Plan 9 from Bell Labs’s /usr/web/sources/plan9/sys/src/libgeometry/qball.c

 ``` /* * Ken Shoemake's Quaternion rotation controller */ #include #include #include #include #include #define BORDER 4 static Point ctlcen; /* center of qball */ static int ctlrad; /* radius of qball */ static Quaternion *axis; /* constraint plane orientation, 0 if none */ /* * Convert a mouse point into a unit quaternion, flattening if * constrained to a particular plane. */ static Quaternion mouseq(Point p){ double qx=(double)(p.x-ctlcen.x)/ctlrad; double qy=(double)(p.y-ctlcen.y)/ctlrad; double rsq=qx*qx+qy*qy; double l; Quaternion q; if(rsq>1){ rsq=sqrt(rsq); q.r=0.; q.i=qx/rsq; q.j=qy/rsq; q.k=0.; } else{ q.r=0.; q.i=qx; q.j=qy; q.k=sqrt(1.-rsq); } if(axis){ l=q.i*axis->i+q.j*axis->j+q.k*axis->k; q.i-=l*axis->i; q.j-=l*axis->j; q.k-=l*axis->k; l=sqrt(q.i*q.i+q.j*q.j+q.k*q.k); if(l!=0.){ q.i/=l; q.j/=l; q.k/=l; } } return q; } void qball(Rectangle r, Mouse *m, Quaternion *result, void (*redraw)(void), Quaternion *ap){ Quaternion q, down; Point rad; axis=ap; ctlcen=divpt(addpt(r.min, r.max), 2); rad=divpt(subpt(r.max, r.min), 2); ctlrad=(rad.xxy)); q=*result; for(;;){ *m=emouse(); if(!m->buttons) break; *result=qmul(q, qmul(down, mouseq(m->xy))); (*redraw)(); } } ```