In message <3cc9ce28@news.povray.org>, Vic <let### [at] fwhu> writes
>Hi Pov fans!
>
>I've made a shutter glass interface hardware and created some stereo
>pictures with PovRay. I'm faced with a serious shortage of Pov, namely the
>unability to "shift" the camera viewport. The right and up vectors are
>always at 0.5, so I've to shift the resulting images after rendering to
>match the left and right eye views at the screen plane. (see "The
>perspective camera" in Pov help at "Placing the Camera" topic)
>
>Is there any way to shift the image plane during the rendering process?
>
I was going to suggest changing the direction vector, but unfortunately 
this does not work at all logically.
The docs say "Note: that the up, right, and direction vectors should 
always remain perpendicular to each other or the image will be 
distorted.", but knocking up a short test scene I find the povray (3.5) 
*forces* the vectors to be perpendicular. Whilst non-perpendicular 
vectors do distort the scene, it does not do it in the way I would 
expect (and which allow the camera viewport to be shifted).

Slime suggests using the matrix transformation, which will allow you to 
shift the image plane.

matrix <1,0,sf,
         0,1,0,
         0,0,1,
         0,0,0>

where sf is the shear required (use +/- for R/L images respectively

    x
   ____      sf=x/z
   \   |
    \  |  z
     \ |
      \|

But this doesn't quite work as expected, causing a slight stretch in the 
image along the x-axis. The fix is to use

matrix <sqrt(1+sf^2),0,sf,
                    0,1,0,
                    0,0,1,
                    0,0,0>
>
>camera {
>  #local pp=clock*2-1;
>  #local dd=0.06;
>  #local xx=-dd*0.5*pp;
>  #local yy=0.4;
>  #local zz=0.6;
>  location  <xx, yy, -zz>
>  right     x*image_width/image_height
>  look_at   <xx, 0.0,  0.0>
>}
>

This ought to work:

camera {
   #local pp=cl0ck*2-1;
   #local dd=0.06;
   #local xx=-dd*0.5*pp;
   #local yy=0.4;
   #local zz=0.6;
   #local sf=xx/zz;
   location  <xx, yy, -zz>
   right     x*image_width/image_height
   look_at   <0, 0.0,  0.0>

   matrix <sqrt(1+sf^2),0,-sf,
                    0,1,0,
                    0,0,1,
                    0,0,0>
}

The other thing that doesn't work as expected is the look_at. I would 
have expected to look_at <xx,0,0> (parallel to the z-axis) and the 
matrix to deform the camera to look at the origin.


Can I have a new camera please, this one's broken (shakes it and hears 
glass rattling about inside).
-- 
Alex

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