diff --git a/engine/MUNGA_L4/L4VB16.h b/engine/MUNGA_L4/L4VB16.h
index f6dc0a1..bf7d6e1 100644
--- a/engine/MUNGA_L4/L4VB16.h
+++ b/engine/MUNGA_L4/L4VB16.h
@@ -419,6 +419,12 @@ public:
 	// dwords; the image is written TOP-DOWN.  *outW/*outH receive the produced size.
 	void ExpandPlaneToBGRA(int mask, int paletteID, int monoTint, int rotateQuadrant,
 						   unsigned long *dst, int *outW, int *outH);
+
+	// GLASS dirty-skip (L4GLASSWIN, 2026-08-09): FNV-1a checksum of the shared
+	// gauge pixelBuffer, masked to the bits a given window can show.  Lets the
+	// glass repaint pump re-blit ONLY the windows whose plane actually changed
+	// (idle MFDs / static panels skip; the sweeping radar keeps updating).
+	unsigned long PlaneChecksum(int mask) const;
 };
 
 //########################################################################