Poly-decomp and Uncaught TypeError: Cannot read properties of undefined (reading 'type')

[richbrill]

When generating a body from vertices which represent a concave polygon I see the following error in the console in a Webpack/Typescript project using matterjs running in Chrome browser Version 98.0.4758.80 (Official Build) (x86_64) on an Intel Mac:

bundle.js?84db9c7fcca2acde6e77:307 Uncaught TypeError: Cannot read properties of undefined (reading 'type') at Object.Composite.add (matter.js?b8bf:1798:1) at eval (index.ts?ffb4:29:1) at Module../src/index.ts (bundle.js?84db9c7fcca2acde6e77:105:1) at __webpack_require__ (bundle.js?84db9c7fcca2acde6e77:304:33) at bundle.js?84db9c7fcca2acde6e77:1360:37 at bundle.js?84db9c7fcca2acde6e77:1362:12

but only when I use Common.setDecomp(require('poly-decomp')); in the code.

I've debugged matterjs and I can see poly-decomp is imported and is being used by decomp.removeCollinearPoints and decomp.removeDuplicatePoints but it's reducing the number of sets of vertices to 6, rather than the 7 sets I have in this minimal reproduction use case: https://github.com/richbrill/matterjs-poly-decomp-bug

Uncommenting this line https://github.com/richbrill/matterjs-poly-decomp-bug/blob/afa97bec01ffc3fb87966469936b59f3e6151fdb/src/index.ts#L7 causes the issue

It feels like this might be a problem with poly-decomp but thought I may glean valuable insight by posting here.

[richbrill]

For anyone who might stumble across this I worked around it by ensuring my concave polygon vertices were listed counter-clockwise in the first place as described in poly-decomp readme: https://github.com/schteppe/poly-decomp.js

I think there might still be an issue though because even after matterjs calls decomp.makeCCW(concave); (passing in a clockwise concave polygon) matterjs still errored as above.

[liabru]

Thanks for the example case, hopefully it is just an issue with them not being CCW. Do you know if makeCCW gives different results to your manual fix?

[richbrill]

I can see why it's reducing the set of vertices from 7 to 6 since it calling decomp.removeDuplicatePoints so no problem there but after debugging and passing in a set of counter-clockwise vertices the decomp.makeCCW(concave); line in the Bodies.fromVertices method transforms those vertices into a set of clockwise vertices which matterjs doesn't mind.

If I pass in a set of clockwise vertices it transforms them into a set of counter-clockwise vertices which matter errors on even though matterjs mentions vertices should be in clockwise order: https://brm.io/matter-js/docs/classes/Vertices.html (mind you, that is only for creating vertices by themselves rather than passing as a parameter to the fromVertices method).

(I've updated the repo with a newer commit which includes a commented out set of clockwise vertices): https://github.com/richbrill/matterjs-poly-decomp-bug/commit/efdea9252af8f0e5f32b1c7983c8bf775daa9f08

It does not mention clockwise/counter-clockwise ordering on https://brm.io/matter-js/docs/classes/Bodies.html#method_fromVertices though so wondering if it should be added there for future readers?