Math problem when comparing vector (3d) angles [Solved]

@RIL wrote:

I’m comparing two 3d Vectors using Dot Product, but I keep getting strange results. I compare the yellow Vector3d (n), a face normal, with the “cube-direction” Vector3d a. The Box is planar (XY) and therefore it regards only the Planar (XY) angle of vector a. But when I try calculate the angle between the two vectors I don’t get expected results(n is not parallel to n (0 degree) where it should be parallel):

bild.png892x412 55.4 KB

In the code below the variable m_dirToDelete is the vector “a” pictured above :

if ( m_dirToDelete.Length > 0 )
{
	// Test the face normal direction angle against the m_dirToDelete direction.
	// Remove the MeshFace if less than x° degree tolerance (+/- 1°).
	//
	Vector3d n = normal;
	Vector3d a = m_dirToDelete;

	// Project both vectors down to XY plane (planarize)
	//
	a.Z = 0;
	n.Z = 0;

	// Get the Dot product
	//
	a.Unitize();
	n.Unitize();

	double dot = a.X * n.X + a.Y * n.Y; // + a.Z * n.Z;  0-values are meaningless.
	double deg = Math.Acos(dot) * (180/Math.PI);

	// Record  MeshFace index, if the vectors are within angle tolerance of each other
	//
	if ( deg < 1 && deg > -1 )
	{
	    //face index to be deleted
	    FaceIndiciesToDelete.Add(i);
	}
}

So, something is wrong with the math, since I know that there are several yellow faces (n) having the same (planar XY) direction as the red arrow (a).

What am I doing wrong?

// Rolf

Posts: 4

Participants: 3

Read full topic