1 files changed, 108 insertions, 0 deletions
diff --git a/qball.c b/qball.c
new file mode 100644
index 0000000..1075767
--- /dev/null
+++ b/qball.c
@@ -0,0 +1,108 @@
+/*
+ * Ken Shoemake's Quaternion rotation controller
+ */
+#include <u.h>
+#include <libc.h>
+#include <draw.h>
+#include "dat.h"
+#include "fns.h"
+
+typedef struct Point2 Point2;
+struct Point2 {
+	double x, y;
+};
+
+static Point2
+Pt2(double x, double y)
+{
+	return (Point2){x, y};
+}
+
+static Point2
+addpt2(Point2 a, Point2 b)
+{
+	return (Point2){a.x+b.x, a.y+b.y};
+}
+
+static Point2
+subpt2(Point2 a, Point2 b)
+{
+	return (Point2){a.x-b.x, a.y-b.y};
+}
+
+static Point2
+divpt2(Point2 p, double s)
+{
+	return (Point2){p.x/s, p.y/s};
+}
+
+static Point2 ctlcen;		/* center of qball */
+static double ctlrad;		/* radius of qball */
+static Quaternion *axis;	/* constraint plane orientation, 0 if none */
+
+static double
+fmin(double a, double b)
+{
+	return a < b? a: b;
+}
+
+/*
+ * Convert a mouse point into a unit quaternion, flattening if
+ * constrained to a particular plane.
+ */
+static Quaternion
+mouseq(Point2 p)
+{
+	double qx = (p.x-ctlcen.x)/ctlrad;
+	double qy = (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 != nil){
+		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, Point mxy, Quaternion *orient, Quaternion *ap)
+{
+	Quaternion down;
+	Point2 rmin, rmax;
+	Point2 rad;
+
+	if(orient == nil)
+		return;
+
+	axis = ap;
+	rmin = Pt2(r.min.x, r.min.y);
+	rmax = Pt2(r.max.x, r.max.y);
+	ctlcen = divpt2(addpt2(rmin, rmax), 2);
+	rad = divpt2(subpt2(rmax, rmin), 2);
+	ctlrad = fmin(rad.x, rad.y);
+	down = invq(mouseq(Pt2(mxy.x, mxy.y)));
+	*orient = mulq(*orient, mulq(down, mouseq(Pt2(mxy.x, mxy.y))));
+}