joint_test.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 "box2d/box2d.h"
#include "doctest.h"
#include <stdio.h>

DOCTEST_TEST_CASE("joint reactions")
{
        b2Vec2 gravity(0, -10.0f);
        b2World world = b2World(gravity);

        b2BodyDef bodyDef;
        b2Body* ground = world.CreateBody(&bodyDef);

        b2CircleShape circle;
        circle.m_radius = 1.0f;

        b2FixtureDef fixtureDef;

        // Disable collision
        fixtureDef.filter.maskBits = 0;
        fixtureDef.density = 1.0f;
        fixtureDef.shape = &circle;

        bodyDef.type = b2_dynamicBody;
        bodyDef.position.Set(-2.0f, 3.0f);

        b2Body* bodyA = world.CreateBody(&bodyDef);
        b2Body* bodyB = world.CreateBody(&bodyDef);
        b2Body* bodyC = world.CreateBody(&bodyDef);

        b2MassData massData;
        circle.ComputeMass(&massData, fixtureDef.density);
        const float mg = massData.mass * gravity.y;

        bodyA->CreateFixture(&fixtureDef);
        bodyB->CreateFixture(&fixtureDef);
        bodyC->CreateFixture(&fixtureDef);

        b2DistanceJointDef distanceJointDef;
        distanceJointDef.Initialize(ground, bodyA, bodyDef.position + b2Vec2(0.0f, 4.0f), bodyDef.position);
        distanceJointDef.minLength = distanceJointDef.length;
        distanceJointDef.maxLength = distanceJointDef.length;

        b2PrismaticJointDef prismaticJointDef;
        prismaticJointDef.Initialize(ground, bodyB, bodyDef.position, b2Vec2(1.0f, 0.0f));

        b2RevoluteJointDef revoluteJointDef;
        revoluteJointDef.Initialize(ground, bodyC, bodyDef.position);

        b2DistanceJoint* distanceJoint = (b2DistanceJoint*)world.CreateJoint(&distanceJointDef);
        b2PrismaticJoint* prismaticJoint = (b2PrismaticJoint*)world.CreateJoint(&prismaticJointDef);
        b2RevoluteJoint* revoluteJoint = (b2RevoluteJoint*)world.CreateJoint(&revoluteJointDef);

        const float timeStep = 1.f / 60.f;
        const float invTimeStep = 60.0f;
        const int32 velocityIterations = 6;
        const int32 positionIterations = 2;

        world.Step(timeStep, velocityIterations, positionIterations);

        const float tol = 1e-5f;
        {
                b2Vec2 F = distanceJoint->GetReactionForce(invTimeStep);
                float T = distanceJoint->GetReactionTorque(invTimeStep);
                CHECK(F.x == 0.0f);
                CHECK(b2Abs(F.y + mg) < tol);
                CHECK(T == 0.0f);
        }

        {
                b2Vec2 F = prismaticJoint->GetReactionForce(invTimeStep);
                float T = prismaticJoint->GetReactionTorque(invTimeStep);
                CHECK(F.x == 0.0f);
                CHECK(b2Abs(F.y + mg) < tol);
                CHECK(T == 0.0f);
        }

        {
                b2Vec2 F = revoluteJoint->GetReactionForce(invTimeStep);
                float T = revoluteJoint->GetReactionTorque(invTimeStep);
                CHECK(F.x == 0.0f);
                CHECK(b2Abs(F.y + mg) < tol);
                CHECK(T == 0.0f);
        }
}