--- ray/src/common/fvect.c	1989/05/08 08:40:56	1.3
+++ ray/src/common/fvect.c	1991/03/19 13:14:19	1.5
@@ -83,6 +83,16 @@ register FVECT  vres, v1, v2;
 }
 
 
+fvsum(vres, v0, v1, f)		/* vres = v0 + f*v1 */
+FVECT  vres, v0, v1;
+double  f;
+{
+	vres[0] = v0[0] + f*v1[0];
+	vres[1] = v0[1] + f*v1[1];
+	vres[2] = v0[2] + f*v1[2];
+}
+
+
 double
 normalize(v)			/* normalize a vector, return old magnitude */
 register FVECT  v;
@@ -105,4 +115,27 @@ 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;
+{
+	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;
 }