apply_force.cpp

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// MIT License

// Copyright (c) 2019 Erin Catto

// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:

// The above copyright notice and this permission notice shall be included in all
// copies or substantial portions of the Software.

// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
// SOFTWARE.

#include "test.h"

// This test shows how to apply forces and torques to a body.
// It also shows how to use the friction joint that can be useful
// for overhead games.
class ApplyForce : public Test
{
public:
        ApplyForce()
        {
                m_world->SetGravity(b2Vec2(0.0f, 0.0f));

                const float k_restitution = 0.4f;

                b2Body* ground;
                {
                        b2BodyDef bd;
                        bd.position.Set(0.0f, 20.0f);
                        ground = m_world->CreateBody(&bd);

                        b2EdgeShape shape;

                        b2FixtureDef sd;
                        sd.shape = &shape;
                        sd.density = 0.0f;
                        sd.restitution = k_restitution;

                        // Left vertical
                        shape.SetTwoSided(b2Vec2(-20.0f, -20.0f), b2Vec2(-20.0f, 20.0f));
                        ground->CreateFixture(&sd);

                        // Right vertical
                        shape.SetTwoSided(b2Vec2(20.0f, -20.0f), b2Vec2(20.0f, 20.0f));
                        ground->CreateFixture(&sd);

                        // Top horizontal
                        shape.SetTwoSided(b2Vec2(-20.0f, 20.0f), b2Vec2(20.0f, 20.0f));
                        ground->CreateFixture(&sd);

                        // Bottom horizontal
                        shape.SetTwoSided(b2Vec2(-20.0f, -20.0f), b2Vec2(20.0f, -20.0f));
                        ground->CreateFixture(&sd);
                }

                {
                        b2Transform xf1;
                        xf1.q.Set(0.3524f * b2_pi);
                        xf1.p = xf1.q.GetXAxis();

                        b2Vec2 vertices[3];
                        vertices[0] = b2Mul(xf1, b2Vec2(-1.0f, 0.0f));
                        vertices[1] = b2Mul(xf1, b2Vec2(1.0f, 0.0f));
                        vertices[2] = b2Mul(xf1, b2Vec2(0.0f, 0.5f));
                        
                        b2PolygonShape poly1;
                        poly1.Set(vertices, 3);

                        b2FixtureDef sd1;
                        sd1.shape = &poly1;
                        sd1.density = 2.0f;

                        b2Transform xf2;
                        xf2.q.Set(-0.3524f * b2_pi);
                        xf2.p = -xf2.q.GetXAxis();

                        vertices[0] = b2Mul(xf2, b2Vec2(-1.0f, 0.0f));
                        vertices[1] = b2Mul(xf2, b2Vec2(1.0f, 0.0f));
                        vertices[2] = b2Mul(xf2, b2Vec2(0.0f, 0.5f));
                        
                        b2PolygonShape poly2;
                        poly2.Set(vertices, 3);

                        b2FixtureDef sd2;
                        sd2.shape = &poly2;
                        sd2.density = 2.0f;

                        b2BodyDef bd;
                        bd.type = b2_dynamicBody;

                        bd.position.Set(0.0f, 3.0);
                        bd.angle = b2_pi;
                        bd.allowSleep = false;
                        m_body = m_world->CreateBody(&bd);
                        m_body->CreateFixture(&sd1);
                        m_body->CreateFixture(&sd2);

                        float gravity = 10.0f;
                        float I = m_body->GetInertia();
                        float mass = m_body->GetMass();

                        // Compute an effective radius that can be used to
                        // set the max torque for a friction joint
                        // For a circle: I = 0.5 * m * r * r ==> r = sqrt(2 * I / m)
                        float radius = b2Sqrt(2.0f * I / mass);

                        b2FrictionJointDef jd;
                        jd.bodyA = ground;
                        jd.bodyB = m_body;
                        jd.localAnchorA.SetZero();
                        jd.localAnchorB = m_body->GetLocalCenter();
                        jd.collideConnected = true;
                        jd.maxForce = 0.5f * mass * gravity;
                        jd.maxTorque = 0.2f * mass * radius * gravity;

                        m_world->CreateJoint(&jd);
                }

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

                        b2FixtureDef fd;
                        fd.shape = &shape;
                        fd.density = 1.0f;
                        fd.friction = 0.3f;

                        for (int i = 0; i < 10; ++i)
                        {
                                b2BodyDef bd;
                                bd.type = b2_dynamicBody;

                                bd.position.Set(0.0f, 7.0f + 1.54f * i);
                                b2Body* body = m_world->CreateBody(&bd);

                                body->CreateFixture(&fd);

                                float gravity = 10.0f;
                                float I = body->GetInertia();
                                float mass = body->GetMass();

                                // For a circle: I = 0.5 * m * r * r ==> r = sqrt(2 * I / m)
                                float radius = b2Sqrt(2.0f * I / mass);

                                b2FrictionJointDef jd;
                                jd.localAnchorA.SetZero();
                                jd.localAnchorB.SetZero();
                                jd.bodyA = ground;
                                jd.bodyB = body;
                                jd.collideConnected = true;
                                jd.maxForce = mass * gravity;
                                jd.maxTorque = 0.1f * mass * radius * gravity;

                                m_world->CreateJoint(&jd);
                        }
                }
        }

        void Step(Settings& settings) override
        {
                g_debugDraw.DrawString(5, m_textLine, "Forward (W), Turn (A) and (D)");
                m_textLine += m_textIncrement;

                if (glfwGetKey(g_mainWindow, GLFW_KEY_W) == GLFW_PRESS)
                {
                        b2Vec2 f = m_body->GetWorldVector(b2Vec2(0.0f, -50.0f));
                        b2Vec2 p = m_body->GetWorldPoint(b2Vec2(0.0f, 3.0f));
                        m_body->ApplyForce(f, p, true);
                }

                if (glfwGetKey(g_mainWindow, GLFW_KEY_A) == GLFW_PRESS)
                {
                        m_body->ApplyTorque(10.0f, true);
                }

                if (glfwGetKey(g_mainWindow, GLFW_KEY_D) == GLFW_PRESS)
                {
                        m_body->ApplyTorque(-10.0f, true);
                }

                Test::Step(settings);
        }

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

        b2Body* m_body;
};

static int testIndex = RegisterTest("Forces", "Apply Force", ApplyForce::Create);