diff --git a/th03/math/subpixel.hpp b/th03/math/subpixel.hpp
new file mode 100644
index 00000000..3e0b5d1e
--- /dev/null
+++ b/th03/math/subpixel.hpp
@@ -0,0 +1,36 @@
+// Fixed-point format for expressing world-space coordinates, with 4 bits of
+// fractional resolution.
+
+typedef int subpixel_t;
+
+inline subpixel_t to_sp(float screen_v) {
+	return static_cast<subpixel_t>(screen_v * 16.0f);
+}
+
+class Subpixel {
+public:
+	// Code generation will require direct access to v, if performing
+	// arithmetic with a local variable...
+	subpixel_t v;
+
+	void operator +=(float screen_v) {
+		this->v += to_sp(screen_v);
+	}
+
+	void operator -=(float screen_v) {
+		this->v -= to_sp(screen_v);
+	}
+
+	void operator =(float screen_v) {
+		v = to_sp(screen_v);
+	}
+};
+
+struct SPPoint {
+	Subpixel x, y;
+
+	void set(float screen_x, float screen_y) {
+		x = screen_x;
+		y = screen_y;
+	}
+};
diff --git a/th04/shared.hpp b/th04/shared.hpp
index be13a4c5..5d461047 100644
--- a/th04/shared.hpp
+++ b/th04/shared.hpp
@@ -7,39 +7,7 @@
 
 /// Math
 /// ----
-typedef int subpixel_t;
-
-inline subpixel_t to_sp(float screen_v) {
-	return static_cast<subpixel_t>(screen_v * 16.0f);
-}
-
-class Subpixel {
-public:
-	// Code generation will require direct access to v, if performing
-	// arithmetic with a local variable...
-	subpixel_t v;
-
-	void operator +=(float screen_v) {
-		this->v += to_sp(screen_v);
-	}
-
-	void operator -=(float screen_v) {
-		this->v -= to_sp(screen_v);
-	}
-
-	void operator =(float screen_v) {
-		v = to_sp(screen_v);
-	}
-};
-
-struct SPPoint {
-	Subpixel x, y;
-
-	void set(float screen_x, float screen_y) {
-		x = screen_x;
-		y = screen_y;
-	}
-};
+#include "th03/math/subpixel.hpp"
 
 typedef struct {
 	SPPoint cur;