Google is working on wallpaper support for the always-on display feature of Android 9 Pie, according to a number of source code snippets discovered by XDA Developers. The functionality still appears to be mid-development and it's unclear when it might end up hitting the stable channel, though the newly discovered backend indicates Google opted for a relatively basic implementation that won't be as versatile as the one Samsung has been offering on its Galaxy-branded smartphones for several years now.
While the stock version of Android will eventually allow static images to be used as always-on-display wallpapers, the default system behavior will see them hidden after a minute. The transition between a wallpaper and a traditional AOD background will be conducted via a cross-fading animation which lasts 400 milliseconds by default, as per the same source. For comparison, Samsung's AOD framework can display static images as wallpapers indefinitely and even allows for shorter GIFs to be used in the same manner. Using AOD wallpapers as opposed to a conventional clock is likely to negatively affect the battery life of a given device due to the way OLED panels work - the more complex the graphic, the more pixels will have to be backlit by the device, which consequently requires more energy.
ADB commands can be used for customizing the default behavior of Android 9 Pie's AOD wallpapers but Google is unlikely to deliver many user-facing modification options in the near future. The latest major edition of the world's most popular operating system hit the stable channel earlier this week and is already available on the first two generations of the Pixel flagships, as well as the Essential PH-1. Sony, Vivo, HMD Global (Nokia), OnePlus, and a number of other handset manufacturers already confirmed they'll start bringing the new OS to their mobile products in the final quarter of the year, i.e. early fall.