  Multimedia retrieval plays an indispensable role in big data utilization.
Past efforts mainly focused on single-media retrieval. However, the
requirements of users are highly flexible, such as retrieving the relevant
audio clips with one query of image. So challenges stemming from the "media
gap", which means that representations of different media types are
inconsistent, have attracted increasing attention. Cross-media retrieval is
designed for the scenarios where the queries and retrieval results are of
different media types. As a relatively new research topic, its concepts,
methodologies and benchmarks are still not clear in the literatures. To address
these issues, we review more than 100 references, give an overview including
the concepts, methodologies, major challenges and open issues, as well as build
up the benchmarks including datasets and experimental results. Researchers can
directly adopt the benchmarks to promptly evaluate their proposed methods. This
will help them to focus on algorithm design, rather than the time-consuming
compared methods and results. It is noted that we have constructed a new
dataset XMedia, which is the first publicly available dataset with up to five
media types (text, image, video, audio and 3D model). We believe this overview
will attract more researchers to focus on cross-media retrieval and be helpful
to them.
