Physics.cpp

Physics.cpp

//
//  Physics.cpp
//  UDPTEST
//
//  Created by Christian Hohlfeld on 22.09.18.
//

#include "Physics.h"
#include <cocos2d.h>
using namespace cocos2d;

Physics::Physics() : fixedTimestepAccumulator_(0)
{
    b2Vec2 gravity(0, -9.8); //normal earth gravity, 9.8 m/s/s straight down!

    world = new b2World(gravity);
    world->SetAutoClearForces(false);
}
void Physics::tick(const float dt)
{
    static const float FIXED_TIMESTEP = 1.0f / 60.0f;
    static const int MAX_STEPS = 5;
    static const int velocityIterations = 8;
    static const int positionIterations = 1;
    
    fixedTimestepAccumulator_ += dt;
    int nSteps = static_cast<int>(floor(fixedTimestepAccumulator_/FIXED_TIMESTEP));
    if (nSteps > 0) {
        fixedTimestepAccumulator_ -= nSteps*FIXED_TIMESTEP;
    }
    
    assert("Invalid time accumulator state." &&  fixedTimestepAccumulator_ < FIXED_TIMESTEP + FLT_EPSILON);
    
    if (nSteps > MAX_STEPS) {
        nSteps = MAX_STEPS;
    }
    for (int i = 0; i < nSteps; ++i) {
        if (i == nSteps - 1) {
            renderFullStep();
        }
        world->Step(FIXED_TIMESTEP, velocityIterations, positionIterations);
    }
    world->ClearForces();
    renderPartStep(fixedTimestepAccumulator_/FIXED_TIMESTEP);
}
void Physics::renderFullStep(){
    for (b2Body *body = world->GetBodyList(); body; body = body->GetNext()) {
        if (body->GetType() != b2_staticBody) {
            CCNode *node = (CCNode *)body->GetUserData();
            if (node != NULL) {
                const b2Vec2 &bodyPos = body->GetPosition();
                node->setPosition(CCPoint(bodyPos.x * PTM, bodyPos.y * PTM));
                node->setRotation(-1 * CC_RADIANS_TO_DEGREES(body->GetAngle()));
            }
        }
    }
}
void Physics::renderPartStep(const float alpha){
    const float reminder = 1.0f - alpha;
    for (b2Body *body = world->GetBodyList(); body; body = body->GetNext()) {
        if (body->GetType() != b2_staticBody) {
            Node *node = (Node *)body->GetUserData();
            if (node != nullptr) {
                const Point &prevPos = node->getPosition();
                const b2Vec2 &bodyPos = body->GetPosition();
                const Point renderPos((bodyPos.x * alpha + prevPos.x/PTM * reminder) * PTM,
                                        (bodyPos.y * alpha + prevPos.y/PTM * reminder) * PTM);
                node->setPosition(renderPos);
                const float prevAngle = node->getRotation();
                const float bodyAngle = -1 * CC_RADIANS_TO_DEGREES(body->GetAngle());
                const float renderAngle = bodyAngle * alpha + prevAngle * reminder;
                node->setRotation(renderAngle);
            }
        }
    }
}