Slime wrote:

 > Oh, wait. I hadn't thought about the case where the up and right
 > vectors
 > weren't already perpendicular. *Does* it straighten them out in this
 > case,
 > or does it align the right vector as I described, or does it align the
 > up
 > vector as I described?

I just did some testing:

* look_at straightens all vectors (even when direction-location = look_at).
* sky alone has no effect.
* I can't figure what really happens when right, sky and look_at are
aligned - but I am under the impression this defines a degenerated
camera that traces only one "point" (not pixel, one ray only). Not very 
important.



I tried to match POV behavior in SDL - this is not thoroughly tested but 
it seems to work quite nicely.

// --- BEGIN SDL ---
// Trying to reproduce POV original sky/look_at behavior in SDL
camera{
   // Declare camera parameters
   #local Camera_location  = <0,.1,-3> ;
   #local Camera_direction = 1*vnormalize(<0,0,1>) ;
   #local Camera_up        = 1*vnormalize(<0,1,2>) ;
   #local Camera_right     = 1*vnormalize(<1,0,0>) ;
   #local Camera_sky       = 1*vnormalize(<0,1,0>) ;
   #local Camera_look_at   = <-.5,1,.5> ;

   #if (0) // choose between POV coded behaviour and SDL behaviour
     location  Camera_location
     direction Camera_direction
     up        Camera_up
     right     Camera_right
     sky       Camera_sky
     look_at   Camera_look_at
   #else

     // new look_at to account for location
     #declare New_look_at=Camera_look_at-Camera_location ;

     // examine handedness
     #declare Camera_handedness = vdot( Camera_direction, vcross( 
Camera_right, Camera_up ) );
     #declare Camera_handedness = Camera_handedness / abs( 
Camera_handedness );

     // new direction is look_at, conserve length (=zoom) and orientation
     #declare New_direction = vnormalize( New_look_at) * vlength( 
Camera_direction );

     // if look_at and sky are aligned
     #if (vlength(vcross(New_look_at, Camera_sky))<1e-6)
       // do nothing ;
       #declare New_right = Camera_right ;
     #else
       // else, new right is normal to the look_at-sky plane, conserve
       // length and orientation
       #declare New_right= vnormalize( vcross ( Camera_sky, New_look_at 
) ) * vlength( Camera_right ) * Camera_handedness;
     #end

     // new up is normal to the look_at-right plane, conserve length
     // and orientation
     #declare New_up = vnormalize ( vcross ( New_look_at, New_right ) ) 
* vlength( Camera_up )* Camera_handedness;
     // look_at and New_right can't be aligned

     location  Camera_location
     direction New_direction
     up        New_up
     right     New_right
   #end

}
// --- END SDL ---


Povingly,

Philippe

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