Somewhere in the distant past I had a set up where I could render an 
image in tiles by rotating the camera towards the region to be rendered 
and shearing the camera to compensat for the 'distortions'. It required a 
'long lens' so wasn't extremly usefull. This code is lost though and the 
sad part is that I can't code it again as I don't remeber how.

Anyway, I thought to do it with the user_defined camera. No rotations or 
shearing (I hope), just select a small section of the image and render it 
to the proper resolution. But I fail.

In the scene below, two camera's, the user_defined one gives an identical 
image to the commented out perpective one, pixel perfect says imagemagick 
(without AA). With the commented out part of the user_defined camera I 
intend select the top left quadrant and render that to a square image. I 
get an image of a quarter sphere, not in the position I want and 
stretched in strange ways. It looks as if the "canvas" needs to be moved. 
But how?

ingo

---%<------%<------%<---

#version 3.8;
global_settings {assumed_gamma 1}
// +w400 +h400 +a0.001 +am3

camera {
  user_defined
  location 0
  direction{
    function{u*tan(radians(90)/2)}
    function{v*tan(radians(90)/2)}
    //function{select((u>-0.5 & u<0  ),0,0,u*radians(90)/2)}
    //function{select((v> 0   & v<0.5),0,0,v*radians(90)/2)}
    function{.5}
  }
}

//camera{
//  perspective
//  location 0
//  look_at <0,0,1>
//  direction z
//  right x
//  up y
//  angle 90
//}


background{rgb 1}
light_source {<100,100,-100> rgb 1}
sphere{<0,0  , 1>,0.5 texture{pigment{checker}} scale 5}
sphere{<0,0.5, 1>,0.2 pigment{rgb z} scale 5}

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