angle calculation in 3D space

Let's say I have a point, t1(x1 y1 z1) somewhere in space. And I have another point g1(x2 y2 z2) that lies on a plane somewhere in space. If I have the normal direction associated with that plane,and I want to calculate the angle of point t1 relative to that plane at point g1, is the following code legit? (basically finding the angle between the vector g1-t1 and the normal of the plane)And does my vector from g1 to t1 have to be normalized? Or the normal direction of the plane, for that matter?

t1 = [1 4 3];
g1 = [2 4 3];
% normal associated with g1
n1 = [-1 0 0]; %<-- does this have to be normalized??
% get vector from g1 to t1:
v1 = g1-t1;
angle = atan2(norm(cross(v1,n1)),dot(v1,n1)).*(180/pi)

Thanks!

[Iman Alsharkawi]

Clearly it's been a long day for me. I think it just dawned on me that the normalization doesn't have to happen because all I care about are the directions of the vectors. But, I'd still like feedback on my method for calculating the angle.

Answer 1

[Sean de Wolski]

Your equation looks right to me. Well, that is it looks like what Roger recommends frequently:

CSSM: Roger Stafford on Angles

Answer 2

[Jan]

You can try it:

v = [1 4 3];
n = [-1 0 0];
angle1 = atan2(norm(cross(v,n)), dot(v,n)).*(180/pi)

n = [-1000 0 0];
angle2 = atan2(norm(cross(v,n)), dot(v,n)).*(180/pi)

I did not use g1 - t1, because it is [1,0,0] by accident, which might hide problems.

Usually this kind of gunshot debugging is not reliable. But if it helps... ;-)

Take into account, that a normalization can help to control rounding errors, even if the algorithm does not demands for a normalization mathematically, e.g. if n is [1e12, 0, 0] or [1e-12, 0, 0].

[Iman Alsharkawi]

=) like I said... it's been a long day... Thanks to both of you!