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Source is a 3D video game engine developed by Valve Corporation. The engine made its debut in June 2004, where it was used for the development of Counter-Strike: Source and shortly followed by Half-Life 2. It has been in an active development ever since.
Respawn Entertainment used a heavily modified version of the Source engine to develop Titanfall and its sequel. This is not dissimilar to the IW engine many of the team members were familiar with, which was and remains to this day a heavily modified id Tech 3 engine. Respawn added a completely new lighting system with support for Physically Based Rendering, and all of the core gameplay systems are completely different from the ones seen in Valve made titles.
Despite all these changes, the elements used to build the game are very similar, using similar formats such as VTF for images, BSP for maps, albeit heavily modified, and NUT for scripting. There are also console commands that are similar to Source, although some have been changed to be similar to commands used in Call of Duty 4.