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From: tomt@maui.coral.COM (Tom Tulinsky)
Newsgroups: comp.windows.x
Subject: 3d rotation in room coords?
Message-ID: <9103050342.AA20065@maui.coral.com>
Date: 5 Mar 91 03:42:39 GMT
Sender: daemon@athena.mit.edu (Mr Background)
Organization: The Internet
Lines: 76


I'd like to allow the user to do 3d rotations of an object in the
'room' coordinate system; i.e. y is toward the ceiling, x is to his
right, and z is out of the screen (rt-handed).  I want this to be true
even after he has done some rotations....  The naive way of
implementing rotations accumulates them so that a 90 deg. y rotation,
the x axis is now pointing out of the screen, where the room z axis
is.  

I've tried everything I can think of, and can't get it to work.

My latest version, (using the Vogle package from uunet, somewhat like
SGI GL) does the y rotation, then tries to calculate where the xroom
vector is, and uses an arbitrary rotation routine to rotate about it.
The result: it just interchanges the order of rotations:  the X
axis is constant but the Y axis is rotated by xrot.  (also it only
works for y<0).

        while((but = slocator(&x, &y)) != 44) {    /* get mouse x,y */

              pushmatrix(); 
              {
                 color(BLACK);
                 clear();
                 yrot = 100.0*-x;
                 xrot = 100.0*-y;

                 rotate(yrot, 'y');    /* do y rot */

                 /* calc. xroom vector = Ry((1,0,0), -yrot) */

                 xroom[0] = cos(-yrot*D2R);
                 xroom[1] = 0.0;
                 xroom[2] = -sin(-yrot*D2R);
                 
                 /* do x rot */
                 rotarb (xroom[0],xroom[1],xroom[2],xrot);

                 callobj(obj);
                 callobj(GNOMON);
              }
              popmatrix();

/* rotarb--do rotation about an arbitrary vector 
 */
rotarb (a,b,c, theta)
float a,b,c,theta;
{
   float L, V, I, J;

   /* from Computer Graphics, Harrington, p 261. (modified) */

   L = sqrt(a*a + b*b + c*c);    /* lenght of vector */

   V = sqrt(b*b + c*c);          /* len of proj. on y-z plane */

   I = R2D*asin(b/V);      /* this is diff. from Harrington because I can't
   J = R2D*-asin(a/L);      * set rot mtx directly */
   
   rotate (I,'x');
   rotate (J,'y');
   rotate (theta,'z');
   rotate (-J,'y');
   rotate (-I,'x');
};



 Coral
     * **	Tom Tulinsky                508 460-6010
  *  **		Coral Network Corporation   fax 508 481-6258
*  ** 		734 Forest St               net: tomt@coral.com
 ***		   Marlboro, MA 01752		
  **		   U S A
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