On Mon, 30 Oct 2000 08:18:34 +0200, Peter Popov <pet### [at] usanet>
wrote:

>>Now, I keep tripping over these silly little spheres and boxes.
>
>Try using arrows made of cylinders and cones. I think there's a sample
>scene included with POV-Ray and/or an item in the Insert menu in the
>Windows version.

Unfortunately, this method leads to the same problem as the spheres
and boxes.  It creates shapes that distract from the object I'm
working on...

>
>>Now, I'm trying to do this with text.  (I find the little Xs, Ys and
>>Zs less distracting...)  Unfortunately, the letters are just a bit
>>hard to read except from directly in front or back.  I've managed to
>>find a little macro using atan() that allows me to aim objects at a
>>focal point.  (Someone on the POV-Ray Study Forum at Delphi worked it
>>out for aiming eyeballs...)  Problem is that it doesn't work if the
>>camera (and focal point) are on the Z plane and things seem to forget
>>where to look if I get near the Y axis.
>>
>>Anybody got ideas?
>
>Try using John VanSickle's Reorient macro, part of his Throroughly
>Useful Macros package. There's a link to his site on the povray.org
>link section.

This method still requires that I work out the axes to point the
letters along.  I think I've got this worked out now.  But I keep
finding that things like to point the wrong way.  I'm starting to
think that, maybe, there's a bug someplace in PoV...

I've created a little sample scene to test the algorithm and it's
giving me odd results.

The following bit of code draws the axes and places a little yellow
ball at the position indicated by HereItIs.  Then it draws a line
starting at the origin and running through HereItIs.  I haven't quite
worked out how to deal with the ball sitting on the X, Y or Z axis,
but it tells me the angles are right for all other cases.  Just
doesn't seem to draw the line there...

-------------------------------------------------
example code:
-------------------------------------------------

#include "colors.inc"
#include "macs.inc"

	#declare ViewPoint = <50, -50, -50> ;

	camera { location ViewPoint look_at   <0,  0, 0> }

//////////////////////  Draw the axes  /////////////////////

sphere   {<0,0,0>,1             
	 pigment { White } finish { ambient 1 } }
cylinder {<-49,0,0>,<49,0,0>,.05 
	pigment { Red   } finish { ambient 1 } }
cylinder {<0,-49,0>,<0,49,0>,.05 
	pigment { Green } finish { ambient 1 } }
cylinder {<0,0,-49>,<0,0,49>,.05 
	pigment { Blue  } finish { ambient 1 } }

//////////////////////////////////////////////////////////////////

#macro LookHere(from_pos, to_pos)
  #local Dx = to_pos.x - from_pos.x * sgn(to_pos.x) ;
  #local Dy = to_pos.y - from_pos.y * sgn(to_pos.y) ;
  #local Dz = to_pos.z - from_pos.z * sgn(to_pos.z) ;

  #if (Dy)
    #local Ax = degrees(atan2(Dz,Dy)) ;
  #else
    #local Ax = 0 ;
  #end
  
  #if (Dz)
    #local Ay = degrees(atan2(Dx,Dz)) ;
  #else
    #local Ay = 0 ;
  #end
  
  #if (Dx)
    #local Az = degrees(atan2(Dy,Dx)) ;
  #else
    #local Az = 0 ;
  #end

#render concat("Dx: ",str(Dx, 5, 2),"\n",
	"Dy: ",str(Dy, 5, 2),"\n",
	"Dz: ",str(Dz, 5, 2),"\n")
#render concat("Ax: ",str(Ax, 10, 7),"\n",
	"Ay: ",str(Ay, 10, 7),"\n",
	"Az: ",str(Az, 10, 7),"\n\n")

  Reorient(y,<Ax,Ay,Az>)

#end


#declare HereItIs = <10,15,10> ;

sphere   {HereItIs,1 pigment { Yellow } finish { ambient 1 } }

cylinder {<0,0,0>,<0,45,0>.1 
	pigment { Orange } finish { ambient 1 }
	LookHere(<0,0,0>,HereItIs)      }

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