 Thief II was built with the third iteration of the Dark Engine , which had been used previously for Thief and System Shock 2 . According to Pearsall , the Dark Engine had become " a very well understood development environment " , which made for an easier production process . Engine updates created for System Shock 2 , such as support for 16 @-@ bit color , were carried over to Thief II . The average character model in Thief II was given close to double the polygons of the average model in Thief , with much of the added detail focused on characters ' heads . This was an attempt to give the characters a " more organic " look . Certain artificial intelligence ( AI ) routines written into the Dark Engine , which allowed enemies to notice changes in the environment such as open doors , had not been used in Thief or in System Shock 2 but were implemented in Thief II . Weather effects such as fog and rain were added , and technology from Flight Unlimited III was used to generate the sky and clouds . 
