  Grounding referring expressions aims to locate in an image an object referred
to by a natural language expression. The linguistic structure of a referring
expression provides a layout of reasoning over the visual contents, and it is
often crucial to align and jointly understand the image and the referring
expression. In this paper, we propose a scene graph guided modular network
(SGMN), which performs reasoning over a semantic graph and a scene graph with
neural modules under the guidance of the linguistic structure of the
expression. In particular, we model the image as a structured semantic graph,
and parse the expression into a language scene graph. The language scene graph
not only decodes the linguistic structure of the expression, but also has a
consistent representation with the image semantic graph. In addition to
exploring structured solutions to grounding referring expressions, we also
propose Ref-Reasoning, a large-scale real-world dataset for structured
referring expression reasoning. We automatically generate referring expressions
over the scene graphs of images using diverse expression templates and
functional programs. This dataset is equipped with real-world visual contents
as well as semantically rich expressions with different reasoning layouts.
Experimental results show that our SGMN not only significantly outperforms
existing state-of-the-art algorithms on the new Ref-Reasoning dataset, but also
surpasses state-of-the-art structured methods on commonly used benchmark
datasets. It can also provide interpretable visual evidences of reasoning. Data
and code are available at https://github.com/sibeiyang/sgmn
