Material combine mode hash table

[erincatto]

Store default on world. Each shape can have a material index 0-255 which 0 being default and using the world setting for 0-0.

I don't have a hash table now, just a hash set.

[MelvMay-Unity]

mixing-patch.patch

I think it would be more than adequate to do what PhysX does here for mixing friction/restitution per-shape by simply using the mixing rule that has the max value rather than a look-up table/hash: https://nvidia-omniverse.github.io/PhysX/physx/5.4.2/_api_build/struct_px_combine_mode.html?highlight=pxcombinemode#

They could also be compressed to a byte each and combined into a uint16 if memory was a concern.

[amytimed]

Rapier also has per-collider mixing rules like PhysX: https://rapier.rs/docs/user_guides/rust/colliders#friction

[MelvMay-Unity]

To add, the above patch would like use the following instead:

static inline float b2MixRules( float value1, float value2, b2MixingRule rule1, b2MixingRule rule2 )
{
	b2MixingRule mixingRule = (b2MixingRule)b2MaxInt( rule1, rule2 );

	switch ( mixingRule )
	{
		case b2_mixAverage:
			return 0.5f * ( value1 + value2 );

		case b2_mixGeometricMean:
			return sqrtf( value1 * value2 );

		case b2_mixMultiply:
			return value1 * value2;

		case b2_mixMinimum:
			return value1 < value2 ? value1 : value2;

		case b2_mixMaximum:
			return value1 > value2 ? value1 : value2;

		default:
			B2_ASSERT( false );
			return 0.0f;
	}
}

[erincatto]

Is there any justification for the precedence rule? It seems arbitrary. Why not have a hash table to look up the combine rule for two materials?

[erincatto]

There are now shape material identifiers and callbacks, so the user can mix friction and restitution however they like. #873