diff --git a/rfb/ScaledPixelBuffer.h b/rfb/ScaledPixelBuffer.h
index b4b5a8e..8342a58 100644
--- a/rfb/ScaledPixelBuffer.h
+++ b/rfb/ScaledPixelBuffer.h
@@ -68,6 +68,10 @@
     // using bilinear interpolation
     virtual void scaleRect(const Rect& r);
 
+    // Calculate the scaled image rectangle which depend on the source 
+    // image rectangle.
+    inline Rect calculateScaleBoundary(const Rect& r);
+
   protected:
     virtual void recreateScaledBuffer();