Index: trunk/psModules/src/imcombine/pmSubtractionMask.h
===================================================================
--- trunk/psModules/src/imcombine/pmSubtractionMask.h	(revision 15756)
+++ trunk/psModules/src/imcombine/pmSubtractionMask.h	(revision 17729)
@@ -22,4 +22,15 @@
     );
 
+/// Apply the subtraction mask to an image and weight.
+///
+/// Unfortunately, image subtraction may result in a bi-modal image in masked areas, which can upset image
+/// statistics (very important for quantising images so that a product can be written out!).  This function
+/// sets masked areas to NAN in the image and weight.
+bool pmSubtractionMaskApply(psImage *image, ///< Image to which to apply mask
+                            psImage *weight, ///< Weight map to which to apply mask (or NULL)
+                            const psImage *mask, ///< Subtraction mask
+                            pmSubtractionMode mode ///< Subtraction mode
+    );
+
 
 #endif