Warp,
  Ah, I think I understand now. What you're saying is that Pov needs to 
calculate one angle at a time and get a sum total.
  Thanks for the help, I'll try the example and see how it all works, 
but it does make more sense now.
Les

Warp wrote:

> Les Patterson <les### [at] txcybercom> wrote:
> : There's something I don't understand about rotation.
> : In the case of an aircraft positioned at 0,0,0(facing camera) then X, 
> : and Z work fine, but rotate the aircraft to a right angle to the camera, 
> : 0,-90,0 and then X & Z don't rotate as expected. I understand that this 
> : is a 360degree universe, but if that's the case then why wouldn't the 
> : rotational angles be the same regardless of which direction the aircraft 
> : is pointing?
> 
>   If you tell povray to rotate an object around the x-axis, it will rotate
> the object around the x axis. There's no reason for it to rotate it around
> any other axis. It rotates the object around the x-axis regardless of the
> current orientation of the object.
>   Also you have to understand that the effect of consecutive rotations are
> summed in the order they are given. This means that if you do this:
> 
>   rotate <50,0,0>
>   rotate <30,0,0>
> 
> the final orientation of the object will be equivalent to
> 
>   rotate <80,0,0>
> 
>   Now, with these two things in mind, think about these consecutive rotations:
> 
>   rotate <0,45,0>
>   rotate <45,0,0>
>   rotate <0,-45,0>
>   rotate <-45,0,0>
> 
>   It might not be clear at first, but the net effect of those rotations
> is *not* as if no rotations were made at all.
> 
>   Suppose we have a regular box { -1,1 } which we are rotating that way.
>   First we rotate it 45 degrees around the y-axis. Now the face which was
> facing the camera is oriented so that it's 45 degrees to the left (the side
> of the box which was facing right is now 45 degrees to the right).
>   Then we rotate 45 degrees around the x-axis. Here it's important to
> understand that the rotation is done around the x-axis, not any other axis.
> The cube is rotated so that now we see three of its faces (the new face is
> the lower face which became visible). The corner of the box is pointing
> approximately at the camera.
>   The next rotation is more difficult to imagine in our head (thus it's
> better to do it in povray to see it). We rotate -45 degrees around the y-axis.
> The box is rotate so that the corner which was pointing at the camera now
> points 45 degrees to the right. We still see the lower face and the original
> front face of the box, but it's tilted
>   Finally we rotate -45 degrees around the x-axis. The final orientation
> of the box looks awkward because it still is tilted. It's oriented
> approximately so that the original front face of the box is again facing
> the camera, but the whole box is tilted.
> 
>   I know that this is pretty confusing, but it's a direct consequence of
> the fact that rotations are always made around the axes. You should test
> with the box example to see what happens. Here's a simple scene:
> 
> camera { location -z*7 look_at 0 angle 35 }
> light_source { <100,200,-300>, 1 }
> 
> box
> { -1,1 pigment { rgb <1,.5,0> }
>   rotate <0,45,0>
>   rotate <45,0,0>
>   rotate <0,-45,0>
>   rotate <-45,0,0>
> }
> 
>   Note that the order of the rotations matter. If we shuffle the rotations
> above, we will get a different result.
> 
>   With your aircraft you have to make the rotations in the right order
> to get the correct result. You probably want to rotate first around the
> main longitudinal axis of the plane, then around the "pitch" axis and then
> the axis which orients the plane to wherever it is heading.

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