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Clipping vs Substepping

· 4 min read
raysplaceinspace
raysplaceinspace
Creator of Easel

Like many game engines, Easel simulates physics in discrete time steps, or "ticks". There are 60 ticks per second.

Sometimes, if you have a fast-moving object, it can move so far in 1/60th of a second that it can have already passed through to the other side of a wall before the physics engine has a chance to notice it. This can cause problems like:

  • Tunneling: A bullet moving fast could pass right through a thin wall and hit someone on the other side. Ouch.
  • Penetration: For a hero jumping and landing on the ground, you want the hero to land level on the ground, and not partially sink into it before the physics engine catches the collision and pushes them back up.

A Physics Engine with Incremental Rollback

· 9 min read
raysplaceinspace
raysplaceinspace
Creator of Easel

We want Easel to be powerful enough to make the kinds of games you would play for hours. Popular multiplayer games like Among Us let you walk around an entire spaceship, completing tasks and evading impostors. Unfortunately, up to this point, games of that scale were out of reach for Easel, because the off-the-shelf physics engine would have to snapshot and roll back the entire world to support Easel's predictive multiplayer architecture. It's too much to do every frame.

Until this point, you were required to keep your world small. But not anymore!

Easel's new custom-built physics engine only snapshots and rolls back the parts of the world that change. That big spaceship might have thousands of objects forming the walls, the control panels, the vents, and so on. However, each frame, a surprisingly few number of objects actually change - perhaps less than 30 per frame as the players walk around and interact with the world. A smart implementation keeps objects sleeping while they are offscreen.

With only 30 objects out of thousands needing to be snapshotted each frame, a factor of 30-50x fewer than before, multiplayer Easel games with large worlds suddenly becomes feasible. Release the feral hogs!