They "took a lot for granted" with Half-Life 1's engine, but Spark is their creation. It's always being compared to Valve's Source, it's a "complicated process".
"We took a lot for granted being on the Half-life 1 engine. The core marine player movement was easier back - it was just tweaking a couple constants and we’re done," said Unknown Worlds' Charlie Cleveland, referring to Half-Life mod Natural Selection.
"Now player movement is a much bigger problem involving general engine performance and smoothness, input tweaks, physics, networking and Lua script work. The aliens are another huge layer on top of that with Skulk wall-walking, Fade “blinking”, etc."
"We are now at the point where the basic movement is pretty solid but it’s definitely still being worked on. We are constantly loading Natural Selection 1, Team Fortress 2 and other games to compare Natural Selection 2 side-by-side with them," he continued.
"Often there’s something that feels wrong, but it is unclear if it’s input, performance, script bugs, weapon cones of fire or even enemy player speeds. It is just a complicated process and we’re going to be refining it for a long time and it feels amazing."
Unknown Worlds has its digs in San Francisco having moved from Boston. So far the new studio has had "over 20,000 pre-orders to date" for Natural Selection 2. It's a game that mixes multiplayer shooting and even strategy - 'Counter-strike meets StarCraft'.