Index: trunk/ppImage/src/ppImageReplaceBackground.c
===================================================================
--- trunk/ppImage/src/ppImageReplaceBackground.c	(revision 15928)
+++ trunk/ppImage/src/ppImageReplaceBackground.c	(revision 15928)
@@ -0,0 +1,70 @@
+#ifdef HAVE_CONFIG_H
+#include <config.h>
+#endif
+
+# include "ppImage.h"
+
+// In this function, we perform the psphot analysis routine for the chip-mosaicked images
+bool ppImageReplaceBackground (pmConfig *config, pmFPAview *view, ppImageOptions *options) {
+
+    bool status;
+    pmCell *cell;
+    pmReadout *readout;
+
+    // find the reference source image
+    pmFPAfile *input = psMetadataLookupPtr (&status, config->files, "PPIMAGE.CHIP");
+    if (!status) {
+        psError(PSPHOT_ERR_CONFIG, false, "PSPHOT.INPUT I/O file is not defined");
+        return false;
+    }
+
+    // find the model data, if it exists yet (if not, we build it below)
+    pmFPAfile *modelFile = psMetadataLookupPtr(&status, config->files, "PSPHOT.BACKMDL");
+
+    // select the appropriate recipe information
+    psMetadata *recipe  = psMetadataLookupPtr (&status, config->recipes, PSPHOT_RECIPE);
+
+    // define the mask value to be used in ppImage
+    psMaskType maskVal  = options->satMask | options->badMask | options->flatMask | options->blankMask;
+
+    // iterate over the cells and readout for this chip
+    while ((cell = pmFPAviewNextCell (view, input->fpa, 1)) != NULL) {
+	psLogMsg ("ppImagePhotom", 4, "Cell %d: %x %x\n", view->cell, cell->file_exists, cell->process);
+	if (! cell->process || ! cell->file_exists) { continue; }
+
+	// process each of the readouts
+	while ((readout = pmFPAviewNextReadout (view, input->fpa, 1)) != NULL) {
+	    if (! readout->data_exists) { continue; }
+	    if (! readout->mask) { continue; }
+
+	    // replace masked pixels with values from model (unbinning not needed)
+
+	    // we are using this pmFPAfile as an I/O file: select readout or create
+	    pmReadout *modelReadout = NULL;
+	    if (modelFile) {
+		modelReadout = pmFPAviewThisReadout (view, modelFile->fpa);
+	    }
+	    if (!modelReadout) {
+		psphotImageMedian (config, view, maskVal);
+		modelFile = psMetadataLookupPtr(&status, config->files, "PSPHOT.BACKMDL");
+		assert (modelFile);
+		modelReadout = pmFPAviewThisReadout (view, modelFile->fpa);
+		assert (modelReadout);
+	    }
+
+	    psImageBinning *binning = psMetadataLookupPtr(&status, recipe, "PSPHOT.BACKGROUND.BINNING");
+
+	    psImage *mask = readout->mask;
+	    psImage *image = readout->image;
+	    psImage *model = modelReadout->image;
+
+	    for (int iy = 0; iy < image->numRows; iy++) {
+		for (int ix = 0; ix < image->numCols; ix++) {
+		    if (!(mask->data.U8[iy][ix] && maskVal)) continue;
+		    image->data.F32[iy][ix] = psImageUnbinPixel_V2(ix, iy, model, binning);
+		}
+	    }
+	}
+    }
+    return true;
+}