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pybox2d

Sep 01, 2021, 6 min read

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基本概念

  • Shape 形状
  • Rigid body/body 刚体
  • Fixture 夹具
    • 一个Shape可以绑定多个Fixture
  • 单位制：
    • 千克米秒，物体应在0.1到10米之间
    • 世界尺寸小于2km
  • 角度：逆时针为正向

创建物体

概述

  • 流程

Body

  • StaticBody

  • 可以先创建body再添加fixture

    self.item=self.world.CreateStaticBody(
    shape = someShape()
)
self.item.CreateEdgeFixture(vertices=[], density=, friction=)

  • DynamicBody 创建的同时添加fixture

    self.item=self.world.CreateDynamicBody(
    position(x, y),
    angle = ,
    fixtures = fixtureDef(
        shape = someShape(),
        density = ,
        friction = ,
        categoryBits = ,
        maskBits = , # collide only with something
        resitution = , # 0, no resitution, 1, very bouncy
)

    • 添加多个fixture:

      fixture=[fixture1, fixture2, ...]

    • 位置信息 可直接通过obj.position读取位置信息，可以直接和向量进行操作，如pos = obj.position+(x,y) 设置位置信息直接给obj.position赋值

    • 密度

      The density, usually in kg/m^2.

      The second parameter is the shape density in kilograms per meter squared. A static body has zero mass by definition, so the density is not used in this case.

      • 物体的质量与重量
        • 计算方法：物体的物理面积*density*gravity
        • Gravity在初始化world的时候配置 self.world = Box2D.b2World(gravity=(0, -self.g))

  • Body Type（可读写）:

    • 通过访问body的type属性，可以获知其是dynamic还是static type = bd.type type的定义为： Box2D.b2_staticBody=0 Box2D.b2_dynamicBody=2

  • fixtures（只读）

  • b2Bod类型数据所包含的属性(如world.contacts[0].fixtureA.body)

        b2Body(active=True,
       angle=0.0,
       angularDamping=0.0,
       angularVelocity=0.0,
       awake=False,
       bullet=False,
       contacts=[b2ContactEdge(contact=b2Contact(childIndexA=0,
                                                childIndexB=0,
                                                enabled=True,
                                                fixtureA=b2Fixture(body=(max recursion depth hit),...  )],
       fixedRotation=True,
       fixtures=[b2Fixture(body=b2Body(active=True,
                                      angle=0.0,
                                      angularDamping=0.0,
                                      angularVelocity=0.0,
                                      awake=False,...  )],
       inertia=0.0,
       joints=[b2JointEdge(joint=b2RevoluteJoint(active=True,
                                                anchorA=b2Vec2(7.93249,18.3333),
                                                anchorB=b2Vec2(7.93249,18.3333),
                                                angle=0.0,...  )],
       linearDamping=0.0,
       linearVelocity=b2Vec2(0,0),
       localCenter=b2Vec2(0,0),
       mass=1.0,
       massData=I=0.0,center=b2Vec2(0,0),mass=1.0,),
       position=b2Vec2(7.93249,18.3333),
       sleepingAllowed=True,
       transform=R=<Box2D.Box2D.b2Rot; proxy of <Swig Object of type 'b2Rot *' at 0x00000206F1E1AAB0> >,angle=0.0,position=b2Vec2(7.93249,18.3333),),
       type=2,
       userData='gripper',
       worldCenter=b2Vec2(7.93249,18.3333),
       )

Fixture

  • friction: 一般为0到1（由光滑到粗糙），可以是任何非负数
  • categoryBits, maskBits, groupIndex
    • categoryBits：物体是什么类型？
    • maskBits：会与哪个类碰撞？
    • 碰撞检测：参见碰撞检测
  • 设置为传感器之后（isSensor=True）碰撞设置失效，将穿过其它物体
  • Resitution：0~1（无弹性到非常弹）

形状

  • ChainShape()
  • CircleShape()
    • pos(), radius()
    circle = circleShape(pos=(1,2), radius=0.5)
  • PolygonShape()

控制

  • 施加一个作用力 obj.ApplyForceToCenter((1, 0,), True, ) 第一个数据，正值为施加的力向左，第二个数据，正值为施加的力向上
  • 修改位置 obj.position+=(1, 2) 第一个数据，正值向右移动，第二个数据，正值向上移动

关节

revoluteJointDef

weldJointDef

Instead it is better to create breakable bodies starting with a single body with multiple fixtures. When the body breaks, you can destroy a fixture and recreate it on a new body. See the Breakable example in the testbed.

创建与销毁关节

self.joint = self.world.CreateJoint(rjd) self.world.DestroyJoint(self.joint)

只读属性？

  • 读取关节角度（只读） rj.angle
  • 读取速度、扭矩? 
    rj.speed
rj.GetMotorTorque(inverse_dt)

可读写属性

rj.motorEnable = True
rj.motorSpeed = 5.0 (read-write)
rj.maxMotorTorque = 0 # (write-only)
rj.lowerLimit = -math.pi/2
rj.upperLimit =  math.pi/2

技巧

  • 模拟关节的摩擦阻力？

    You can use a joint motor to simulate joint friction. Just set the joint speed to zero, and set the maximum torque to some small, but significant value. The motor will try to prevent the joint from rotating, but will yield to a significant load.

  • 如果将gravity设置为0的同时通过position属性移动物体，关节可能不能正常工作
  • 通过joint motor达到模拟关节角度跟踪的效果——实测不一定好用

    You can also use joint motors to track a desired joint angle. For example:

    # ... Game Loop Begin ...
angleError = myJoint.angle - angleTarget
gain = 0.1
myJoint.SetMotorSpeed(-gain ** angleError)
# ... Game Loop End ...

碰撞检测

Contact（接触）

  • 碰撞是如何被判定的： 判定范例（A的类与B的掩码比较，B的类与A的掩码比较）： 
    uint16 catA = fixtureA.filter.categoryBits;
uint16 maskA = fixtureA.filter.maskBits;
uint16 catB = fixtureB.filter.categoryBits;
uint16 maskB = fixtureB.filter.maskBits;
 
if ((catA & maskB) != 0 && (catB & maskA) != 0)
{
// fixtures can collide
}
    两者（catA & maskB 与catB & maskA）必须皆为True才认定为碰撞
  • 遍历整个world中的碰撞self.world.contacts 获取所有碰撞的对象（list），用for循环遍历 
    for i in self.world.contacts
    访问示例： 
    u1 = i.fixtureA.body.userData
u2 = i.fixtureB.body.userData
    i->fixtureA->body->userData --->fixtureB->body->userData
  • 遍历body上的碰撞 
    for contact_edge in body.contacts:

Sensor

  • Sensor的定义

    Sometimes game logic needs to know when two fixtures overlap yet there should be no collision response. This is done by using sensors. A sensor is a fixture that detects collision but does not produce a response. You can flag any fixture as being a sensor. Sensors may be static or dynamic. Remember that you may have multiple fixtures per body and you can have any mix of sensors and solid fixtures. Sensors do not generate contact points. There are two ways to get the state of a sensor, b2Contact.touching, and during the b2ContactListener callbacks, BeginContact and EndContact.

  • Sensor的使用：
    • 首先通过设置category和mask使得物体之间会产生碰撞而不是相互穿过（穿过时的重叠不会被检测为contact）
    • 将物体的body设置为isSensor=True，此时它可以穿过一切物体
    • density等属性的设置依然有效

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