The setting “Ambient mode” controls whether bubbles show in Always-ON Display or not:
“Hollow bubbles”
All bubbles are shown in ambient mode (the bubbles are not filled, just the outlines for burn-in protection)
“Filled bubbles” (may harm OLED)
This option lets you have the same full-color bubbles in ambient mode as you have in active mode.
Note:Do not use this setting unless you are sure your watch has LCD screen which is not damaged by constant pixel use!
For more information please see this post: Advanced tip: LCD screen ambient mode
“Full clock only”
This excludes every other bubble from the ambient screen
“Selective Hollow”
In “selective hollow” mode we have to control each bubble’s ambient visibility individually. Long press on the date bubble and switch its ambient mode on:
Then you should be able to see it both in the active and inactive (ambient) modes:
For more information about “Selective hollow” mode, please see this post: Selective ambient mode
Thank you to Hermano Alexandre for the emailed question prompting this post!
