Inside Velocity 2X: Physics
If at first you don’t succeed…
The physics for Velocity 2X went through at least 3 full iterations, and was a great learning experience for the studio.
We started with a physics system that was just based on circle/line collision and response. This meant we had Kai Tana represented by a tiny circle in the physics world and the environment represented by lines. This worked fine until we got to the point where we had to implement teledashing. Representing the teledash cursor in the physics world was easy, but if the environment is made out of lines, then the teledash cursor can be inside an object without a way for us to know that it’s inside. Lines just create hollow shapes:
The fix was easy: we added support for solid primitives such as Rectangles, Triangles etc. Fantastic, problem solved. This solution worked until we came to implementing throwing telepods.
Because of the speed at which telepods are thrown, they started tunneling through the environment.
At this stage, it became quite obvious that the physics engine for our game was not going to be as simple as we had initially hoped.
Always keen to ‘help’, James (not technical) decided to take our coding woes into his own hands and recommended porting his old Flash ActionScript physics engine to C++…
Those interested, his solution to tunneling: increase the size of the telepod object to twice its size for the first few frames while it’s moving extremely fast! Yes, that is what happens when a designer assumes the role of a programmer – and – believe it or not, I accepted that idea and went down that route.
After a few days of banging my head against the Flash code (trust me it was hell), it became obvious that we couldn’t have been more wrong about porting code written by James (sorry James, no coding for you anymore buddy). [Hey, that code worked fine for Velocity and Coconut Dodge! – JM]
Finally we decided to look at middleware options. After shopping around for a week, Box2D became the popular choice. We integrated Box2D into our engine, adding support for representing the environment as ghosted edge shapes, with polygonal shapes representing the internal hulls. By the end of the project we had multiple collision shapes performing different functions for Kai, as shown in the image below:
Lean and mean
The physics for Velocity 2X is actually quite simple in the sense that we don’t use any joints, fluids or chain based shapes. The complexity for the physics system in Velocity 2X comes from the player performing multiple actions in various configurations at high speed!
Open source saviour
We are pleased that we went through different options before finalizing on Box2D; the more we failed, the more we learned from our mistakes. The most important lesson being never to listen to James when it comes to programming =)
Check out our use of Box2D physics in the trailer below: