--- ray/src/common/fvect.c	1989/10/17 13:35:30	1.4
+++ ray/src/common/fvect.c	1992/10/02 16:02:43	2.2
@@ -10,11 +10,10 @@ static char SCCSid[] = "$SunId$ LBL";
  *     8/14/85
  */
 
+#include  <math.h>
 #include  "fvect.h"
 
-#define  FTINY		1e-7
 
-
 double
 fdot(v1, v2)			/* return the dot product of two vectors */
 register FVECT  v1, v2;
@@ -115,4 +114,26 @@ register FVECT  v;
 	v[1] /= len;
 	v[2] /= len;
 	return(len);
+}
+
+
+spinvector(vres, vorig, vnorm, theta)	/* rotate vector around normal */
+FVECT  vres, vorig, vnorm;
+double  theta;
+{
+	double  sint, cost, normprod;
+	FVECT  vperp;
+	register int  i;
+	
+	if (theta == 0.0) {
+		if (vres != vorig)
+			VCOPY(vres, vorig);
+		return;
+	}
+	cost = cos(theta);
+	sint = sin(theta);
+	normprod = DOT(vorig, vnorm)*(1.-cost);
+	fcross(vperp, vnorm, vorig);
+	for (i = 0; i < 3; i++)
+		vres[i] = vorig[i]*cost + vnorm[i]*normprod + vperp[i]*sint;
 }