Vivo X21 is the first smartphone which includes an in-display sensor. This technology allows to avoid the use of buttons or fingerprint sensors in the side or at the back of a smartphone, the sensor is part of the display.
This optical sensor has been developed by Synaptics and is able to read the ridges of a fingerprint through an OLED display.
A tendency on this type of fingerprint sensors is expected for the next years, as other manufacturers, such as Xiao Mi or Samsung have shown their interest in this technology.
Further information: https://www.synaptics.com/company/news/clear-id-vivo-x21-ud-smartphone