This sample application, called Emojify, demonstrates a Kotlin backend which communicates with a mobile frontend. The backend receives an input image from the frontend and overlays an emoji on each of the detected faces in the image based on facial expressions.
Follow the Extend an Android frontend with a Kotlin backend running on App Engine codelab to build and run it yourself!
Example of source and emojified images:
Picture taken at DroidCon NYC Extended.