Index: trunk/psphot/src/pmCellSetMask.c
===================================================================
--- trunk/psphot/src/pmCellSetMask.c	(revision 6117)
+++ trunk/psphot/src/pmCellSetMask.c	(revision 6311)
@@ -6,4 +6,7 @@
 
     // mask the excluded outer pixels
+    // these coordinates refer to the parent image
+    // these bounds will saturate on the subimage
+    // negative upper bounds will subtract from the *subimage*
     float XMIN  = psMetadataLookupF32 (&status, recipe, "XMIN");
     float XMAX  = psMetadataLookupF32 (&status, recipe, "XMAX");
@@ -17,5 +20,9 @@
         pmReadout *readout = readouts->data[i]; // The readout of interest
 
+	// psRegionForImage creates a region for the subimage, not the parent
+	// XXX I propose that psRegionForImage treat the region as referring to the parent...
 	psRegion keep = psRegionForImage (readout->image, valid);
+
+	// psImageKeepRegion assumes the region refers to the parent coordinates
 	psImageKeepRegion (readout->mask, keep, "OR", PSPHOT_MASK_INVALID);
     }