diff --git a/th01/formats/ptn.cpp b/th01/formats/ptn.cpp
index 53e3304b..fad21105 100644
--- a/th01/formats/ptn.cpp
+++ b/th01/formats/ptn.cpp
@@ -57,15 +57,11 @@ ptn_error_t ptn_load_palette_show(int slot, const char *fn)
 		arc_file_get(ptn->unused_zero);
 		arc_file_get(ptn->planes);
 		for(y = 0; y < PTN_H; y++) {
-			// Color #15 (1111) is always the transparent one, meaning that
-			// transparent dots are 1 in all 4 bitplanes. The alpha mask
-			// therefore simply is the negation of ANDing all bitplanes
-			// together. Nifty!
-			ptn->alpha[y] = ~(
-				ptn->planes.B[y]
-				& ptn->planes.R[y]
-				& ptn->planes.G[y]
-				& ptn->planes.E[y]
+			ptn->alpha[y] = ptn_alpha_from(
+				ptn->planes.B[y],
+				ptn->planes.R[y],
+				ptn->planes.G[y],
+				ptn->planes.E[y]
 			);
 		}
 	}
diff --git a/th01/formats/ptn.hpp b/th01/formats/ptn.hpp
index bf4c72d7..92444768 100644
--- a/th01/formats/ptn.hpp
+++ b/th01/formats/ptn.hpp
@@ -5,6 +5,18 @@
 /// color #15. Can also be used to store the backgrounds of frequently updated
 /// VRAM regions, using the functions for raw allocation and VRAM snapping.
 
+}
+
+// Color #15 (1111) is always the transparent one, meaning that transparent
+// dots are 1 in all 4 bitplanes. The alpha mask therefore simply is the
+// negation of ANDing all bitplanes together. Nifty!
+template <class T> inline T ptn_alpha_from(T B, T R, T G, T E)
+{
+	return ~((B) & (R) & (G) & (E));
+}
+
+extern "C" {
+
 #define PTN_W 32
 #define PTN_H 32