求得的是 开平方的倒数，具体原理不懂

CarmSqrt.cpp

/* 来至 Quake 3 的源码 */
//求得的是 开平方的倒数，具体原理不懂
float CarmSqrt(float x){
	union{
		int intPart;
		float floatPart;
	} convertor;
	union{
		int intPart;
		float floatPart;
	} convertor2;
	convertor.floatPart = x;
	convertor2.floatPart = x;
	convertor.intPart = 0x1FBCF800 + (convertor.intPart >> 1);
	convertor2.intPart = 0x5f3759df - (convertor2.intPart >> 1);
	return 0.5f*(convertor.floatPart + (x * convertor2.floatPart));
}