tests/box2d/Testbed/Tests/RopeJoint.h

/*
* Copyright (c) 2006-2010 Erin Catto http://www.box2d.org
*
* This software is provided 'as-is', without any express or implied
* warranty.  In no event will the authors be held liable for any damages
* arising from the use of this software.
* Permission is granted to anyone to use this software for any purpose,
* including commercial applications, and to alter it and redistribute it
* freely, subject to the following restrictions:
* 1. The origin of this software must not be misrepresented; you must not
* claim that you wrote the original software. If you use this software
* in a product, an acknowledgment in the product documentation would be
* appreciated but is not required.
* 2. Altered source versions must be plainly marked as such, and must not be
* misrepresented as being the original software.
* 3. This notice may not be removed or altered from any source distribution.
*/

#ifndef ROPE_JOINT_H
#define ROPE_JOINT_H

/// This test shows how a rope joint can be used to stabilize a chain of
/// bodies with a heavy payload. Notice that the rope joint just prevents
/// excessive stretching and has no other effect.
/// By disabling the rope joint you can see that the Box2D solver has trouble
/// supporting heavy bodies with light bodies. Try playing around with the
/// densities, time step, and iterations to see how they affect stability.
/// This test also shows how to use contact filtering. Filtering is configured
/// so that the payload does not collide with the chain.
class RopeJoint : public Test
{
public:
	RopeJoint()
	{
		b2Body* ground = NULL;
		{
			b2BodyDef bd;
			ground = m_world->CreateBody(&bd);

			b2EdgeShape shape;
			shape.Set(b2Vec2(-40.0f, 0.0f), b2Vec2(40.0f, 0.0f));
			ground->CreateFixture(&shape, 0.0f);
		}

		{
			b2PolygonShape shape;
			shape.SetAsBox(0.5f, 0.125f);

			b2FixtureDef fd;
			fd.shape = &shape;
			fd.density = 20.0f;
			fd.friction = 0.2f;
			fd.filter.categoryBits = 0x0001;
			fd.filter.maskBits = 0xFFFF & ~0x0002;

			b2RevoluteJointDef jd;
			jd.collideConnected = false;

			const int32 N = 10;
			const float32 y = 15.0f;
			m_ropeDef.localAnchorA.Set(0.0f, y);

			b2Body* prevBody = ground;
			for (int32 i = 0; i < N; ++i)
			{
				b2BodyDef bd;
				bd.type = b2_dynamicBody;
				bd.position.Set(0.5f + 1.0f * i, y);
				if (i == N - 1)
				{
					shape.SetAsBox(1.5f, 1.5f);
					fd.density = 100.0f;
					fd.filter.categoryBits = 0x0002;
					bd.position.Set(1.0f * i, y);
					bd.angularDamping = 0.4f;
				}

				b2Body* body = m_world->CreateBody(&bd);

				body->CreateFixture(&fd);

				b2Vec2 anchor(float32(i), y);
				jd.Initialize(prevBody, body, anchor);
				m_world->CreateJoint(&jd);

				prevBody = body;
			}

			m_ropeDef.localAnchorB.SetZero();

			float32 extraLength = 0.01f;
			m_ropeDef.maxLength = N - 1.0f + extraLength;
			m_ropeDef.bodyB = prevBody;
		}

		{
			m_ropeDef.bodyA = ground;
			m_rope = m_world->CreateJoint(&m_ropeDef);
		}
	}

	void Keyboard(unsigned char key)
	{
		switch (key)
		{
		case 'j':
			if (m_rope)
			{
				m_world->DestroyJoint(m_rope);
				m_rope = NULL;
			}
			else
			{
				m_rope = m_world->CreateJoint(&m_ropeDef);
			}
			break;
		}
	}

	void Step(Settings* settings)
	{
		Test::Step(settings);
		m_debugDraw.DrawString(5, m_textLine, "Press (j) to toggle the rope joint.");
		m_textLine += 15;
		if (m_rope)
		{
			m_debugDraw.DrawString(5, m_textLine, "Rope ON");
		}
		else
		{
			m_debugDraw.DrawString(5, m_textLine, "Rope OFF");
		}
		m_textLine += 15;
	}

	static Test* Create()
	{
		return new RopeJoint;
	}

	b2RopeJointDef m_ropeDef;
	b2Joint* m_rope;
};

#endif