diff --git a/solidify/face.h b/solidify/face.h
index 4d37086..ab6c5cf 100644
--- a/solidify/face.h
+++ b/solidify/face.h
@@ -13,6 +13,8 @@
 #ifndef FACE_H
 #define FACE_H
 
+#include <math.h>
+
 #include "array.h"
 #include "matrix.h"
 
@@ -34,6 +36,30 @@ static inline double face_z0(const struct face *f, int x, int y)
 }
 
 
+static inline double fx_to_angle(const struct face *f)
+{
+	return atan(f->fx)/M_PI*180;
+}
+
+
+static inline double fy_to_angle(const struct face *f)
+{
+	return atan(f->fy)/M_PI*180;
+}
+
+
+static inline void fx_from_angle(struct face *f, double a)
+{
+	f->fx = tan(a/180*M_PI);
+}
+
+
+static inline void fy_from_angle(struct face *f, double a)
+{
+	f->fy = tan(a/180*M_PI);
+}
+
+
 struct face *read_face(const char *name);
 
 #endif /* !FACE_H */
diff --git a/solidify/povray.c b/solidify/povray.c
index b5bcf45..60b8a3c 100644
--- a/solidify/povray.c
+++ b/solidify/povray.c
@@ -109,7 +109,7 @@ static void povray_face(FILE *file, const struct face *f, const char *side,
 "\t}\n", prefix, side,
     f->sx*f->x_step, (f->sz-1)*f->z_step, f->sy*f->y_step,
     -f->sx*f->x_step/2, f->sy*f->y_step/2,
-    -atan(f->fy)/M_PI*180, atan(f->fx)/M_PI*180,
+    -fy_to_angle(f), fx_to_angle(f),
     f->a->min_z*f->z_step,
     -f->z_ref*f->z_step,
     a,
diff --git a/solidify/project.c b/solidify/project.c
index 28a4582..a0991a6 100644
--- a/solidify/project.c
+++ b/solidify/project.c
@@ -72,11 +72,11 @@ static void read_face_data(FILE *file, struct face *f)
 
 	if (fscanf(file, "%f", &v) != 1)
 		return;
-	f->fx = -tan(v/180*M_PI);
+	fx_from_angle(f, v);
 
 	if (fscanf(file, "%f", &v) != 1)
 		return;
-	f->fy = tan(v/180*M_PI);
+	fy_from_angle(f, v);
 
 	if (fscanf(file, "%f", &v) != 1)
 		return;
@@ -163,7 +163,7 @@ static void save_face_data(FILE *file, const char *name, const struct face *f)
 		a = 180-a;
 	if (fprintf(file, "%g %g %g\n%g %g %g\n",
 	    f->z_ref*f->z_step,
-	    -atan(f->fx)/M_PI*180, atan(f->fy)/M_PI*180,
+	    fx_to_angle(f), fy_to_angle(f),
 	    a, f->m.b[0]*f->x_step, f->m.b[1]*f->y_step) < 0) {
 		perror(name);
 		exit(1);