diff --git a/src/p_tick.c b/src/p_tick.c
index 708ed6ca8..ad83abca6 100644
--- a/src/p_tick.c
+++ b/src/p_tick.c
@@ -44,6 +44,14 @@ INT32 P_AltFlip(INT32 n, tic_t tics)
 	return leveltime % (2 * tics) < tics ? n : -(n);
 }
 
+// Please read p_tick.h
+INT32 P_LerpFlip(INT32 n, tic_t tics)
+{
+	const tic_t w = 2 * tics;
+
+	return P_AltFlip(((leveltime % w) - tics) * n, w);
+}
+
 //
 // THINKERS
 // All thinkers should be allocated by Z_Calloc
diff --git a/src/p_tick.h b/src/p_tick.h
index 7b10a5f28..f892942ea 100644
--- a/src/p_tick.h
+++ b/src/p_tick.h
@@ -35,4 +35,12 @@ mobj_t *P_SetTarget(mobj_t **mo, mobj_t *target);   // killough 11/98
 INT32 P_AltFlip(INT32 value, tic_t tics);
 #define P_RandomFlip(value) P_AltFlip(value, 1)
 
+// Multiply value back and forth between -(tics) and +(tics).
+// Example output P_ModulateFlip(2, 2):
+// Tic:  0   1   2   3   4   5   6   7   8
+// Val: -4  -2   0   2   4   2   0  -2  -4
+// A half cycle (one direction) takes 2 * tics.
+// A full cycle takes 4 * tics.
+INT32 P_LerpFlip(INT32 value, tic_t tics);
+
 #endif