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angle calculation in 3D space

Asked by Iman Alsharkawi on 13 Jul 2011

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!

1 Comment

Iman Alsharkawi on 13 Jul 2011

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.

Iman Alsharkawi

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2 Answers

Answer by Sean de Wolski on 13 Jul 2011
Accepted answer

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

CSSM: Roger Stafford on Angles

0 Comments

Sean de Wolski
Answer by Jan Simon on 13 Jul 2011

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].

1 Comment

Iman Alsharkawi on 13 Jul 2011

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

Jan Simon

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