Socializing the Semantic Gap: A Comparative Survey on Image Tag Assignment, Refinement and Retrieval

Where previous reviews on content-based image retrieval emphasize on what can be seen in an image to bridge the semantic gap, this survey considers what people tag about an image. A comprehensive treatise of three closely linked problems, i.e., image tag assignment, refinement, and tag-based image retrieval is presented. While existing works vary in terms of their targeted tasks and methodology, they rely on the key functionality of tag relevance, i.e. estimating the relevance of a specific tag with respect to the visual content of a given image and its social context. By analyzing what information a specific method exploits to construct its tag relevance function and how such information is exploited, this paper introduces a taxonomy to structure the growing literature, understand the ingredients of the main works, clarify their connections and difference, and recognize their merits and limitations. For a head-to-head comparison between the state-of-the-art, a new experimental protocol is presented, with training sets containing 10k, 100k and 1m images and an evaluation on three test sets, contributed by various research groups. Eleven representative works are implemented and evaluated. Putting all this together, the survey aims to provide an overview of the past and foster progress for the near future.Comment: to appear in ACM Computing Survey

Less is MORE: a MultimOdal system for tag REfinement

With the proliferation of image-based social media, an ex-tremely large amount of multimodal data is being produced. Very oftenimage contents are published together with a set of user defined meta-data such as tags and textual descriptions. Despite being very useful toenhance traditional image retrieval, user defined tags on social mediahave been proven to be noneffective to index images because they areinfluenced by personal experiences of the owners as well as their will ofpromoting the published contents. To be analyzed and indexed, multi-modal data require algorithms able to jointly deal with textual and visualdata. This research presents a multimodal approach to the problem of tagrefinement, which consists in separating the relevant descriptors (tags)of images from noisy ones. The proposed method exploits both Natu-ral Language Processing (NLP) and Computer Vision (CV) techniquesbased on deep learning to find a match between the textual informationand visual content of social media posts. Textual semantic features arerepresented with (multilingual) word embeddings, while visual ones areobtained with image classification. The proposed system is evaluated ona manually annotated Italian dataset extracted from Instagram achieving68% of weighted F1-scor