Unexpected loss of precision when dividing doubles

Question

I have a function getSlope which takes as parameters 4 doubles and returns another double calculated using this given parameters in the following way:

double QSw         


        

Answer 1

Could it be that you use DirectX or OpenGL in your project? If so they can turn off double precision and you will get strange results.

You can check your precision settings with

std::sqrt(x) * std::sqrt(x)

The result has to be pretty close to x. I met this problem long time ago and spend a month checking all the formulas. But then I've found

D3DCREATE_FPU_PRESERVE