[vtkusers] Aligning a cylinder between two points

[Kevin Wright]

>I've got a bit of a tranformation/rotation problem at hand. I want to
>draw a cylinder where the top and bottom are aligned to two points,
>p1=(x0, y0, z0) and p2=(x1, y1, z1), something like:
>
>               ---------
>         p1 -> |       | <- p2
>               ---------
>
>I'm using a vtkCylinderSource, vtkPolyDataMapper, and a vtkActor

lets say x = x1-x0 y = y1-y0 z = z1-z0

1. Set the height of the cylinder to be sqrt(x^2 + y ^2 + z^2) using 
vtkCylinderSource->SetHeight
3. Cross the vector (x,y,z) with the default axis of the cylinder (0,1,0) 
to get the axis to rotate about, which would be
         (z,0,-x)
4. Get the angle between the two vectors (using the cosine law where one 
vector is 0,1,0):
         sin(angle) = y / sqrt(x^2 + y^2 + z^2)
5. Create a vtkTransform object
6. Apply a translation to the vtkTransform using Translate() to (x0 + 
x/2,y0 + y/2,z0 + z/2)
7. Apply a rotation to the transform using RotateWXYZ with the angle from 4 
and the vector from 3
8. Create a vtkTransformFilter and apply the vtkTransform from 5 to it.
9. Pass the vtkCylinderSource through the transform filter.

I'm no geometry expert, that may not be the most efficient way to do 
things, but I think it will get the job done for you, unless I've made a 
stupid math error.  If you look at the examples for vtkTransformFilter, 
I'll bet you find some code doing something similar to what you want to do.

Kevin.