Alain <kua### [at] videotronca> wrote:

>
> The faces of the bounding box are always parallel to the reference planes.

Yes, that's the way I understand it as well.

> The same thing apply when you rotate your object : The bounding box
> never get rotated in any way and need to expend to completely contain
> the object.

Yes again. (Although, years ago, I thought the bounding-box got rotated along
with the object-- before I understood the true nature of the box faces being
always parallel to the reference planes.)

> So,
>   cylinder{50*y, -50*y, 1}
> have a tight bounding box, while
>   cylinder{50, -50, 1}
> only touch the bounding box at 6 points (one on each faces near corners)
> and is aligned along one of it's diagonals, leaving the bounding box
> almost empty.

*Ideally*, that is what I would expect; but a rotated object shows that few if
any of the bounding-box faces actually touch the object. There is excess space,
in other words. As Clipka says, the bounding-box cross section could actually
end up being twice the size of the object itself (sorry if I'm not paraphrasing
his words exactly.)

Here's a "bounding-box tester" scene, to show what I mean, using a rotated
stubby cylinder. (Another object could be substituted in the code, just so its
own 'cross section' is approximately 1 unit in size.)

-------
#version 3.7; // or 3.71 or 3.8
global_settings {assumed_gamma 1.0}

#declare CAMERA_POSITION = 1; // 1 -- front
                              // 2 -- right side
                              // 3-- top
#declare OBJECT_ROTATION = 35; // 0.0, or some other single value,
// for simplicity
//----------
#switch(CAMERA_POSITION)
#case(1)
// FRONT view ((i.e., looking at X/Y plane)
camera {
  orthographic
  location  <9.2, 9.2, 2>
  look_at   <9.201, 9.201, 9.200>
  right     x*image_width/image_height  // aspect
  angle 15
}
#break
#case(2)
// SIDE view (i.e., looking at Y/Z plane)
camera {
  orthographic
  location  <16.2, 9.2, 9.2>
  look_at   <9.201, 9.201, 9.201>
  right     x*image_width/image_height  // aspect
  angle 15
}
#break
#case(3)
// TOP view ((i.e., looking at X/Z plane)
camera {
  orthographic
  location  <9.2, 18.2, 9.2>
  look_at   <9.2, 9.2, 9.201>
  right     x*image_width/image_height  // aspect
  angle 15
}
#break
#else
#end

light_source {
  0*x
  color rgb .7
  translate <200, 400, -200>
}

light_source {
  0*x
  color rgb .3
  translate <-200, 100, -200>
}

background{rgb .1}

#declare OBJ =
cylinder{0,.15*y,.5
     no_shadow
     texture{
          pigment{rgb .8}
          finish{ambient .1 diffuse .9}
          }
     scale 1 // or whatever
     rotate OBJECT_ROTATION
     translate 9.2 // also used below
     }

OBJ

// the bounding box (its visible substitute)
box{min_extent(OBJ),max_extent(OBJ)
    no_shadow
    texture{
        pigment{rgbt <.3,.5,1,.7>}
        finish{ambient .3 diffuse .7}
        }
    }

sphere{min_extent(OBJ),.03} // BLACK-- MIN_EXTENT corner
sphere{max_extent(OBJ),.03 pigment {rgb <0,4,0>}} // GREEN-- MAX_EXTENT corner

#declare CENTER_OF_BOUNDING_BOX =
...5*(max_extent(OBJ)-min_extent(OBJ)) + min_extent(OBJ);

// RED sphere
sphere{CENTER_OF_BOUNDING_BOX, .15
      pigment{rgb <2,0,0>}

      }

#debug concat("\n","CENTER OF BOUNDING BOX at <",
    vstr(3,CENTER_OF_BOUNDING_BOX,",",1,3),">","\n\n")

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