This feature allows animators to see a ghost image of the previous frame to ensure smooth motion.

Suddenly, the screen flickered. A system warning popped up: Low Disk Space. Frame capture interrupted.

He adjusted Barnaby’s elbow by a fraction of a millimeter. Click. The shutter of the DSLR camera fired, and the frame blossomed onto the screen. In the "Onion Skin" overlay, the ghost of the previous frame lingered, showing Arthur exactly how far his character had traveled. "Almost there, Barnaby," he whispered.

To anyone else, v3.6.1 was just an older build of stop-motion software—a relic of a few seasons past. To Arthur, it was a time machine.