Index: trunk/psModules/src/imcombine/pmSubtractionMask.c
===================================================================
--- trunk/psModules/src/imcombine/pmSubtractionMask.c	(revision 17727)
+++ trunk/psModules/src/imcombine/pmSubtractionMask.c	(revision 17729)
@@ -7,4 +7,5 @@
 
 #include "pmSubtraction.h"
+#include "pmSubtractionKernels.h"
 
 #include "pmSubtractionMask.h"
@@ -208,2 +209,51 @@
     return true;
 }
+
+
+bool pmSubtractionMaskApply(psImage *image, psImage *weight, const psImage *mask, pmSubtractionMode mode)
+{
+    PS_ASSERT_IMAGE_NON_NULL(image, false);
+    PS_ASSERT_IMAGE_TYPE(image, PS_TYPE_F32, false);
+    if (weight) {
+        PS_ASSERT_IMAGE_NON_NULL(weight, false);
+        PS_ASSERT_IMAGE_TYPE(weight, PS_TYPE_F32, false);
+        PS_ASSERT_IMAGES_SIZE_EQUAL(weight, image, false);
+    }
+    PS_ASSERT_IMAGE_NON_NULL(mask, false);
+    PS_ASSERT_IMAGE_TYPE(mask, PS_TYPE_MASK, false);
+    PS_ASSERT_IMAGES_SIZE_EQUAL(mask, image, false);
+
+    bool maskVal = PM_SUBTRACTION_MASK_BORDER; // Value to mask
+    switch (mode) {
+      case PM_SUBTRACTION_MODE_1:
+        maskVal |= PM_SUBTRACTION_MASK_CONVOLVE_1;
+        break;
+      case PM_SUBTRACTION_MODE_2:
+        maskVal |= PM_SUBTRACTION_MASK_CONVOLVE_2;
+        break;
+      case PM_SUBTRACTION_MODE_DUAL:
+        maskVal |= PM_SUBTRACTION_MASK_CONVOLVE_2 | PM_SUBTRACTION_MASK_CONVOLVE_2;
+        break;
+      case PM_SUBTRACTION_MODE_ERR:
+      case PM_SUBTRACTION_MODE_UNSURE:
+      default:
+        psError(PS_ERR_BAD_PARAMETER_VALUE, true, "Unsuppored subtraction mode: %x", mode);
+        return false;
+    }
+
+    int numCols = image->numCols, numRows = image->numRows; // Size of image
+    psMaskType **maskData = mask->data.PS_TYPE_MASK_DATA; // Dereference mask
+
+    for (int y = 0; y < numRows; y++) {
+        for (int x = 0; x < numCols; x++) {
+            if (maskData[y][x] & maskVal) {
+                image->data.F32[y][x] = NAN;
+                if (weight) {
+                    weight->data.F32[y][x] = NAN;
+                }
+            }
+        }
+    }
+
+    return true;
+}
Index: trunk/psModules/src/imcombine/pmSubtractionMask.h
===================================================================
--- trunk/psModules/src/imcombine/pmSubtractionMask.h	(revision 17727)
+++ 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