声明三维点POINT类，重载运算符“+”“-“ ”=”，实现三维点的加减和赋值-POINT-type three-dimensional point statement, operator overloading "+" "-" " =" , the realization of three-dimensional point of addition and subtraction and assignment

声明三维点POINT类，重载运算符“+”“-“”=”，实现三维点的加减和赋值-POINT-type three-dimensional point statement, operator overloading "+" "-" " =" , the realization of three-dimensional point of addition and subtraction and assignment

简单光线追踪实例,运用到C++中成员函数，运算符重载和引用的知识。(a simple example of ray trace)