webgl中视锥体（投影矩阵，观察矩阵，形成的梯形体）的六个片面计算

计算六个平面的方法：
this.load = function (dataPtr) {
    this._planes[0] = new Plane(dataPtr[12] - dataPtr[0], dataPtr[13] - dataPtr[1], dataPtr[14] - dataPtr[2], dataPtr[15] - dataPtr[3]);
    this._planes[1] = new Plane(dataPtr[12] + dataPtr[0], dataPtr[13] + dataPtr[1], dataPtr[14] + dataPtr[2], dataPtr[15] + dataPtr[3]);

    this._planes[2] = new Plane(dataPtr[12] - dataPtr[4], dataPtr[13] - dataPtr[5], dataPtr[14] - dataPtr[6], dataPtr[15] - dataPtr[7]);
    this._planes[3] = new Plane(dataPtr[12] + dataPtr[4], dataPtr[13] + dataPtr[5], dataPtr[14] + dataPtr[6], dataPtr[15] + dataPtr[7]);

    this._planes[4] = new Plane(dataPtr[12] - dataPtr[8], dataPtr[13] - dataPtr[9], dataPtr[14] - dataPtr[10], dataPtr[15] - dataPtr[11]);
    this._planes[5] = new Plane(dataPtr[12] + dataPtr[8], dataPtr[13] + dataPtr[9], dataPtr[14] + dataPtr[10], dataPtr[15] + dataPtr[11]);

}

平面类：

function Plane(x,y,z,o) {
    this._normal = vec3.create();
    this._distance = 0.0;
    this._normal[0] = x;
    this._normal[1] = y;
    this._normal[2] = z;
    this._distance = o;

    //求任意一个点到这个点的距离
    this.distance = function (pos) {
        return vec3.dot(this._normal,pos)+ this._distance;
    }
}

这个方法它其实求的平面的法向量，用点到平面的距离来判断包围盒在不在视锥体范围内；