ForcedPhot tasks are now responsible for attaching Footrints to sources

Forced photometry previously relied on the MeasureSources class to
transform reference Footprints and attach them to sources, but we've
moved that responsibility here so we can override it at the Python
level in the future.

python/lsst/pipe/tasks/forcedPhotImage.py

@@ -72,6 +72,7 @@ class ForcedPhotImageTask(CmdLineTask):
      - Set the _DefaultName class attribute
      - Implement makeIdFactory
      - Implement fetchReferences
+     - (optional) Implement attachFootprints
     """
 
     RunnerClass = ButlerInitializedTaskRunner
@@ -130,6 +131,24 @@ def fetchReferences(self, dataRef, exposure):
         """
         raise NotImplementedError()
 
+    def attachFootprints(self, dataRef, sources, references, exposure, refWcs):
+        """Hook for derived classes to define how to attach Footprints to blank sources prior to measurement
+
+        Footprints for forced photometry must be in the pixel coordinate system of the image being
+        measured, while the actual detections may start out in a different coordinate system.
+
+        Subclasses for ForcedPhotImageTask must implement this method to define how those Footprints
+        should be generated.
+
+        The default implementation transforms the Footprints from the reference catalog from the refWcs
+        to the exposure's Wcs, which downgrades HeavyFootprints into regular Footprints, destroying
+        deblend information.
+        """
+        exposureWcs = exposure.getWcs()
+        region = exposure.getBBox(lsst.afw.image.PARENT)
+        for refRecord, srcRecord in zip(sources, references):
+            srcRecord.setFootprint(refRecord.getFootprint().transform(refWcs, exposureWcs, region))
+
     def getExposure(self, dataRef):
         """Read input exposure on which to perform the measurements
 
@@ -181,6 +200,7 @@ def run(self, dataRef):
             references = list(self.fetchReferences(dataRef, exposure))
             self.log.info("Performing forced measurement on %d sources" % len(references))
             sources = self.generateSources(dataRef, references)
+            self.attachFootprints(dataRef, sources, references=references, exposure=exposure, refWcs=refWcs)
             self.measurement.run(exposure, sources, references=references, refWcs=refWcs)
             self.writeOutput(dataRef, sources)
             return Struct(sources=sources)