From: Mikko Rasa <tdb@tdb.fi>
Date: Mon, 21 Nov 2022 11:02:03 +0000 (+0200)
Subject: Simulate friction in collisions
X-Git-Url: http://git.tdb.fi/?a=commitdiff_plain;h=9ce882cbb6bb7628d7c9883c43491a75c10625d7;p=libs%2Fgame.git

Simulate friction in collisions
---

diff --git a/examples/bassteroids/source/gamecontroller.cpp b/examples/bassteroids/source/gamecontroller.cpp
index d39b343..b564033 100644
--- a/examples/bassteroids/source/gamecontroller.cpp
+++ b/examples/bassteroids/source/gamecontroller.cpp
@@ -8,7 +8,7 @@ using namespace Msp;
 
 GameController::GameController(Game::Stage &s):
 	System(s),
-	asteroid_setup{ .physical={ .body={ .mass=200 }, .collider={ .type=ColliderType::CIRCLE, .radius=3.0f } },
+	asteroid_setup{ .physical={ .body={ .mass=200, .moment_of_inertia=2160 }, .collider={ .type=ColliderType::CIRCLE, .radius=3.0f } },
 		.mesh={ .object_name="Asteroid 1.object" }}
 { }
 
diff --git a/examples/bassteroids/source/physics.cpp b/examples/bassteroids/source/physics.cpp
index 0fb4326..2453fd9 100644
--- a/examples/bassteroids/source/physics.cpp
+++ b/examples/bassteroids/source/physics.cpp
@@ -70,12 +70,15 @@ void Physics::copy_in(SimulatedEntity &entity)
 	entity.rotation = Geometry::atan2<float>(2*(r.a*r.d+r.b*r.c), 1-2*(r.c*r.c+r.d*r.d));
 
 	if constexpr(is_fixture)
+	{
 		entity.inverse_mass = 0.0f;
+		entity.inverse_momi = 0.0f;
+	}
 	else
 	{
 		Game::Handle<RigidBody> body = entity.entity->get_body();
 		entity.inverse_mass = 1.0f/body->get_mass();
-		entity.moment_of_inertia = body->get_moment_of_inertia();
+		entity.inverse_momi = 1.0f/body->get_moment_of_inertia();
 		entity.external_force = body->get_force();
 		entity.external_torque = body->get_torque();
 		entity.velocity = body->get_velocity();
@@ -105,11 +108,11 @@ void Physics::step(float dt_secs)
 	{
 		SimulatedEntity &entity = entities[i];
 
-		LinAl::Vector<float, 2> new_velocity = entity.velocity+entity.external_force*dt_secs*entity.inverse_mass;
+		LinAl::Vector<float, 2> new_velocity = entity.velocity+entity.external_force*(dt_secs*entity.inverse_mass);
 		entity.position += (entity.velocity+new_velocity)*(dt_secs/2);
 		entity.velocity = new_velocity;
 
-		Geometry::Angle<float> new_angular_velocity = entity.angular_velocity+Geometry::Angle<float>::from_radians(entity.external_torque*(dt_secs/entity.moment_of_inertia));
+		Geometry::Angle<float> new_angular_velocity = entity.angular_velocity+Geometry::Angle<float>::from_radians(entity.external_torque*dt_secs*entity.inverse_momi);
 		entity.rotation = wrap_positive(entity.rotation+(entity.angular_velocity+new_angular_velocity)*(dt_secs/2));
 		entity.angular_velocity = new_angular_velocity;
 	}
@@ -178,12 +181,22 @@ void Physics::apply_impulses()
 	{
 		SimulatedEntity &entity1 = entities[c.body1];
 		SimulatedEntity &entity2 = entities[c.body2];
-		LinAl::Vector<float, 2> v_rel = entity2.velocity-entity1.velocity;
+		LinAl::Vector<float, 2> r1 = c.point-entity1.position;
+		LinAl::Vector<float, 2> r2 = c.point-entity2.position;
+		LinAl::Vector<float, 2> v_p1 = entity1.velocity+LinAl::Vector<float, 2>(-r1.y, r1.x)*entity1.angular_velocity.radians();
+		LinAl::Vector<float, 2> v_p2 = entity2.velocity+LinAl::Vector<float, 2>(-r2.y, r2.x)*entity2.angular_velocity.radians();
+		LinAl::Vector<float, 2> v_rel = v_p2-v_p1;
+		LinAl::Vector<float, 2> tangent = normalize(v_rel-c.normal*inner_product(v_rel, c.normal));
 		float restitution = 1.0f;
+		float friction_coeff = 0.1f;
 		float inv_mass_sum = entity1.inverse_mass+entity2.inverse_mass;
-		float impulse = (1+restitution)*inner_product(v_rel, c.normal)/inv_mass_sum;
-		entity1.velocity += c.normal*(impulse*entity1.inverse_mass);
-		entity2.velocity -= c.normal*(impulse*entity2.inverse_mass);
+		float reaction = (1+restitution)*inner_product(v_rel, c.normal)/inv_mass_sum;
+		float friction = reaction*friction_coeff;
+		LinAl::Vector<float, 2> impulse = c.normal*reaction+tangent*friction;
+		entity1.velocity += impulse*entity1.inverse_mass;
+		entity2.velocity -= impulse*entity2.inverse_momi;
+		entity1.angular_velocity += Geometry::Angle<float>::from_radians(entity1.inverse_momi*(r1.x*impulse.y-r1.y*impulse.x));
+		entity2.angular_velocity -= Geometry::Angle<float>::from_radians(entity2.inverse_momi*(r2.x*impulse.y-r2.y*impulse.x));
 	}
 }
 
diff --git a/examples/bassteroids/source/physics.h b/examples/bassteroids/source/physics.h
index 078d2e6..096e228 100644
--- a/examples/bassteroids/source/physics.h
+++ b/examples/bassteroids/source/physics.h
@@ -17,7 +17,7 @@ private:
 	{
 		Msp::Game::Handle<PhysicalEntity> entity;
 		float inverse_mass = 1.0f;
-		float moment_of_inertia = 1.0f;
+		float inverse_momi = 1.0f;
 		Msp::LinAl::Vector<float, 2> external_force;
 		float external_torque = 0.0f;