There’s a catch, of course. Despite its high marks for creativity, the ad unit doesn’t register as viewable, at least by industry definitions. Celtra said that because the ad unveils bit by bit, half of it is not viewable by a user at any time. The IAB considers ads viewed when more than 50 percent is seen by the user. But core viewable impressions — that some of the ad, at least was seen — will still be registered.