamma && gamma <= 1;

allows detecting 'endpoint intersections'.

With this modification the 'polygonIntersectsPolygon' function gives a correct result.

Tested in Safari Version 13.1 (15609.1.20.111.8).

[HarryStevens]

Hi @robertkovacs. I was unable to reproduce this.

const geometric = require("geometric");

const a = [[180, 140], [180, 205]],
      b = [[160, 180], [170, 181.72], [180, 187.64], [185, 193.41]];

console.log(geometric.polygonIntersectsPolygon(a, b)); // true
console.log(geometric.polygonIntersectsPolygon(b, a)); // true

I've also added it to the test suite: https://github.com/HarryStevens/geometric/blob/master/test/polygonIntersectsPolygon-test.js#L40

[anhkieet]

geometric.polygonIntersectsPolygon also returns true if two overlapping rectangles share two edges. @HarryStevens Can you guide me in this case? I want the function above return false if they share two edges. What should I do?

[HarryStevens]

geometric.polygonIntersectsPolygon also returns true if two overlapping rectangles share two edges. @HarryStevens Can you guide me in this case? I want the function above return false if they share two edges. What should I do?

I can't picture this. What it would it look like for two rectangles to share two edges but not overlap?

[anhkieet]

On Mon, 1 Nov 2021 at 03:55 Harry Stevens @.***> wrote:

geometric.polygonIntersectsPolygon also returns true if two overlapping rectangles share two edges. @HarryStevens https://github.com/HarryStevens Can you guide me in this case? I want the function above return false if they share two edges. What should I do?

I can't picture this. What it would it look like for two rectangles to share two edges but not overlap?

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Yes they share edges but not overlap--

[HarryStevens]

I don't think that is possible. Can you show me a picture of what you mean?