In-App advert for HD Mode.
HD Mode is an add-on for the iOS versions of Marathon, Marathon 2, and Marathon Infinity that replaces the original games' textures with higher resolution equivalents.
Description[]
HD Mode is available as an optional in-app purchase for the iOS versions of Marathon, Marathon 2, and Marathon Infinity. At launch, the price of this add-on was $0.99; the current price is $1.99. The enhanced textures include a complete replacement for all walls, floors, and liquids.
Since Bungie started providing level editing tools to the public early in the history of the Marathon Trilogy, many texture replacements have been available for the games.