Apple Inc. is upgrading its iPad lineup to fend off a growing list of competitors, which are introducing their own tablets at lower prices with snazzier features.

Chief Executive Officer Tim Cook will debut a high-definition iPad mini and a thinner iPad at a San Francisco event Tuesday, people with knowledge of the plans have said. 

Facing two straight quarters of declining profit and a stock that’s down by more than a quarter from a September 2012 record, Apple is facing a similar challenge with the iPad as it has with the iPhone, battling lower-cost rivals and proving that incremental changes to existing products are enough to draw customers. The iPad is Apple’s second-best selling gadget after the iPhone and the new models will be critical as the company seeks to reignite growth.

“Tablets are a maturing market,” said Sarah Rotman Epps, an analyst at Forrester Research Inc. in San Francisco. “It will be difficult for Apple to move the needle on new tablet sales, as the strongest growth is coming from emerging markets where customers are more price sensitive.”

Apple saw its share of the tablet market contract to 32 percent in the second quarter from 60 percent a year earlier, according to researcher IDC. While iPad sales more than doubled every quarter since the 2010 debut, growth of the devices hasn’t topped 66 percent since mid-2012.

Trudy Muller, a spokeswoman for Apple, declined to comment ahead of the event. Cook has said market-share data alone isn’t the best way to judge success.

“Customers continue to love their iPads,” Cook said in July, citing data from analytics firm Chitika Inc. showing that that the iPad makes up 84 percent of Web traffic from tablets.

Samsung Electronics Co., Asustek Computer Inc., Lenovo Group Ltd., Acer Inc. and other competitors are challenging Apple, offering devices with prices starting at less than half of the iPad mini’s $329. Amazon.com Inc. introduced a new Kindle Fire lineup last month with higher-resolution screens at prices starting from $229.

While tablet shipments doubled to 166 million globally in 2012, Counterpoint Research projects that the growth rate will slow to 28 percent in 2014, to 301 million units. (Bloomberg)