The gen3 difference imaging seems to have lost support for some options which are not supposed to be deprecated. For example:

self.config.doAddCalexpBackground == True

and also looking at the code it does not seem that

self.config.doSubtract = False

will work either. Similarly I do not see selectSources and templateSources being passed by the gen3 runQuantum method. 

I have a fix for the `doAddCalexpBackground=True`, which I can send as a PR along with something which will also work for `doSubtract=False` . But I will need some background info on selectSources and templateSources.

To give more information, when I use 

 imageDifferenceTildeVisits:

    class: lsst.pipe.tasks.imageDifference.ImageDifferenceFromTemplateTask

    config:

        doAddCalexpBackground: True

I get an error for line:

 if self.config.doAddCalexpBackground:

    mi = exposure.getMaskedImage()

    mi += calexpBackgroundExposure.getImage()

which says NoneType object has no getImage method, as the calexpBackgroundExposure is not passed by the runQuantum method.