Question

I have been scratching my head on this for quite a while now. Guess I should have paid more attention in the trigonometry math classes when I was younger but here we go:

I have an angle and a point. I then want to put a second point in the direction of the angle, 200 units away from the first point. I use Atan2 to get the angle, then cos and sin to get the third point. But… I think something goes wrong when calculating the Sin for p3.Y.

EDIT: To clarify, I removed p2 and used the angle directly:

        PointF p1 = new PointF(20, 20);
        double angle = 1.3034851559678624f;

        //Create a new PointF in the same direction, 200 pixels away from p1
        //{ X = 199,9482, Y = 4,549629 }
        PointF p3 = new PointF
        {
            X = (float)(Math.Cos(Math.PI * angle / 180.0) * 200),
            Y = (float)(Math.Sin(Math.PI * angle / 180.0) * 200)
        };

        //This is where I would expect 1.3034851559678624 as the first angle 
        //but I get -4.9073849244837184
        double angle2 = Math.Atan2(p3.Y - p1.Y, p3.X - p1.X) * 180 / Math.PI;

Here is a visual representation of the values above. The green line is the first angle.

Answer 1

The problem with your current code is that you don’t calculate p3 relative to p1. You need to add p1.X and p1.Y to the coordinates of p3:

PointF p3 = new PointF
{
    X = p1.X + (float)(Math.Cos(Math.PI * angle / 180.0) * 200),
    Y = p1.Y + (float)(Math.Sin(Math.PI * angle / 180.0) * 200)
};