Drawing a circle segment
Overall, this is pretty good. It's straightforward and to the point. Here are some other thoughts.

Don't Declare PI

You don't need to declare π. It's already declared in math.h as M_PI. Though in C++, it's preferred to #include &lt;cmath&gt; instead, which will put the standard math functions into the std namespace. In C++11 and later, it's suggested to use a calculation to define it such as:

const auto kPi = std::acos(-1.0);


Declare Functions constexpr When Appropriate

You don't say whether you're using C++11 or higher, or not. If you are, I would recommend declaring your degToRad() function constexpr so that it can be used in the definition of constants.

Don't Repeat Yourself

You have some code inside your loop that always calculates the same thing each iteration. The definition of mRelP, and mouseAngle should be moved outside the loop so you do less work in the loop.

Naming

mRelP is a confusing name. In many codebases it's a convention to prepend "m" to member variables. I'd just call it something more readable like mouseRelPos.