I’ve implemented a particle system. I’m drawing their textures on billboards that should be rotated towards the camera.

This works fine except for the case when the angle between particle->camera and the normal comes near to 180 degrees. Then the particle starts rotating around itself many times.

The angle is calculated using cos(angle) = dot(a, b) / (length(a) * length(b), the length are both 1 cause the Vectors are normalized. The axis is calculated using the cross product of those two vectors.

```
glDisable(GL_CULL_FACE);
//calculate rotation
Vector3f normal =newVector3f(0,0,1);
Vector3f dir =Vector3f.sub(newVector3f(GraphicsData.camera.x,GraphicsData.camera.y,GraphicsData.camera.z),newVector3f(x, y, z),null);
if(dir.length()==0)
{
glEnable(GL_CULL_FACE);
return;
}
dir =(Vector3f) dir.normalise();
float angle =(float)Math.toDegrees(Math.acos(Vector3f.dot(normal, dir)));
Vector3f rotationAxis =Vector3f.cross(normal, dir,null);
rotationAxis =(Vector3f) rotationAxis.normalise();
System.out.println("Angle: + "+ angle +" Axis: "+ rotationAxis);
glBindTexture(GL_TEXTURE_2D,ParticleEngine.particleTextures.get(typeId).texture.getTextureID());
glColor4f(1f,1f,1f, time >= lifeTime - decayTime ?((float)lifeTime -(float)time)/((float)lifeTime -(float)decayTime):1f);
shaderEngine.createModelMatrix(newVector3f(x, y, z),newVector3f(angle * rotationAxis.x, angle * rotationAxis.y, angle * rotationAxis.z),newVector3f(sx, sy, sz));
shaderEngine.loadModelMatrix(shaderEngine.particle);
glCallList(ParticleEngine.particleTextures.get(typeId).displayListId + textureIndex);
glEnable(GL_CULL_FACE);
```

[FONT=arial]What am i doing wrong calculating the rotation?

[/FONT]publicstaticvoid createModelMatrix(Vector3f pos,Vector3f rot,Vector3f scale)

{

GraphicsData.camera.modelMatrix =newMatrix4f();

GraphicsData.camera.modelMatrix.setIdentity();

GraphicsData.camera.modelMatrix.translate(pos);

GraphicsData.camera.modelMatrix.rotate((float)Math.toRadians(rot.x),newVector3f(1,0,0));

GraphicsData.camera.modelMatrix.rotate((float)Math.toRadians(rot.y),newVector3f(0,1,0));

GraphicsData.camera.modelMatrix.rotate((float)Math.toRadians(rot.z),newVector3f(0,0,1));

GraphicsData.camera.modelMatrix.scale(scale);

}