axmol/tests/cpp-tests/Source/Box2DTestBed/tests/wrecking_ball.cpp

// 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
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#include "test.h"
#include "ImGui/ImGuiPresenter.h"

/// This test shows how a distance joint can be used to stabilize a chain of
/// bodies with a heavy payload. Notice that the distance joint just prevents
/// excessive stretching and has no other effect.
/// By disabling the distance 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 WreckingBall : public Test
{
public:
    WreckingBall()
    {
        b2Body* ground = NULL;
        {
            b2BodyDef bd;
            ground = m_world->CreateBody(&bd);

            b2EdgeShape shape;
            shape.SetTwoSided(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 float y = 15.0f;
            m_distanceJointDef.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)
                {
                    bd.position.Set(1.0f * i, y);
                    bd.angularDamping = 0.4f;
                }

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

                if (i == N - 1)
                {
                    b2CircleShape circleShape;
                    circleShape.m_radius = 1.5f;
                    b2FixtureDef sfd;
                    sfd.shape               = &circleShape;
                    sfd.density             = 100.0f;
                    sfd.filter.categoryBits = 0x0002;
                    body->CreateFixture(&sfd);
                }
                else
                {
                    body->CreateFixture(&fd);
                }

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

                prevBody = body;
            }

            m_distanceJointDef.localAnchorB.SetZero();

            float extraLength            = 0.01f;
            m_distanceJointDef.minLength = 0.0f;
            m_distanceJointDef.maxLength = N - 1.0f + extraLength;
            m_distanceJointDef.bodyB     = prevBody;
        }

        {
            m_distanceJointDef.bodyA = ground;
            m_distanceJoint          = m_world->CreateJoint(&m_distanceJointDef);
            m_stabilize              = true;
        }
    }

    void UpdateUI() override
    {
        //		ImGui::SetNextWindowPos(ImVec2(Test::g_debugDraw.debugNodeOffset.x, 100.0f));
        ImGui::SetNextWindowSize(ImVec2(200.0f, 100.0f));
        ImGui::Begin("Wrecking Ball Controls", nullptr, ImGuiWindowFlags_NoResize);

        if (ImGui::Checkbox("Stabilize", &m_stabilize))
        {
            if (m_stabilize == true && m_distanceJoint == nullptr)
            {
                m_distanceJoint = m_world->CreateJoint(&m_distanceJointDef);
            }
            else if (m_stabilize == false && m_distanceJoint != nullptr)
            {
                m_world->DestroyJoint(m_distanceJoint);
                m_distanceJoint = nullptr;
            }
        }

        ImGui::End();
    }

    void Step(Settings& settings) override
    {
        Test::Step(settings);

        if (m_distanceJoint)
        {
            DrawString(5, m_textLine, "Distance Joint ON");
        }
        else
        {
            DrawString(5, m_textLine, "Distance Joint OFF");
        }
    }

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

    b2DistanceJointDef m_distanceJointDef;
    b2Joint* m_distanceJoint;
    bool m_stabilize;
};

static int testIndex = RegisterTest("Examples", "Wrecking Ball", WreckingBall::Create);