The paper presents a simple and effective sketch-based algorithm for large scale image retrieval. One of the main challenges in image retrieval is to localize a region in an image which would be matched with the query image in contour. To tackle this problem, we use the human perception mechanism to identify two types of regions in one image: the first type of region (the main region) is defined by a weighted center of image features, suggesting that we could retrieve objects in images regardless of their sizes and positions. The second type of region, called region of interests (ROI), is to find the most salient part of an image, and is helpful to retrieve images with objects similar to the query in a complicated scene. So using the two types of regions as candidate regions for feature extraction, our algorithm could increase the retrieval rate dramatically. Besides, to accelerate the retrieval speed, we first extract orientation features and then organize them in a hierarchal way to generate global-to-local features. Based on this characteristic, a hierarchical database index structure could be built which makes it possible to retrieve images on a very large scale image database online. Finally a real-time image retrieval system on 4.5 million database is developed to verify the proposed algorithm. The experiment results show excellent retrieval performance of the proposed algorithm and comparisons with other algorithms are also given.

We propose a real-time image retrieval system which allows users to search target images whose objects are similar to the query in contour, regardless of their sizes and positions appearing in the images. Even in a complicated scene, as long as the object's contour is most salient in the target image, the system is still able to capture it and lists the image in the retrieval results. Therefore, the system has better retrieval rate than existing systems and algorithms. One typical application of the proposed system is to help the computer understand what does the user draw or upload. It is based on the statistical distributions of tags of retrieved images, and the proposed system feeds back some candidate tags related to the query image. Such tags could be used for further retrieval to refine the result list. In addition, the system provides a friendly interactive interface with multiple queries. These queries are from different combinations of tags, a hand-drawn sketch and a natural image, and could help users search images flexibly and conveniently. The system runs on a database of 1.3 million images and could achieve a real-time retrieval speed. The results in the demonstration show excellent retrieval performance of the proposed system.