test if a point is inside a shape defined by quadratic B-splines

I got it working, based on the idea that if a point is inside it will need to pass through odd number of edges to escape in a linear direction. In my drawing I pick my Y, and get all intersections on that Y. Now I have an array of X values for a given Y. It worked fine, until I started using more complex shapes. I increased precision from float to double, and it made my results much better . But still not good enough.

I don't know how to debug precision. When I step through the program this line right here has issues


// this is from the expanded bezier formula
A = (double)p[0].y + (double)p[2].y - 2.0*(double)p[1].y;
// of course down the line I divide by 2.0*A
// after quadratic formula I get a 't' which I plug into the formula below to get a (x, y) point of intersection

// this is the interpolation formula

Vector2D n;
float t_u;
t_u = 1.0 - t;
n = p[0]*t_u*t_u + p[1]*2*t_u*t + p[2]*t*t;
// p is a vector


'A' goes really close to 0! I've been stuck on this problem for quite some time now. This is in my rendering loop I'm rendering a filled shape defined by this closed curve.