Facebook is training its computers to become seeing-eye guides for blind and visually impaired people as they scroll through the pictures posted on the world's largest online social network, the Associated Press said.

The feature rolling out Tuesday, April 5 on Facebook's iPhone app interprets what's in a picture using a form of artificial intelligence that recognizes faces and objects. The iPhone's built-in screen reader, VoiceOver, must be turned on for Facebook's photo descriptions to be read. For now, the feature will only be available in English.

The descriptions initially will be confined to a vocabulary of 100 words in a restriction that will prevent the computer from providing a lot of details. For instance, the automated voice may only tell a user that a photo features three people smiling outdoors without adding that the trio also has drinks in their hands, AP says.

Facebook is being careful with the technology, called "automatic alternative text," in an attempt to avoid making a mistake that offends its audience.

Eventually, though, Facebook hopes to refine the technology so it provides more precise descriptions and even answers questions that a user might pose about a picture.

Facebook also plans to turn on the technology for its Android app and make it available through Web browsers visiting its site.