Walmart is about to fire a warning shot at Apple’s iTunes: The retail colossus will introduce on Wednesday a way for iPad users to access VUDU’s Web site to buy or rent videos and view them on the tablet. “We’re committed to offering the VUDU experience on as many devices as possible so customers can shop for and access their favorite movies and TV shows however they want, whenever they want,” says VUDU general manager Edward Lichty. Walmart has a mixed track record in video. Although it’s the leading seller of DVDs, the retail chain failed in an effort to create a Netflix-like mail rental service. It also stumbled in its partnership with Hewlett Packard to offer movie downloads. Walmart bought VUDU in 2010 and has slowly been building it into a service that’s too powerful to ignore. The retailer says that VUDU is available on more than 300 electronics devices including the PlayStation 3, making it the most widely accessible broadband pay-per-view service. Its library includes more than 20,000 movies and TV shows and can be streamed at full 1080p high-definition video quality with Dolby Digital Plus 5.1 surround sound.