Valve appears to be working on a next generation version of its hugely versatile Source engine, according to lines of code from the recently released Source Filmmaker tool.

Spotted by, code segments from the movie making tool reference "Source 2".

"Return an str with the current engine version. If key doesn't doesn't exist, assume 'Source', otherwise invalid -- assume next-gen 'Source 2'," reveals one section of code.

Source made its debut in 2004 with Counter-Strike Source and Half-Life 2, but has been updated over the years to enable the tech to produce the likes of Portal 2.

However, references to Source 2 suggests Valve is preparing to roll out a true next generation version of its engine.