Along with a huge notch on the XL variant, the Pixel 3 brought a lot of interesting camera features. Night Sight and Super Res Zoom are nothing short of magic (well, until you compare them to the Huawei P30), Top Shot is a godsend for those who are always a little late or early in tapping the shutter, and Photobooth is a nice addition for group and candid shots. Starting with version 6.2 of Google Camera, Photobooth can also detect kissing.
Until now, Photobooth used Artificial Intelligence and Machine Learning to detect when the people in the frame (one or more) had their eyes open, and one of five expressions: tongues out, smiling, puffy cheeks, making a kissy/ducky face, or looking surprised. When one of these occurred, it automatically snapped a shot.
Now, the team behind the Camera app has added kiss detection, which was an entirely new challenge. People who kiss don't necessarily look straight at the camera or have their eyes open, so a different model had to be used to train the algorithm. The team explains how each frame is scored, more weight is attributed to action happening in the foreground versus the background, and a buffer is used to see if one of the next frames has a higher score than a previous one. That helps in only saving the highest-score frames.
To try it out, all you have to do is make sure you're using Google Camera 6.2 (APK Mirror), turn on Photobooth mode, and start kissing people in front of the cam — preferably not strangers. If you have an older Pixel, you can always download the modded GCam by cstark27 to get Photobooth. Version P3v11 (APK Mirror) is based off v6.2 of Google Camera and should have the same smarts. I've tried it on my Pixel 2 XL, but sadly I'm alone now, and the algorithm wasn't fooled with a pretend kiss between two stuffed animals — hah!