Why meaningful automatic tagging of images is very hard

The paper points out that while automatic image tagging is often studied in connection with content-based image retrieval (CBIR), it is actually a much harder problem. Given the difficulty of the latter, the prospects for automatic image tagging do not appear promising. A brief survey of the current state of the art confirms that conclusion. Then the paper discusses an effort to tag images based on nonpixel data and proceeds with the outline of a case where the difficulty of automatic tagging is taken advantage to construct image based CAPTCHA to distinguish human users from Web-bots. That has led to certain interesting approaches to achieve reliable human tagging that is needed for the CAPTCHA application.