Ars Technica has posted a great video interview with Paul Neurath, the founder of Thief: The Dark Project developer Looking Glass Studios, that charts the creation of the 1998 classic's revolutionary stealth system.
There were plenty of hurdles and speed bumps along the way, and even after a year-and-a-half of development the stealth system still wasn't working correctly.
"As a player, you got confused signals," explains Neurath. "You didn't necessarily know what the guard was thinking, or how well you were hiding."
At that stage, players might be able to walk right past a guard without them noticing, while on other occasions seemingly well-concealed players would be spotted instantly. It was sloppy at best, and created a lot of tension around the office.
One particular frustration related to how players were perceiving light and shadow. It became clear that some were struggling to understand when they were hidden, and when they might be at risk of being discovered. A solution was sorely needed, and the fix was surprisingly simple.
"It was one of the things we didn't fully understand. We had to feed back to the player their stealth state in terms of visibility. Showing that in a very clear way is important, because if you don't know wether you're in shadow a lot of [the system] just doesn't work. The player is like 'why did the guard just detect me? I don't know. I don't know how hidden I am.'
"One of the team members came up with the concept of a light gem that shows up at the bottom of the screen. It's just a user interface icon, but the light gem gets brighter or dimmer depending on how visibly hidden you are, and there's also an audio component. That really helped, and you could just quickly look at it and intuitively figure out 'oh, I'm really well hidden now.'"
You can learn more about how the system was refined and implemented by checking out the full video over on Ars Technica's YouTube channel. It's well worth a watch.