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Asked: 2026-05-10T17:37:49+00:00

I’ve been playing with some algorithms on the internet for a while and I

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I’ve been playing with some algorithms on the internet for a while and I can’t seem to get them to work, so I’m tossing the question out here;

I am attempting to render a velocity vector line from a point. Drawing the line isn’t difficult: just insert a line with length velocity.length into the graph. This puts the line centered at the point in the y-axis direction. We need to get this now in the proper rotation and translation.

The translational vector is not difficult to calculate: it is half the velocity vector. The rotational matrix, however, is being exceedingly elusive to me. Given a directional vector <x, y, z>, what’s the matrix I need?

Edit 1: Look; if you don’t understand the question, you probably won’t be able to give me an answer.

Here is what I currently have:

                    Vector3f translation = new Vector3f();                     translation.scale(1f/2f, body.velocity);                      Vector3f vec_z = (Vector3f) body.velocity.clone();                     vec_z.normalize();                      Vector3f vec_y; // reference vector, will correct later                     if (vec_z.x == 0 && vec_z.z == 0) {                         vec_y = new Vector3f(-vec_z.y, 0f, 0f); // could be optimized                     } else {                         vec_y = new Vector3f(0f, 1f, 0f);                     }                     Vector3f vec_x = new Vector3f();                     vec_x.cross(vec_y, vec_z);                     vec_z.normalize();                      vec_y.cross(vec_x, vec_z);                     vec_y.normalize();                     vec_y.negate();                      Matrix3f rotation = new Matrix3f(                         vec_z.z, vec_z.x, vec_z.y,                         vec_x.z, vec_x.x, vec_x.y,                         vec_y.z, vec_y.x, vec_y.y                     );                      arrowTransform3D.set(rotation, translation, 1f);

based off of this article. And yes, I’ve tried the standard rotation matrix (vec_x.x, vec_y.x, etc) and it didn’t work. I’ve been rotating the columns and rows to see if there’s any effect.

Edit 2:

Apologies about the rude wording of my comments.

So it looks like there were a combination of two errors; one of which House MD pointed out (really bad naming of variables: vec_z was actually vec_y, and so on), and the other was that I needed to invert the matrix before passing it off to the rendering engine (transposing was close!). So the modified code is:

                    Vector3f vec_y = (Vector3f) body.velocity.clone();                     vec_y.normalize();                      Vector3f vec_x; // reference vector, will correct later                     if (vec_y.x == 0 && vec_y.z == 0) {                         vec_x = new Vector3f(-vec_y.y, 0f, 0f); // could be optimized                     } else {                         vec_x = new Vector3f(0f, 1f, 0f);                     }                      Vector3f vec_z = new Vector3f();                     vec_z.cross(vec_x, vec_y);                     vec_z.normalize();                      vec_x.cross(vec_z, vec_y);                     vec_x.normalize();                     vec_x.negate();                      Matrix3f rotation = new Matrix3f(                         vec_x.x, vec_x.y, vec_x.z,                         vec_y.x, vec_y.y, vec_y.z,                         vec_z.x, vec_z.y, vec_z.z                     );                     rotation.invert();
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1 Answer

  1. Dupe.

    The question there involves getting a rotation to a certain axis, whereas I’m concerned with getting a rotation matrix.

    Gee, I wonder if you could turn convert one to the other?

    BTW, your current solution of picking an arbitrary y axis and then reorthogonalising should work fine; it looks bugged though, or at least badly written. ‘z_vec‘ is not a good variable-name for the y-axis. What’s with the ‘z,x,y’ ordering, anyway?

    If it still doesn’t work, try making random changes until it does – transpose the matrix, negate vectors until you have an even number of sign errors, that kind of thing.

    Also your tone of voice comes across as sort-of rude, given that you’re asking strangers to spend their time helping you.

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