I am trying to do the same thing - I need to grab a raw still image on demand. I am currently using the currentImage in ARSession, but would like to get better resolution and colour depth. Ideally I would just like to add an output device e.g. AVCaptureOutput to the ARSession and grab a shot whenever needed. I have tried setting up my own AVCaptureSession but they don't seem to play nicely together. I will try momentarily pausing the ARSession while I grab the shot.

OK, I have been using ARImkageTrackingConfiguration - that doesn't track anything except Image anchors. Unfortunately that provides me with a reason why this doesn't work, but unfortunately the ARWorldTrackingConfiguration does't work for my image requirements, as it does not update frequently enough. Hmmmm.