--- ray/src/common/fvect.c	1989/10/17 13:35:30	1.4
+++ ray/src/common/fvect.c	1991/03/19 13:14:19	1.5
@@ -116,3 +116,26 @@ register FVECT  v;
 	v[2] /= len;
 	return(len);
 }
+
+
+spinvector(vres, vorig, vnorm, theta)	/* rotate vector around normal */
+FVECT  vres, vorig, vnorm;
+double  theta;
+{
+	extern double  sin(), cos();
+	double  sint, cost, dotp;
+	FVECT  vperp;
+	register int  i;
+	
+	if (theta == 0.0) {
+		VCOPY(vres, vorig);
+		return;
+	}
+	sint = sin(theta);
+	cost = cos(theta);
+	dotp = DOT(vorig, vnorm);
+	fcross(vperp, vnorm, vorig);
+	for (i = 0; i < 3; i++)
+		vres[i] = vnorm[i]*dotp*(1.-cost) +
+				vorig[i]*cost + vperp[i]*sint;
+}