PEOPLE have long used webcams on their laptop and desktop computers to add live video to Internet calls. But the face-to-face chats often include grainy, low-resolution images and much crowding around the computer when the whole family wants to get in the picture. Now wide-angle cameras that pop onto large-screen televisions are on the market; they capture high-definition video and a generous stretch of the living room sofa, too. Several devices, including the TV Cam HD ($199.99) from Logitech , are already on sale, with at least a half-dozen others expected in time for the holiday shopping season, said Richard Doherty, research director of the Envisioneering Group, a market research company in Seaford, N.Y. The new TV cams are for people who want to add an Internet-based feature to their high-definition TV’s. “You can add this capability for a few hundred dollars or less,” Mr. Doherty said. “Lots of people have HDTVs they’ve bought in the last few years, a...