Index: trunk/psModules/src/imcombine/pmSubtractionMask.c
===================================================================
--- trunk/psModules/src/imcombine/pmSubtractionMask.c	(revision 27086)
+++ trunk/psModules/src/imcombine/pmSubtractionMask.c	(revision 27365)
@@ -42,11 +42,27 @@
                            pmSubtractionMode mode)
 {
-    PM_ASSERT_READOUT_NON_NULL(ro1, NULL);
-    PM_ASSERT_READOUT_IMAGE(ro1, NULL);
-    PM_ASSERT_READOUT_MASK(ro1, NULL);
-    PM_ASSERT_READOUT_NON_NULL(ro2, NULL);
-    PM_ASSERT_READOUT_IMAGE(ro2, NULL);
-    PM_ASSERT_READOUT_MASK(ro2, NULL);
-    PS_ASSERT_IMAGES_SIZE_EQUAL(ro1->image, ro2->image, NULL);
+    int numCols = 0, numRows = 0;       // Size of the images
+    if (ro1) {
+        PM_ASSERT_READOUT_NON_NULL(ro1, NULL);
+        PM_ASSERT_READOUT_IMAGE(ro1, NULL);
+        PM_ASSERT_READOUT_MASK(ro1, NULL);
+        numCols = ro1->image->numCols;
+        numRows = ro1->image->numRows;
+            }
+    if (ro2) {
+        PM_ASSERT_READOUT_NON_NULL(ro2, NULL);
+        PM_ASSERT_READOUT_IMAGE(ro2, NULL);
+        PM_ASSERT_READOUT_MASK(ro2, NULL);
+        numCols = ro2->image->numCols;
+        numRows = ro2->image->numRows;
+    }
+    if (ro1 && ro2) {
+        PS_ASSERT_IMAGES_SIZE_EQUAL(ro1->image, ro2->image, NULL);
+    }
+    if (!ro1 && !ro2) {
+        psError(PS_ERR_UNEXPECTED_NULL, true, "No image provided.");
+        return false;
+    }
+    psAssert(numCols > 0 && numRows > 0, "There should be an image provided");
     PS_ASSERT_INT_NONNEGATIVE(size, NULL);
     PS_ASSERT_INT_NONNEGATIVE(footprint, NULL);
@@ -56,10 +72,9 @@
     }
 
-    int numCols = ro1->image->numCols, numRows = ro1->image->numRows; // Size of the images
-
     // Dereference inputs for convenience
-    psF32 **imageData1 = ro1->image->data.F32, **imageData2 = ro2->image->data.F32;
-    psImageMaskType **maskData1 = ro1->mask->data.PS_TYPE_IMAGE_MASK_DATA,
-        **maskData2 = ro2->mask->data.PS_TYPE_IMAGE_MASK_DATA;
+    psF32 **imageData1 = ro1 ? ro1->image->data.F32 : NULL;
+    psF32 **imageData2 = ro2 ? ro2->image->data.F32 : NULL;
+    psImageMaskType **maskData1 = ro1 ? ro1->mask->data.PS_TYPE_IMAGE_MASK_DATA : NULL;
+    psImageMaskType **maskData2 = ro2 ? ro2->mask->data.PS_TYPE_IMAGE_MASK_DATA : NULL;
 
     // First, a pass through to determine the fraction of bad pixels
@@ -69,9 +84,9 @@
         for (int y = 0; y < numRows; y++) {
             for (int x = 0; x < numCols; x++) {
-                if ((maskData1[y][x] & maskVal) || !isfinite(imageData1[y][x])) {
+                if (ro1 && ((maskData1[y][x] & maskVal) || !isfinite(imageData1[y][x]))) {
                     numBad++;
                     continue;
                 }
-                if ((maskData2[y][x] & maskVal) || !isfinite(imageData2[y][x])) {
+                if (ro2 && ((maskData2[y][x] & maskVal) || !isfinite(imageData2[y][x]))) {
                     numBad++;
                     continue;
@@ -128,8 +143,8 @@
     for (int y = 0; y < numRows; y++) {
         for (int x = 0; x < numCols; x++) {
-            if (maskData1[y][x] & maskVal) {
+            if (ro1 && maskData1[y][x] & maskVal) {
                 maskData[y][x] |= PM_SUBTRACTION_MASK_BAD_1;
             }
-            if (maskData2[y][x] & maskVal) {
+            if (ro2 && maskData2[y][x] & maskVal) {
                 maskData[y][x] |= PM_SUBTRACTION_MASK_BAD_2;
             }
@@ -144,12 +159,12 @@
 
     // Pixels that will be bad (or poor) if we convolve with a bad reference pixel
-    if (!psImageConvolveMask(mask, mask, PM_SUBTRACTION_MASK_BAD_1, PM_SUBTRACTION_MASK_CONVOLVE_1,
-                             -size, size, -size, size)) {
+    if (ro1 && !psImageConvolveMask(mask, mask, PM_SUBTRACTION_MASK_BAD_1, PM_SUBTRACTION_MASK_CONVOLVE_1,
+                                    -size, size, -size, size)) {
         psError(psErrorCodeLast(), false, "Unable to convolve bad pixels from mask 1.");
         psFree(mask);
         return NULL;
     }
-    if (!psImageConvolveMask(mask, mask, PM_SUBTRACTION_MASK_BAD_2, PM_SUBTRACTION_MASK_CONVOLVE_2,
-                             -size, size, -size, size)) {
+    if (ro2 && !psImageConvolveMask(mask, mask, PM_SUBTRACTION_MASK_BAD_2, PM_SUBTRACTION_MASK_CONVOLVE_2,
+                                    -size, size, -size, size)) {
         psError(psErrorCodeLast(), false, "Unable to convolve bad pixels from mask 2.");
         psFree(mask);
@@ -174,6 +189,6 @@
         psAbort("Unsupported subtraction mode: %x", mode);
     }
-    if (!psImageConvolveMask(mask, mask, maskRej, PM_SUBTRACTION_MASK_REJ,
-                             -footprint, footprint, -footprint, footprint)) {
+    if (ro1 && ro2 && !psImageConvolveMask(mask, mask, maskRej, PM_SUBTRACTION_MASK_REJ,
+                                           -footprint, footprint, -footprint, footprint)) {
         psError(psErrorCodeLast(), false, "Unable to convolve bad pixels.");
         psFree(mask);