v3.1 Tasks

[erincatto]

• [ ] sample character mover
• [ ] smooth manifold merger
• [ ] world serialization
• [x] floating point determinism
• [ ] rolling resistance
• [ ] infinite planes
• [x] 64 bit category and mask bits
• [ ] clang-cl support in VS

[erincatto]

https://box2d.org/posts/2024/08/determinism/

[erincatto]

I investigated using realloc, but alignment is not well supported.

[erincatto]

Character movement reference: https://x.com/MaddyThorson/status/1238338574220546049

[erincatto]

world serialization in 3.2

[erincatto]

Removed infinite planes. Only one user asked for it so it is not widely useful or worth the effort.

[erincatto]

Removed smooth manifold merger because I took a different approach with the character mover

[erincatto]

Here are some of the technical problems with infinite planes:

• it is a new shape type
• it cannot be added to the broad-phase because it is too big, this means it needs special logic in many places
• contact points can be very far from the plane body's origin, leading to precision problems

I already recommend people don't create bodies bigger than ~20meters for precision reasons.

If someone makes a fork and shows how all these problems can be addressed without much complexity then I will reconsider.

[amytimed]

if most worlds only need one plane (the ground), wouldn't the performance hit of special-casing collision checks for a single, unique shape type be an acceptable trade-off?

even with precision limitations far from the origin, it might still be a good enough and super convenient solution for ground and other boundaries instead of needing huge finite boxes

[erincatto]

It would be an interesting to test a huge box and see how the stability looks far from the origin compared to a smaller ground box with an origin closer to the active bodies.

This would be somewhat similar to an infinite plane for stability.

A single infinite plane per world would be easier. It would be in the b2WorldDef.

[erincatto]

I tested the card house with sleeping disabled. It is a bit less stable with a huge ground box offset from the origin by 20km.

#if 1
		{
			b2BodyDef bodyDef = b2DefaultBodyDef();
			bodyDef.position = { 20000.0f, -5000.0f };
			b2BodyId groundId = b2CreateBody( m_worldId, &bodyDef );

			b2ShapeDef shapeDef = b2DefaultShapeDef();
			shapeDef.material.friction = 0.7f;

			b2Polygon groundBox = b2MakeBox( 30000.0f, 5000.0f );
			b2CreatePolygonShape( groundId, &shapeDef, &groundBox );
		}
#else
		{
			b2BodyDef bodyDef = b2DefaultBodyDef();
			bodyDef.position = { 0.0f, -2.0f };
			b2BodyId groundId = b2CreateBody( m_worldId, &bodyDef );

			b2ShapeDef shapeDef = b2DefaultShapeDef();
			shapeDef.material.friction = 0.7f;

			b2Polygon groundBox = b2MakeBox( 40.0f, 2.0f );
			b2CreatePolygonShape( groundId, &shapeDef, &groundBox );
		}
#endif