.    Imagine browsing a website when an attractive ad for lingerie catches your eye. You don't click on it, merely smile and go to another page. Yet it follows you, putting up more racy pictures, perhaps even the offer of a discount. Finally, annoyed by its persistence, you frown. "Sorry for taking up your time," says the ad, and promptly stops further disturbance. Creepy. But making online ads that not only know you are looking at them but also respond to your emotions will soon be possible, thanks to the power of image-processing software and the common existence of tiny cameras in computers and mobile devices.
    Uses for this technology would not, of course, be confined to advertising. There is ample scope to apply it in areas like security, computer gaming, education and health care. But advertisers are among the first to embrace the idea in earnest. Advertising firms already film how people react to ads, usually in an artificial setting. The participants' faces are studied for positive or negative feelings. A lot of research has been done into ways of categorizing the emotions behind facial expressions. Some consumer-research companies also employ cameras to track eye movements so they can be sure what their subjects are looking at. This can help determine which ads attract the most attention and where they might be placed for the best effect on a web page.
    One of the companies doing such work, Realeyes, which is based in London, has been developing a system that combines eye-spying webcams with emotional analysis. Mihkel Jaatma, who founded the company in 2007, says that his system is able to detect a person's mood by plotting the position of facial features, such as eyebrows, mouth and nostrils, and employing clever programs to interpret changes in their alignment—as when eyebrows are raised in surprise, say. Add eye-movement tracking, hinting at which display ads were overlooked and which were studied for any period of time, and the approach offers precisely the sort of quantitative data brand managers yearn for.
    At present the system is being used on purpose-built websites with, for instance, online research groups testing the effect of various display ads. The next step is to make interactive ads. Because they can spot the visual attention given to them, as well as the emotional state of the viewer, these ads could change their responses.
    As similar technologies become widespread, privacy concerns will invariably increase. People would need to give consent to their webcams being used in this way, Mr. Jaatma admits. One way to persuade Internet users to grant access to their images would be to offer them discounts on goods or subscriptions to websites.6.  The imagined situation in Paragraph 1 introduces ______.