  The use of explicit object detectors as an intermediate step to image
captioning - which used to constitute an essential stage in early work - is
often bypassed in the currently dominant end-to-end approaches, where the
language model is conditioned directly on a mid-level image embedding. We argue
that explicit detections provide rich semantic information, and can thus be
used as an interpretable representation to better understand why end-to-end
image captioning systems work well. We provide an in-depth analysis of
end-to-end image captioning by exploring a variety of cues that can be derived
from such object detections. Our study reveals that end-to-end image captioning
systems rely on matching image representations to generate captions, and that
encoding the frequency, size and position of objects are complementary and all
play a role in forming a good image representation. It also reveals that
different object categories contribute in different ways towards image
captioning.
