Juzzz - 10 months ago 89

Java Question

I am working on showing a 3D model (IFC) in Java3D. I need to rotate the object with 2 vectors, a direction vector and a axis vector that are given by the IFC model.

But I can't figure out how to get the right angle.

Code:

`// Create vectors`

Vector3f directionVector = new Vector3f(dx, dy, dz);

Vector3f axisVector = new Vector3f(ax, ay, az);

//Calculate angle

float angle = axisVector.angle(directionVector);

//create AxisAngle4f

AxisAngle4f axisAngle = new AxisAngle4f(axisVector, angle);

The axisVector is always (0.0, 0.0, 1.0), so it needs to be rotated on the Z-axis

But when I calculate the angle it seems always 1.5707964 (90°):

`Example 1:`

dir: (-1.0, 0.0, 0.0)

axis: (0.0, 0.0, 1.0)

angle: 1.5707964 (90.00000250447816)

AA: (0.0, 0.0, 1.0, 1.5707964)

Example 2:

dir: (0.0, 1.0, 0.0)

axis: (0.0, 0.0, 1.0)

angle: 1.5707964 (90.00000250447816)

AA: (0.0, 0.0, 1.0, 1.5707964)

Example 3:

dir: (1.0, 0.0, 0.0)

axis: (0.0, 0.0, 1.0)

angle: 1.5707964 (90.00000250447816)

AA: (0.0, 0.0, 1.0, 1.5707964)

I know through testing that -1.0 means inverted so 180°.

Can some one help me understanding what I am doing wrong?

Documentation for the placement object (Direction and axis)

Screenshots of the results:

- Orange: are the floors
- Green: is the roof
- Red: is de rotation point
- Left: Side perspective
- Right: Top perspective
- The group of 3 slabs have direction direction (0.0, 1.0, 0.0)
- The group of 2 slabs have direction direction (1.0, 0.0, 0.0)
- Roof has direction: (-1.0, 0.0, 0.0)

I did the *2 test to simulate the 180° of -1.0. as you can see in the last example the roof is correctly drawn.

Answer

You don't need the angle between the direction vector and the axis vector. Since your axis vector is always (0.0, 0.0, 1.0), you can define a constant vector of (1.0, 0.0, 0.0) to which you can compare the direction vector, i.e.:

```
// Create vectors
Vector3f directionVector = new Vector3f(dx, dy, dz);
Vector3f axisVector = new Vector3f(ax, ay, az);
Vector3f constantBaseVector = new Vector3f(1, 0, 0);
//Calculate angle
float angle = constantBaseVector.angle(directionVector);
//create AxisAngle4f
AxisAngle4f axisAngle = new AxisAngle4f(axisVector, angle);
```

Source (Stackoverflow)