You could also use direct trigonometry on this, which Pov allows.

Grim
"Spock" <spo### [at] nospamcom> wrote in message
news:3a929765$1@news.povray.org...
> Here is a macro I didn't write.  I have lost the original comments but I
> think it might be attributable to Giles Tran.
>
> In any case I use it a lot and it works nicely for placing objects in
> arbitrary orientations.  I don't have a big brain so I can't comment on
your
> image issue, but maybe you will get lucky and the image will be aligned
> along with the cylinder :-)
>
> Would the author of this please stand up to take credit?  Its wonderful!
>
> //
> // used to align an object to an arbitrary orientation
> //
> // a1 - the axis in the object before transformation
> // a2 - the desired orientation after transformation
> //
> #macro Align( a1, a2 )
>
>     #local vX1 = vnormalize( a1 );
>     #local vX2 = vnormalize( a2 );
>     #local vY = vnormalize( vcross( vX1, vX2 ) );
>     #local vZ1 = vnormalize( vcross( vX1, vY ) );
>     #local vZ2 = vnormalize( vcross( vX2, vY ) );
>     matrix < vX1.x, vY.x, vZ1.x, vX1.y, vY.y, vZ1.y, vX1.z, vY.z, vZ1.z,
0,
> 0, 0 >
>     matrix < vX2.x, vX2.y, vX2.z, vY.x, vY.y, vY.z, vZ2.x, vZ2.y, vZ2.z,
0,
> 0, 0 >
>
> #end
>
> "Matt Moose" <mrr### [at] sphericademoncouk> wrote in message
> news:3a90fab8.9496988@news.povray.org...
> > (Using Povray v. 3.1g.watcom.win32)
> >
> > Dear brainy types,
> >
> > Problem: Got a unit cylinder lying on the x-axis, from x=-0.5 to 0.5,
> > with a bitmap projected onto its side in the xz plane. Good so far,
> > very happy with that. However I would then like to take the
> > illustrated cylinder's start & end points and move them to 2 totally
> > different unrelated  coordinates p1 & p2, which can be a variable
> > distance apart. The image must face the camera (towards -Z), whatever
> > the final orientation of the object.
> >
> > It's required to keep the image locked to the side of the cylinder so
> > it can point in the right direction.
> >
> > I can easily translate the object's start point to p1, but can't get
> > my head around determining the amount of pre-rotation and pre-scaling
> > required at the origin in order to move the endpoint to p2,  _and_
> > have the image still facing -Z !
> >
> > Have I missed an easier way?
> >
> > Thanks in advance,
> >
> > Matt.
>
>

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