Box2dの任意多边形[Ispooky]

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本文详细介绍了在Box2D中创建刚体的过程，包括刚体定义、形状定义、创建刚体、创建形状和计算质量等关键步骤，并提供了源码实例。重点展示了如何设置位置、角度、阻尼、贴图、形状属性以及静态物体的密度，以及如何使用Box2D的基本图形（圆形、矩形、多边形）创建不同类型的刚体。

一般来讲，创建一个刚体有五个步骤，具体为：

刚体定义，设置位置，角度，阻尼，贴图等相关属性

形状定义，高宽，密度，摩擦力，弹力等相关属性

创建刚体

创建形状

计算质量

注：

Box2D有三种基本图形，分别为圆形，矩形，多边形

刚体默认原点是在中心

SetAsBox函数接收的是半个宽度和半个高度，圆接收的是半径

创建多边形首先要指出顶点数（最多8个），在指点顶点位置即可，指定顶点必须以顺时针方向指定，否则会变成静态物体，因为其面积计算为负，导致质量=密度*面积为负

Box2D只支持凸多边形，因此要创建凹多边形需将其拆分为多个凸多边形

静态物体的密度为0

角度为弧度制

贴源码：
var bodyDef:b2BodyDef = new b2BodyDef();
var boxDef:b2PolygonDef = new b2PolygonDef();
var circleDef:b2CircleDef = new b2CircleDef();
var body:b2Body;

bodyDef.position.Set((Math.random() * 200 - 100 + 275) / RATIO, (Math.random() * -100 - 100) / RATIO);
circleDef.radius = (Math.random() * 10 + 10) / RATIO;
circleDef.density = 1.0;
circleDef.friction = 0.3;
circleDef.restitution = 0.3;
body = world.CreateBody(bodyDef);
body.CreateShape(circleDef);
body.SetMassFromShapes();

bodyDef.position.Set((Math.random() * 200 - 100 + 275) / RATIO, (Math.random() * -100 - 100) / RATIO);
boxDef.SetAsBox((Math.random() * 15 + 20) / 2 / RATIO, (Math.random() * 15 + 10) / 2 / RATIO);
boxDef.density = 1.0;
boxDef.friction = 0.3;
boxDef.restitution = 0.3;
body = world.CreateBody(bodyDef);
body.CreateShape(boxDef);
body.SetMassFromShapes();

bodyDef.position.Set((Math.random() * 200 - 100 + 275) / RATIO, (Math.random() * -100 - 100) / RATIO);
boxDef.vertexCount = 3;
boxDef.vertices[0].Set(0, (Math.random() * 10 + 15) / RATIO);
boxDef.vertices[1].Set((Math.random() * -10 - 10) / RATIO, (Math.random() * -10 - 15) / RATIO);
boxDef.vertices[2].Set((Math.random() * 10 + 10) / RATIO, (Math.random() * -10 - 15) / RATIO);
boxDef.density = 1.0;
boxDef.friction = 0.3;
boxDef.restitution = 0.3;
body = world.CreateBody(bodyDef);
body.CreateShape(boxDef);
body.SetMassFromShapes();

bodyDef.position.Set((Math.random() * 200 - 100 + 275) / RATIO, (Math.random() * -100 - 100) / RATIO);
boxDef.vertexCount = 4;
boxDef.vertices[0].Set((Math.random() * -10 - 10 ) / RATIO, (Math.random() * 10 + 10) / RATIO);
boxDef.vertices[1].Set((Math.random() * -10 - 5) / RATIO, (Math.random() * -10 - 10) / RATIO);
boxDef.vertices[2].Set((Math.random() * 10 + 5) / RATIO, (Math.random() * -10 - 10) / RATIO);
boxDef.vertices[3].Set((Math.random() * 10 + 10) / RATIO, (Math.random() * 10 + 10) / RATIO);
boxDef.density = 1.0;
boxDef.friction = 0.3;
boxDef.restitution = 0.3;
body = world.CreateBody(bodyDef);
body.CreateShape(boxDef);
body.SetMassFromShapes();

bodyDef.position.Set((Math.random() * 200 - 100 + 275) / RATIO, (Math.random() * -100 - 100) / RATIO);
boxDef.vertexCount = 5;
boxDef.vertices[0].Set(0, (Math.random() * 10 + 10) / RATIO);
boxDef.vertices[2].Set((Math.random() * -10 - 15) / RATIO, (Math.random() * -10 - 10) / RATIO);
boxDef.vertices[3].Set((Math.random() * 10 + 5) / RATIO, (Math.random() * 10 - 10) / RATIO);
boxDef.vertices[1].Set((boxDef.vertices[0].x + boxDef.vertices[2].x), (boxDef.vertices[0].y + boxDef.vertices[2].y));
boxDef.vertices[1].Multiply(Math.random() / 2 + 0.8);
boxDef.vertices[4].Set((boxDef.vertices[3].x + boxDef.vertices[0].x), (boxDef.vertices[3].y + boxDef.vertices[0].y));
boxDef.vertices[4].Multiply(Math.random() / 2 + 0.8);
boxDef.density = 1.0;
boxDef.friction = 0.3;
boxDef.restitution = 0.3;
body = world.CreateBody(bodyDef);
body.CreateShape(boxDef);
body.SetMassFromShapes();

运行结果：