更新v1

Question

在上部点上的棱柱形接头（b2PrismaticJointDef）上的提升块给动力体提供了冲击力，即使提升块根本不移动，也会放置在提升块表面上。

也许是javascript端口的bug。但是我想修理它，因为我需要在游戏中使用电梯。

更新v1

我使用了Box2DFlash 2.1a https://code.google.com/p/box2dweb/的 box2dweb javascript端口。

更新v2

以下是关于flash http://hyzhak.github.com/darlingjs/performance/box2dweb/的类似演示，它有同样的问题，所以可能是flash或原装Box2D引擎上的这个问题。

var b2Vec2 = Box2D.Common.Math.b2Vec2,
    b2BodyDef = Box2D.Dynamics.b2BodyDef,
    b2Body = Box2D.Dynamics.b2Body,
    b2FixtureDef = Box2D.Dynamics.b2FixtureDef,
    b2World = Box2D.Dynamics.b2World,
    b2PolygonShape = Box2D.Collision.Shapes.b2PolygonShape,
    b2CircleShape = Box2D.Collision.Shapes.b2CircleShape,          
    b2DebugDraw = Box2D.Dynamics.b2DebugDraw,
    b2PrismaticJointDef = Box2D.Dynamics.Joints.b2PrismaticJointDef;

(function() {    
    var world = buildWorld();

    var leftBlock = buildBlock({world: world, x: 2, y: 8, width: 2, height: 12, static: true});
    var rightBlock = buildBlock({world: world, x: 12, y: 8, width: 2, height: 12, static: true});
    var bottomBlock = buildBlock({world: world, x: 7, y: 13, width: 8, height: 2, static: false});
    var box = buildBlock({world: world, x: 7, y: 10, width: 2, height: 2, static: false});

    var joint = buildPrismaticJoint({world: world, 
                                     anchorA: new b2Vec2(7, 13), 
                                     axis: new b2Vec2(0, 1), 
                                     bodyA: bottomBlock, 
                                     bodyB: world.GetGroundBody()});

    var debugDraw = buildDebugDraw(world);

    setInterval(function(){
        world.Step(1 / 60, 10, 10);
        world.DrawDebugData();
        world.ClearForces();
    },1000/60);
})();

function buildWorld() {    
    return new b2World(
        new b2Vec2(0, 10), //gravity vector
        true
    );
}

function buildBlock(state) {
    var fixDef = new b2FixtureDef;
    fixDef.shape = new b2PolygonShape;
    fixDef.density = 1.0;
    fixDef.friction = 0.5;
    fixDef.restitution = .5;         
    fixDef.shape.SetAsBox(state.width / 2, state.height / 2);
    var bodyDef = new b2BodyDef;
    bodyDef.type = state.static?b2Body.b2_staticBody:b2Body.b2_dynamicBody;
    bodyDef.position.Set(state.x, state.y);
    var body = state.world.CreateBody(bodyDef);
    body.CreateFixture(fixDef);
    return body;
}

//buildPrismaticJoint(world, 9, 15, 0, 1, bottomBlock, world.GetGroundBody());
function buildPrismaticJoint(state) {
    var jointDef = new b2PrismaticJointDef();
    jointDef.Initialize(state.bodyA, state.bodyB, state.anchorA, state.axis);
    jointDef.collideConnected = false;
    jointDef.lowerTranslation = 0.0;
    jointDef.upperTranslation = 5.0;
    jointDef.enableLimit = true;
    jointDef.maxMotorForce = 400.0;
    jointDef.motorSpeed = 3.0;
    jointDef.enableMotor = true;
    return state.world.CreateJoint(jointDef);
}

function buildDebugDraw(world) {
    var debugDraw = new b2DebugDraw();
    debugDraw.SetSprite(document.getElementById("playground").getContext("2d"));
    debugDraw.SetDrawScale(20.0);
    debugDraw.SetFillAlpha(0.5);
    debugDraw.SetLineThickness(1.0);
    debugDraw.SetFlags(
        b2DebugDraw.e_shapeBit | 
        b2DebugDraw.e_jointBit |
        b2DebugDraw.e_aabbBit |
        b2DebugDraw.e_pairBit |
        b2DebugDraw.e_centerOfMassBit |
        b2DebugDraw.e_controllerBit
    );
    world.SetDebugDraw(debugDraw);

    return debugDraw;
}

Answer 1

我使用Box2D v2.2.1 C ++代码遇到了同样的问题。我在静态物体和移动（动态）物体之间使用b2PrismaticJoint来模拟“升力”（或电梯）。

当电梯沿着棱柱接头到达平移的上端时，它似乎停止了。然而，当另一个车身坐在升降机的顶部并且它到达平移的上端时，升降机顶部的车身开始“反弹”，好像升力机仍在施加力。（我相信这正是发生的事情，即电梯的身体不会沿着棱柱关节进一步平移，但是棱柱关节的马达仍然在向身体施加向上的力量。）

我最初通过在达到平移的上端时手动将棱柱关节的电机速度设置为0.0来解决此问题。在我的主游戏循环中，即以帧速率调用的函数（Box2D中的“tick：”或Cocos2D中的“update：”），我插入了以下内容（我在这里使用Objective-C而kLiftJointValue是一个任意整数常量）：

// Iterate over all joints (used to prevent upper end oscillations from psimaticJoints on lifts)
for (b2Joint* j = world->GetJointList(); j; j = j->GetNext())
{
    if ((int)(j->GetUserData()) == kLiftJointValue) // check to see if lift is at upper end of translation 
    {
        CGFloat currentTranslation = ((b2PrismaticJoint*)j)->GetJointTranslation();
        CGFloat lowerLimit = ((b2PrismaticJoint*)j)->GetLowerLimit();
        CGFloat upperLimit = ((b2PrismaticJoint*)j)->GetUpperLimit();
        b2Body *bodyA = j->GetBodyA();
        b2Body *bodyB = j->GetBodyB();
        BOOL notStopped = (bodyA->GetType() == b2_dynamicBody || bodyB->GetType() == b2_dynamicBody);

        if (fabsf(currentTranslation - lowerLimit) <= 0.01*(upperLimit - lowerLimit) && notStopped )
        {
            CCLOG(@"Stopping platform");
            (j->GetBodyA())->SetType(b2_staticBody);
            (j->GetBodyB())->SetType(b2_staticBody);
            ((b2PrismaticJoint*)j)->SetMotorSpeed(0.0);
        }
    }
}

上面的代码简单地迭代了世界上的所有关节，并寻找适当标记的关节，如果身体已经转换到允许的最大平移的1％以内，那么移动体将变为静态并且电机被禁用将电机速度设置为0.您也可以使用Box2D碰撞检测并将关节的电机速度设置为0 ...

然后我意识到实现这种升力的最佳方式（不会扰乱坐在升降机顶部的任何东西）是逐渐减慢棱柱关节的电机速度。我就这样做了：

// Iterate over all joints (used to prevent upper end oscillations from psimaticJoints on lifts)
for (b2Joint* j = world->GetJointList(); j; j = j->GetNext())
{
    if ((int)(j->GetUserData()) == kLiftJointValue) // check to see if lift is at upper end of translation 
    {
        CGFloat currentTranslation = ((b2PrismaticJoint*)j)->GetJointTranslation();
        CGFloat lowerLimit = ((b2PrismaticJoint*)j)->GetLowerLimit();
        CGFloat upperLimit = ((b2PrismaticJoint*)j)->GetUpperLimit();
        b2Body *bodyA = j->GetBodyA();
        b2Body *bodyB = j->GetBodyB();
        BOOL notStopped = (bodyA->GetType() == b2_dynamicBody || bodyB->GetType() == b2_dynamicBody);

        if (fabsf(currentTranslation - lowerLimit) <= 0.25*(upperLimit - lowerLimit) && notStopped)
        {
            // Get current motor speed and update
            CGFloat currentSpeed = ((b2PrismaticJoint*)j)->GetMotorSpeed();
            CGFloat newSpeed;

            if (currentSpeed < 0.0)
            {
                if (fabsf(currentTranslation - lowerLimit) <= 0.01*(upperLimit - lowerLimit) && notStopped )
                {
                    CCLOG(@"Stopping platform");
                    (j->GetBodyA())->SetType(b2_staticBody);
                    (j->GetBodyB())->SetType(b2_staticBody);
                    ((b2PrismaticJoint*)j)->SetMotorSpeed(0.0);
                }
                else
                {
                    CCLOG(@"Decelerating: speed=%f", currentSpeed);
                    newSpeed = 0.95*currentSpeed;
                }
            }
            else if (currentSpeed > 0.0)
            {
                CCLOG(@"Accelerating: speed=%f", currentSpeed);

                newSpeed = 1.05*currentSpeed;

                if (newSpeed > 20.0)
                    newSpeed = 20.0;
            }

            // update motor speed
            ((b2PrismaticJoint*)j)->SetMotorSpeed(newSpeed);
        }            
    }
}

你需要使用各种常数（特别是常数0.95和1.05），因为它们针对特定的电机属性进行了微调（在我的情况下，maxMotorForce = 1000，motorSpeed = 20）。

当然，在我的项目的其他地方，我有定时器运行，根据需要重置升降机机身和关节电机属性（使其向上移动，然后通过以正值反转接头的电机速度来向下移动）。

当电梯在另一个（底部）极限附近转换时，我没有实施加速/减速技术，因为当电梯停止时我不关心乘坐电梯的东西在这个方向上施加的力。但是，当电梯从底部离开或返回底部时，可以使用与上述相同的方法平稳地加速或减速......

Box2D引擎javascript端口Prismatic Joint的奇怪行为

更新v1

更新v2

1 个答案: