SharedShape 2D convex decomposition produces bad results if polygon is clockwise

[xlambein]

Hi!

I don't know whether this is a bug or a problem in the docs, but if you do a convex decomposition with a clockwise polygon, each shape in the decomposition will also be clockwise, and therefore will not work properly with SharedShape::convex_polyline. In turn, this means that the resulting compound shape will not work as expected.

parry/src/shape/shared_shape.rs

Lines 200 to 208 in d4b01c5

	let mut parts = vec![];
	let decomp = VHACD::decompose(params, &vertices, &indices, true);
	#[cfg(feature = "dim2")]
	for vertices in decomp.compute_exact_convex_hulls(&vertices, &indices) {
	if let Some(convex) = Self::convex_polyline(vertices) {
	parts.push((Isometry::identity(), convex));
	}
	}

The docs for SharedShape::convex_decomposition don't mention that shapes have to be counter-clockwise, and neither do the user guides. Similarly, the docs for SharedShape::convex_polyline don't mention that either, and it's only when looking at ConvexPolygon::from_convex_polyline that the counter-clockwise condition is ever mentioned.

Thanks for the amazing engine btw :-) people who played my LDJam game kept saying they were impressed by the physics, for which y'all are 100% responsible.

[sebcrozet]

Hi! Could you please try again with the master branch? It should be fixed by a recent PR (#53), but a new version of parry including this fix hasn’t been released yet.

[xlambein]

Ah sorry, I looked up the problem but only in issues, not in PRs! I'll try it out.

There's still the (small) detail about the docs for SharedShape::convex_polyline, but not sure it's worth keeping the issue open for that?

[sebcrozet]

Mmh, you are correct, the docs should be made more precise about the expected orientation!

but not sure it's worth keeping the issue open for that?

Let’s make another issue with a more suited title for that.

[xlambein]

Cool, I'll do that if you want :-)